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' i. O to & 
 
 [From the Annals and Magazine of Natural History for 
 September 1887.] 
 
 The minute structure of the eye in the Cymothoidge has been 
 treated of by Johannes Muller and more recently by J. F. 
 Bullar f ; the observations of the older author principally 
 concern the cuticular lenses and the vitreous body, and are 
 immaterial to the present note. Bullar has described and 
 figured the eye of Cymothoa in some detail ; his results on 
 the whole show no great difference from the eye of Porcellio y 
 which has been investigated by Grenacher and described in 
 
 CU3 
 
 % 
 
 Note on a neio Type of Compound Eye. 
 By F. E. Beddard, M.A., F.Z.S. 
 
 * Meckel’s 1 Archiv,’ 1829. 
 t Phil. Trans. 1878. 
 
 542407 
 
234 
 
 Mr. E. E. Beddard on a 
 
 the eye in several 
 
 his important memoir * on the Arthropod eye. The vitrellaf 
 in both types consists of two cells, which secrete a round or 
 pear-shaped crystalline cone ; this crystalline cone is evidently 
 composed of two halves closely applied together, each half 
 being formed from a single cell of the vitrella. 
 
 The retinula in both types is seven-celled ; each cell secretes 
 a chitinous refracting rod — the rhabdomere ; these become 
 fused into an axial structure — the rhabdom — in Porcellio ; in 
 Cymotlnoa each rhabdomere remains separate and within the 
 retinula-cell of which it is a product. 
 
 I have recently studied the structure of 
 species of JEga and allied genera, 
 and find some notable differences 
 from the types already mentioned 
 as well as from all other Tsopods, 
 excepting the genus Serolis. In 
 Ser oil's | the retinula differs from 
 that of Porcellio &c. in being 
 composed of only four cells ; each 
 cell secretes at its upper extremity a 
 chitinous rhabdomere : the rhabdo- 
 meres are more or less completely 
 fused together along their inner faces, 
 but the rhabdom is not imbedded 
 between the retinula-cells ; on the 
 contrary, each of these cells, owing 
 to its peculiar shape, is only in con- 
 tact with the upper part of the 
 rhabdom ; the lower portion is sur- 
 rounded by two large spherical trans- 
 parent cells , which fit in closely 
 between the four retinula-cells (see 
 woodcut). These cells are distinctly 
 nucleated (h), the nucleus possessing 
 a well-defined nucleolus. In sections 
 it can be readily seen that the rhab- 
 dom, which at its inferior extremity 
 becomes divided into four separate 
 pieces (corresponding of course to 
 the four rhabdomeres of which it is 
 composed), is imbedded in, or at least is entirely surrounded 
 by, the substance of these large clear cells. 
 
 * 1 Sehorgan der Arthropoden/ Gottingen, 1879. 
 
 t This term has been introduced by Profs. Lankester and Bourne 
 (Quart. Journ. Micr. Sci. 1883, p. 177). 
 
 X “ Report on the Isopoda collected during the Voyage of H.M.S. 
 ‘ Challenger,’ ” Zool. Chall. Kxp. pt. xxxiii. 
 
 Ommatidiuin 
 
 Schythei. 
 
 of Serolis 
 vitrella- 
 
 cells ; n, retinula-cells ; 
 r, rhabdom ; n, hyaline 
 cells. 
 
new Type of Compound Eye. 
 
 235 
 
 In several species of Cymothoidce I have been able to recog- 
 nize the presence of these same hyaline cells both in sections 
 and in teased preparations : I invariably found two present, 
 and their relation to the retinula-cells and to the rhabdom 
 was precisely as described above in Serolis. EEga, however, 
 agrees with Cymothoa and other Isopods and differs from 
 Serolis in the fact that there are seven cells to each retinula ; 
 but in the presence of these remarkable hyaline cells, as well 
 as in their structure and position, EEga exhibits a striking 
 resemblance to Serolis , and differs, so far as our knowledge 
 goes, from all other Isopods. This structural resemblance 
 between EEga and Serolis tends further to confirm the view, 
 held by many carcinologists, of the close relationship between 
 the Serolidae and Oymothoidae. 
 
 In one of my figures of the structure of the eye in Serolis 
 Schythei ( loc . cit. pi. ix. fig. 5) I have depicted the rhabdom 
 as ending in a fine filament which passes through the hyaline 
 cell as far back as the membrane which bounds the omma- 
 teum posteriorly ; I have also (figs. 3, 4) noted a similar 
 prolongation of the rhabdom in Serolis cornuta. 
 
 On again referring to my preparations of both these species 
 I find that those figures are not quite accurate. In Serolis 
 Schythei the rhabdom has not the conical form which I have 
 erroneously given to it in my drawing ; it ends in four blunt 
 points (cf. woodcut) : just below the termination of the rhab- 
 dom is a bundle of delicate fibrils which unite into a single 
 fibre (r) ; this passes through the substance of the hyaline cells 
 and can be traced back as far as the ommateal membrane. 
 In S. cornuta the arrangement is identical. 
 
 In some young examples of S. Schythei , taken from the 
 brood-pouch of the mother, this bundle of delicate fibres, ter- 
 minating in a single long fibre, was present, and appeared 
 from its position to be a product of the four pigmented 
 retinula-cells.. At this stage the thickened masses which 
 form the; greater portion of the rhabdom in the adult eye were 
 not developed. If it were not for this fact the bundle of 
 fibrils (r in woodcut) in the adult eye would seem to have 
 nothing to do with the rhabdom of the pigmented retinula- 
 cells, but to be anteriorly formed by the hyaline cells. It is 
 indeed quite possible that it is in part formed by these cells. 
 If this be so, the retinula in Serolidas and Cymothoidas is 
 composed of six cells, two transparent cells surrounded by 
 four pigmented cells, all of which secrete chitinous rods. The 
 central transparent cells, however, do not appear to end in 
 nerve-fibres, unless the axial chitinous rod contains nerve- 
 fibrils, which is of course a mere suggestion. 
 
233 
 
 On a new Type of Compound Eye. 
 
 The structure of each retinula is therefore clearly very 
 similar to that of the retinula of many mollusks as described 
 by Patten, and, which is more important for purposes of com- 
 parison, to Nereis among Annelids if Patten’s interpretation * 
 of Carribre’s figures be allowed. The two. central clear cells 
 are Patten’s 1 retinophorae.’ It will be observed, however, 
 that apart from these two problematical hyaline cells the 
 minute structure of the eyes of the Serolidae and Cymotho- 
 idae bear out Grenadier’s conclusions rather than Patten’s 
 with regard to the morphology of the Crustacean eye. There 
 can be no doubt that the crystalline cone is independent of 
 the rhabdom and formed by different cells. 
 
 The specialization of the retinula-cells is, however, a new 
 feature, and distinguishes the eye of these Isopods. 
 
 * Mitth. Zool. Stat. Neap 1, 1886. 
 

V ~*J ) 
 
1890.] ON THE STRUCTURE OF THE EYE IN ARCTURUS. 
 
 365 
 
 [ From the Proceedings of the Zoological Society of 
 London, May 6, 1890.] 
 
 On the Minute Structure of the Eye in some Shallow- 
 Water and Deep-Sea Species of the Isopod Genus 
 Arcturus. By Frank E. Beddard, M.A., Prosector to 
 the Society. 
 
 (Plate XXXI.) 
 
 Three years ago I communicated a paper to the Royal Society of 
 Edinburgh upon the structure of the Eye in the two Isopodan families 
 
 [i] 
 
366 
 
 MR. F. E. BEDDARD ON THE STRUCTURE 
 
 [May 6, 
 
 of the Serolidae and the Cymothoidse, which was published in the 
 ‘ Transactions.’ The present paper is a continuation of the same 
 subject, but deals with the genus Arcturus. The material, like that of 
 my former paper, consists of teased preparations and of sections of the 
 eyes of species obtained during the voyage of H.M.S. ‘Challenger,’ 
 all of which species have been described by me in my Report (3). 
 
 In my paper on the structure of the eye in the Cymothoidse, I 
 mentioned the principal papers dealing with the Isopodan eye, which 
 are not many in number. Since the appearance of that paper but 
 little has been published upon the Isopodan eye. I am, indeed, only 
 acquainted with a single memoir upon the subject, one by Mr. S. 
 Watase (11); this paper deals largely with Serolis, but it contains 
 also some very weighty observations upon the morphology and 
 pedigree of the Arthropod eye in general. 
 
 It is gratifying to me personally to find that Mr. Watase has 
 “ verified all the chief results ” of my own research. This fact 
 also gives me greater confidence in laying the present paper before 
 the Society. If the state of preservation of the specimens of Serolis 
 was so good as to enable me to state accurately the principal facts 
 in the anatomy of the eye, it seems likely that the Arcturi, which 
 were preserved in an identical fashion, will also furnish reliable data. 
 In any case our knowledge of this particular genus is at present, so 
 far as I am aware, absolutely nil ; and it is almost unnecessary to 
 state that the deep-sea forms are as little known as those which in- 
 habit the shallower waters. Mr. Watase, in his description of the 
 eye of Serolis, which occupies the first five pages of the special part, 
 refers to the presence of a “ corneagen” 1 (a term introduced by 
 Patten, 13) below the cornea and above the cells of the vitrella ; he 
 also figures a row of pigmented cells surrounding the vitrella 2 . 
 These structures were not figured or described by myself, but I am 
 not prepared to dispute the probable justice of Mr. Watase’s addition 
 to my own account. 
 
 It seems to me to be very probable that this corneagen layer is, 
 as Patten has particularly insisted, always present in eyes of these 
 types ; and Watase has shown a very strong raison d'etre for its 
 presence. 
 
 The present paper, however, only professes to be a very small 
 contribution to the morphology of the Isopodan eye ; the main 
 object is to compare the minute structue of the eye of species living 
 in shallow water with that of their deep-sea allies. 
 
 The questions involved are interesting and lead to some rather 
 important conclusions about the life of these deep-sea forms. 
 
 In the first part of my ‘ Challenger ’ Report, dealing only with the 
 very remarkable genus Serolis (2), I gave some figures and a brief 
 description of the structure of the eyes in two deep-sea species, viz. 
 Serolis bromleyuna and Serolis necera. Without recapitulating all 
 the results here, I may point out that the eyes in those forms showed 
 very considerable traces of degeneration ; this degeneration was 
 
 1 PI. xxix. fig. 1 eg , fig. l a a. 
 
 2 This term was introduced by Lankester and Bourne. 
 
 [ 2 ] 
 
OF THE EYE IN ARCTURTJS. 
 
 367 
 
 1890.] 
 
 shown to have affected all the component parts of the eye. The 
 cornea was little (S. necera) or hardly at all ( S . bromleyana ) convex 
 below ; the lens was granular, and could hardly have been transparent 
 during life ; the rhabdom and retinules were not recognizable — 
 at least in the form which they present in other (shallow-water) 
 species. The amount of pigment present was comparatively small, 
 or, as in S. bromleyana and S. gracilis, completely absent. I hope 
 to show in the present paper a somewhat similar though less marked 
 series of changes in the eyes of the deep-water Arcturi. 
 
 Before the appearance of my preliminary account of the genus 
 Serolis ( 1 ), which contained a summary of observations upon the 
 structure of the eye, but little had been done in investigating the 
 histology of that organ in deep-sea Crustacea. Dr. P. P. C. Hoek, 
 in his Report on the ‘ Challenger ’ Pycnogonida (6), mentioned that 
 pigment is often absent from the eyes of deep-sea forms, and 
 that the retina may be replaced by a mass of connective tissue, 
 though the lens be present. The details given by Hoek are not 
 very numerous. Since the publication of my Report several other 
 groups of deep-sea animals have been reported on. Mr. S. I. Smith 
 (12) found that in the majority of species of Atlantic deep-sea 
 Decapods the eyes have undergone certain structural changes ; these 
 changes are partly in the alteration of the pigment, which becomes 
 lighter coloured in the abyssal species, and partly in the reduction 
 of ihe number of the visual elements. 
 
 A considerable number of deep-sea Mollusca according to Pelseneer 
 (8) have rudimentary eyes ; some are totally blind. 
 
 Henderson found (7) with regard to the Auomura that degenera- 
 tion was common in the eyes of abyssal forms ; this degeneration 
 was largely shown by the absence or reduction in quantity of the 
 pigment. Here, however, there is no elaboration of detail and the 
 points raised are not illustrated by figures. 
 
 Animals that dwell in caves are, so far as absence of sunlight is 
 concerned, subjected to the same conditions as are deep-sea animals. 
 Packard (10), in investigating animals from the Kentucky caves, 
 found various conditions of degeneration in the eyes, culminating in 
 the total blindness of some species. 
 
 The result, then, of all these investigations has been to show that 
 the deep-sea fauna is chiefly made up of animals which are either 
 blind or — if they have eyes — show evident traces of degeneration in 
 these eyes. 
 
 I attempted to show, in considering the deep-sea Isopods, that 
 the blind deep-sea genera were, at any rate for the most part, peculiar 
 genera, and that those deep-sea Isopods with apparently well-developed 
 eyes were closely allied to, if not identical with, forms living in 
 shallow water. Thus it appeared reasonable to assume that the 
 eyed forms were comparatively recent immigrants into deep water. 
 This view has already, I find, been considered by Prof. Semper 1 to 
 
 1 ‘Animal Life,’ Int. Scient. Series, p. 84. “ We have become acquainted .... 
 with a wonderful deep-sea fauna, showing the same striking mixture of blind 
 and seeing animals as the fauna of the caves. This case is all the more 
 
368 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, 
 
 account for the presence of animals with eyes in dark caves and the 
 deep-sea, but rejected. It is accepted, however, by Henderson. 
 This being the case it is unnecessary to make any further use of the 
 ingenious “ theory of abyssal light,” and it is impossible to build up 
 any theories with regard to the brilliant coloration of deep-sea animals. 
 These colours must be absolutely without any secondary meaning, as 
 must also the frequent phosphorescence of Alcyonarians and other 
 animals living in great depths. 
 
 If there were no intermediate stages between Crustacea and other 
 animals of the deep sea with well-developed eyes and those without 
 any trace of eyes at all, such theories might be put forward with 
 some plausibility. It might be urged that the eyeless forms were 
 simply peculiar in this respect ; that is to say, that just as among 
 shallow-water genera, and even surface forms, eyes may be absent 
 and characterize a particular genus or species by their absence, such 
 might also be the case with genera inhabiting the deeper waters of 
 the oceans. The numerous stages of degeneration appear to me to 
 render this view untenable. 
 
 I shall now proceed to describe, in as much detail as my prepara- 
 tions allow of, the minute structure of the eye in a number of species 
 of Arcturus. 
 
 (1) Arcturus furcatus, Studer. 
 
 The eye of this species is quite a typical Isopodan eye, though 
 differing in certain details from any type that has been hitherto 
 studied. 
 
 The vitreous body is rounded conical in form and is distinctly 
 made up of two halves. As is illustrated (Plate XXXI. fig. 4), there 
 appear to be four nuclei corresponding to each vitreous body and 
 lying above it. These are, I imagine, the nuclei of Semper and the 
 nuclei of the corneagen cells. 
 
 The retinula of each eyelet is made up of six? cells, which is not 
 a number that has been hitherto met with among the Isopods. In 
 insects this number appears to be common according to Grenacher’s 
 figures (5). 
 
 The rhabdom secreted by these retinula-cells is in certain respects 
 rather remarkable. 
 
 It is conspicuous on account of its size ; it has the clear amber- 
 yellow colour of the vitreous body ; peripherally (see Plate XXXI. 
 figs. 5, 14-16) the rhabdom is markedly a very densely pigmented 
 band. Towards its upper extremity the rhabdom is, as shown by 
 
 puzzling, because the chief part of such deep-sea animals as can see are extra- 
 ordinarily unlike their nearest congeners living at the surface and in the light, 
 so that we are forbidden to suppose that they may be species that have only 
 lately migrated from the surface to great depths.” It is unnecessary to point 
 out that this statement does not allow for such cases as I refer to, where the 
 eyes, although apparently like those of others, are really in various stages of 
 degeneration. There are no doubt plenty of species in which, as in Serolis 
 necera, the facetted cornea is the last part of the eye to disappear. Hence totally 
 blind animals may seem to have well-developed eyes. 
 
1890.] 
 
 OF THE EYE IN ARCTURUS. 
 
 369 
 
 transverse sections (figs. 14-16), of an oblong shape, the corners are 
 sharply marked and the sides are perfectly parallel with, or at right 
 angles to each other. Lower down, at about the level of the nuclei 
 of the retinula-cells (fig. 14), the rhabdom becomes indented, and 
 shows obvious traces of its orgin from six rhabdomeres. Lower 
 down still (fig. 15) the six rhabdomeres diverge from each other. 
 
 Each rhabdomere becomes surrounded by a dense pigmented 
 sheath. 
 
 When the eyes are teased in glycerine after depigmentation by 
 nitric acid, the rhabdom shows a tendency to break up into squarish 
 blocks (fig. 8), as has frequently been noticed in other Arthropods. 
 
 (2) Arcturus spinosus, F. E. Beddard. 
 
 The eye of this species, which is from deep water, contrasts in 
 many points with that of Arcturus furcatus — a typically shallow- 
 water form. 
 
 The lens has the peculiar form shown in the drawing (Plate 
 XXXI. fig. 10), which represents a semidiagrammatic longitudinal 
 section through an eye-element. It is somewhat muffin-shaped, 
 being depressed on both sides in the middle. In some other slides 
 which are labelled “ Arcturus spinosus ,” and which I have no reason 
 for doubting are really preparations from this species, the lens has 
 the form shown in another drawing (figs. 6, 11); it is pear-shaped, 
 and in the middle it is decidedly more opaque than peripherally, 
 where it is quite transparent. This central opacity may be due to 
 a precipitated and coagulated fluid occupying the interior of the lens, 
 such as Watase (11) has described and figured in Serolis 1 . I have 
 not, however, observed anything similar in the shallow-water species 
 of Serolis which I myself investigated. Perhaps it will turn out to 
 be a commencing degeneration in the eyes of the species described 
 which is carried out more fully in Serolis necera. 
 
 The rhabdom of Arcturus spinosus is very large, and in longi- 
 tudinal sections of the undepigmented eye shows the characters ex- 
 hibited in the drawing (fig. 10) ; it is of roughly conical form, the 
 apex of the cone lying towards the ommateal membrane. In some 
 examples of this species which I referred to above in connection with 
 the peculiar difference in the structure of their lenses, the rhabdom 
 also shows a departure from the ordinary condition. As indicated 
 in fig. 6, its upper extremity embraces the lens, which is sunk into a 
 depression of what is really the broad end of the conical rhabdom ; 
 although in such preparations as those illustrated in figs. 6, 11, the 
 vitreous body and the rhabdom appear to be very nearly if not quite 
 in actual contact, there is not the least difficulty in distinguishing 
 between them. 
 
 The rhabdom in both forms of eye is by no means so clear and 
 transparent as in Arcturus furcatus , and it is proportionately very 
 much larger than in that species. Its form varies much, but is 
 usually more or less bent. 
 
 1 Loc. cit. p. 290, pi. xxix. fig. 1 a. 
 
 [ 5 ] 
 
370 
 
 MR. F. E. BEDDARD ON THE STRUCTURE 
 
 [May 6, 
 
 The retinula-cells are, on the other hand, very much smaller pro- 
 portionally, and are only well developed and conspicuous at the end 
 of the rhabdom, which, by the way, shows no traces of division into 
 six rhabdomeres. 
 
 As may be seen by longitudinal sections (fig. 10), a coating of 
 dense pigment covers the rhabdom, and occasionally pierces into its 
 interior for a short distance ; this I presume to be the upper portion 
 of the retinula-cells. The nuclei of these cells are placed on a level 
 with the posterior end of the rhabdom instead of near the upper 
 extremity of this structure, as they are in Arcturus furcatus. The 
 pigment, although deep black in colour, is very much less in amount 
 than it is in A. furcatus. It is clear, therefore, that the eye of 
 Arcturus furcatus differs from that of A. spinosus in many points. 
 
 I do not possess so many preparations of other species of Arcturus 
 as of the two which I have just described. The following notes 
 therefore show many lacunae which I see no chance of being able to 
 fill up. They are largely but not entirely based upon sketches 
 which were made some five years ago, when I commenced to work 
 at this subject. These sketches, unfortunately, do not show all the 
 points which I have since ascertained to be important. 
 
 (3) Arcturus anna, F. E. Beddard. 
 
 In this species the lens has an ellipsoidal form, the long axis coin- 
 ciding with that of the ommatidium when the lens is in position. 
 
 The lens agrees, however, with that of Arcturus spinosus — at least 
 with some individuals of that species — in being composed of a clearer 
 peripheral portion and a granular-looking opaque middle. 
 
 The rhabdom is large and solid, it is not prolonged into six 
 separate rhabdomeres at the posterior extremity as in A. furcatus ; 
 after treatment with nitric acid, however, it shows distinct traces of 
 longitudinal division into a number of pieces which no doubt corre- 
 spond with the cells of the retinula. 
 
 The retinula-cells themselves, as in A. spinosus , are only clearly 
 distinguishable as such behind the rhabdom where their nuclei are 
 situated ; a coating of pigment which covers the rhabdom up to very 
 nearly the lens is doubtless deposited in a forward prolongation of 
 the retinula-cells. 
 
 (4) Arcturus cornutus, F. E. Beddard. 
 
 In most respects the eye of this species agrees with that of 
 Arcturus anna ; so close is this agreement, that I need not enter into 
 any description of the ommatidium. All that I shall do is to call atten- 
 tion to one rather important point of difference between this species 
 and Arcturus anna. 
 
 This point of difference concerns the vitreous body, which appears 
 to be even less fitted as a refracting medium in this species than in 
 the last. 
 
 The opacity, which is quite a noticeable feature of the lens in 
 A. anna , is exaggerated in A. cornutus , until" there is not even a 
 [ 6 ] 
 
OF THE EYE IN ARCTURUS. 
 
 1890.] 
 
 371 
 
 narrow peripheral band left which is clear. The whole vitreous body 
 appears to be more or less granular and opaque. 
 
 (5) Arcturus brunneus, F. E. Beddard. 
 
 In this species the vitreous bodies or lenses of the ommatidia have 
 the form which is illustrated in the drawing exhibited (Plate XXXI. 
 fig. 13) ; their shape is usually that of a buffet with the convex 
 outer surface and a straightish margin where the lens comes into 
 actual or at any rate very near contact with the rhabdom. As a 
 rule the lens is decidedly smaller. 
 
 The rhabdom, on the other hand, is particularly large. 
 
 (6) Arcturus glacialis, F. E. Beddard. 
 
 The eye of this species calls for no lengthy description, the vitreous 
 bodies have the same curious muffin-shape that they have in 
 A. spinosus ; the rhabdom is large, and the nuclei of the retinula- 
 cells are placed below it. 
 
 (7) Arcturus studeri, F. E. Beddard. 
 
 This is the only shallow- water species besides A. furcatus and 
 A. americanus that I have studied ; unfortunately in this case, as in 
 that of A. americanus , I am dependent upon a single sketch which I 
 made some years ago ; the preparations themselves are missing. This 
 is particularly to be regretted, as A. studeri resembles in some par- 
 ticulars species that only occur in deep water. The vitreous bodies, 
 however, are quite like those of A. furcatus in their perfect trans- 
 parency and in their general shape. I am unable to give any details 
 about the rhabdom ; it does not, however, seem to be particularly 
 large ; the retinula-cells are unlike those of A. furcatus, and like 
 those of A. spinosus and all the deep-sea species described in the 
 present paper in that their nuclei are placed below the posterior ex- 
 tremity of the rhabdom. Whether there is much or little pigment 
 I cannot say. 
 
 The following table indicates the depths at which the various 
 species described in the present paper were met with : — 
 
 1. Arcturus furcatus. 7-127 fathoms (one specimen in 1675 
 
 fathoms). 
 
 2. Arcturus americanus. 55 fathoms. 
 
 3. Arcturus studeri. 25-127 fathoms. 
 
 4. Arcturus glacialis. 1675 fathoms. 
 
 5. Arcturus brunneus. 1600 fathoms. 
 
 6. Arcturus anna. 600 fathoms. 
 
 7. Arcturus cornutus. 500 fathoms. 
 
 8. Arcturus spinosus. 1375 fathoms. 
 
 The first three species are therefore to be looked upon as shallow- 
 water forms ; the remainder as true deep-sea species. 
 
 [ 7 ] 
 
372 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, 
 
 It is noteworthy that all the shallow-water species, viz. Arcturus 
 furcatus , A. americanus, A. studeri, have lenses which are perfectly 
 clear and transparent, and are characteristically pear-shaped. 
 
 On the other hand, all those species which have an apparently 
 partly opaque lens are deep-water forms 1 ; these are Arcturus spi- 
 nosus , A. anna , A. cornutus. This list is not exhaustive of the deep- 
 sea forms which I have been able to examine ; but there are no 
 others in which the lens appears to be getting opaque. It is re- 
 markable, however, that in the other deep-sea species which I have 
 examined, viz. Arcturus brunneus and A. glacialis , and some specimens 
 of A. spinosus, the lens should show a reduction is size and an alter- 
 ation in shape which must impair its perfection as an organ for the 
 passage of rays of light, if the form best suited for that purpose be 
 that exhibited by A. furcatus. 
 
 The retinula-cells appear to be best developed in A. furcatus, where, 
 as shown in my drawing (fig. 8), the nucleus is placed high up, not 
 far from the commencement of the rhabdom. This may also be the 
 case with A. americanus , but mv sketches are unfortunately not con- 
 clusive as to this point and the preparations have been since spoiled. 
 
 In all the other species of Arcturus examined by me, the retinula- 
 cells are relatively small, and the nuclei are situated (e. g. fig. 13, n) 
 below the extremity of the rhabdom. It is possible that this re- 
 duction of the retinula-cells (which I believe with Grenacher and 
 others to be the essential visual cells) is correlated with a commencing 
 degeneration of the eye. If it were not for the single exception 
 offered by A. studeri (a shallow-water species from Kerguelen), I 
 should be disposed to lay considerable weight upon this series of 
 facts. As it is, it does not appear to me to be safe to make any 
 such assertion in at all a positive way. 
 
 The rhabdom does seem in several of the deep-sea species, par- 
 ticularly in A. spinosus, to be undergoing degeneration. This is 
 shown by its less perfect transparency and by its irregular form, 
 and perhaps also by its very large size. It may not perhaps seem 
 very reasonable to adduce increase of bulk in an organ as indi- 
 cation of degeneration. If we are to regard the rhabdom as formed 
 by the retinula-cells, the large size of the former may be connected 
 with the diminished size of the latter ; it may therefore be a sort of 
 degeneration. On the Lamarckian view of evolution, the increase in 
 size of the media for concentrating the light might seem to be an 
 attempt to keep up with the diminishing supply of light. I myself 
 should be disposed to regard this phenomenon as a kind of “ running 
 to seed ” of the non-essential part of the eye. 
 
 Another point of very considerable importance in relation to the 
 supposed degeneration of the eye is the smaller amount of pigment 
 which occurs in the eyes of most of the deep-sea species examined 
 by me. In teased preparations the rhabdom was always perfectly 
 distinct, the yellowish-brown colour being quite visible ; and in sec- 
 tions of A. spinosus the amount of pigment covering the rhabdom is 
 seen to be not great (cf. Plate XXXI. figs. 10 and 5). On the other 
 1 I. e, occurring at depths greater than 500 fathoms. 
 
 [ 8 ] 
 
1890.] 
 
 OF THE EYE IN ARCTURUS. 
 
 373 
 
 hand, in teased preparations of A. furcntus the rhabdom always 
 appeared as a densely black mass in the centre of the retinula-cells, 
 its outline only being recognizable ; although in these deep-sea forms 
 the amount of the pigment is very decidedly less than that which is 
 found in the shallow-water species A. furcatus, its colour is the 
 same ; in all forms it had a dense black appearance. These facts 
 are simil; r to those which I stated with reference to Serolis necera ; 
 in that species (a deep-sea form) the pigment is just as densely black 
 as in the shallow-water Serolis cornuta, but less in amount 1 . On 
 the other hand, it has been several times observed that in other 
 deep-sea Crustacea the pigment is of an orange colour. This I 
 suppose only means that the pigment-granules are less dense in 
 those forms ; for in the species of Arcturus which I describe in the 
 present paper the pigment when dissolved by means of nitric acid 
 showed an orange-brown colour. Unfortunately I am not able to 
 state what is the amount of pigment, as compared with other forms, 
 present in the ommatidia of Arcturus studeri. It agrees, as I have 
 pointed out, with other deep-water forms in the small size of the 
 retinula-cells and in the position of their nuclei below the level of 
 the extremity of the rhabdom, but it has a large clear vitreous body 
 like that which is found in each oinmatidium of the eye of A. fur- 
 catus and A . americanus. 
 
 In any case I have been able to describe in this paper for the first 
 time certain interesting differences of structure in the eyes of a 
 number of species of Arcturus. 
 
 These differences fall into two main categories : — 
 
 (1) In A. furcatus and A. americanus (?) the rhabdom is com- 
 paratively small (though large compared with other Crustacea), and 
 the retinula-cells are very large, the nuclei being situated at the level 
 of the anterior end of the rhabdom. 
 
 (2) In A. spinosus, A. anna , A. cornutus, A. brunneus, A. glaci- 
 alis, and A. studeri the rhabdom is very large and the retinula-cells 
 are comparatively small, their nuclei 2 being situated below the ex- 
 tremity of the rhabdom, near to the basement membrane of the 
 ommateum. Besides these morphological differences in the retinula- 
 cells, which perhaps have no reference to the conditions under which 
 the animals live, the second division shows various peculiarities iu 
 most species which seem to be correlated with a deep-sea habit. 
 Thus in some forms the lens is reduced in size and altered in form 
 or has become partially opaque and the pigment is small in amount ; 
 these statements apply to all the species in the second list except 
 Arcturus studeri. 
 
 List of Memoirs referred to. 
 
 1. Beddard, F. E. — Preliminary Notice of Isopoda collected 
 
 1 It will be remembered that in the case of this deep-sea Serolis the small 
 amount of pigment is also correlated with degeneration of the retinula. 
 
 2 This position of the nuclei, though unusual, is not unparallelled. They 
 occur in that position in Talorchestia, even below the ommateal membrane 
 (Watase), and in other Amphipods. 
 
 Proc. Zool. Soc. — 1890, No. XXVI. 26 
 
 [ 9 ] 
 
374 MR. F. E. BEDDARD ON THE STRUCTURE [May 6, 
 
 during the Voyage of H.M.S. * Challenger.’ — Part I. Serolis. 
 P.Z.S. 1884, p. 330. 
 
 2. Beddard, F. E. — Report on the Isopoda collected by H.M.S. 
 ‘Challenger’ during the years 1873-76. — Part I. The genus 
 Serolis . Zool. Chall. Exp. pt. xxxiii. 
 
 3. Beddard, F. E. — Report on the Isopoda collected by H.M.S. 
 ‘Challenger’ during the years 1873-76. — Part II. Zool. Chall. 
 Exp. pt. xlviii. 
 
 4. Beddard, F. E. — On the Minute Structure of the Eye in the 
 Cymothoidse. Trans. Roy. Soc. Edinb. vol. xxxiii. p. 443. 
 
 5. Grenacher, H. — Untersuchungen iiber das Sehorgan der 
 Arthropoden. Gottingen, 1879. 
 
 6. Hoek, P. P. C. — Report on the Pycnogonida collected by 
 H.M.S. ‘ Challenger ’during the years 1873-76. Zool. Chall. 
 Exp. pt. x. 
 
 7. Henderson, J. R. — Report on the Anomura collected by 
 
 H.M.S. ‘Challenger’ during the years 1873-76. Zool. Chall. 
 Exp. pt. Ixix. 
 
 8. Pelseneer, P. — Report on the Anatomy of the Deep-sea Mol- 
 lusca collected by H.M.S. ‘Challenger’ in the years 1873-76. 
 Zool. Chall. Exp. pt. lxxiv. 
 
 9. Lankester, E. R., and Bourne, A. G. — On the Minute 
 Structure of the Lateral and Central Eyes of Limulus and 
 Scorpio. Q. J. Micr. Sci. vol. xxiii. p. 177. 
 
 V 10- Packard, A. S. — The Cave Fauna of North America, with 
 Remarks on the Anatomy of the Brain and Origin of the Blind 
 Species. Mem. Nat. Acad. Sci. vol. iv. 
 
 11. Watase, S. — On the Morphology of the Compound Eyes of 
 Arthropods. Stud. Biol. Lab. Johns Hopkins Univ. vol. iv. 
 no. 6, p. 287 et seq. 
 
 12. Smith, S. I. — Abyssal Decapod Crustacea of the ‘Albatross’ 
 Dredgings in the North Atlantic. Ann. & Mag. Nat. Hist. (5) 
 xvii. p. 187 et seq. 
 
 13. Patten, W. — Studies on the Eyes of Arthropods. — I. De- 
 velopment of the Eyes of Vespa , &c. Journ. Morph, vol. i. 
 no. 1. 
 
 EXPLANATION OF PLATE XXXI. 
 
 The following letters have the same significance in all the figures: — v., vitreous 
 body; rh., rhabdom; r., retinula; n., nuclei of retinula-cells. The ohitinous 
 parts of the eye (vitreous body and rhabdom) are for the most part coloured 
 yellow. 
 
 Fig. 1. A number of ommatidia of Arcturus spinosus, from above. 
 
 2, 3. Cross sections at different levels through rhabdom of A. spinosus. 
 
 4. A number of ommatidia of A. furcatus, from above. 
 
 5. Longitudinal section through ommatidium of A furcatus. 
 
 6. Vitreous body and rhabdom of A. spinosus , from a teased and depig- 
 
 mented preparation. 
 
 7. Partially depigmented retinula of A. furcatus. 
 
 8. Depigmented retinula of A. furcatus. 
 
 9. Transverse section of ommatidium of A. furcatus below extremity of 
 
 rhabdom ; one rhabdomere is seen. 
 
 10. Longitudinal section through ommatidium of A. spinosus. 
 
 [ 10 ] 
 
OF THE EYE IN ARCTURUS. 
 
 375 
 
 1890.] 
 
 Fig. 11. Ommatidium of A. spinosus, from a teased and depigmented prepa- 
 ration. 
 
 12. Vitreous body of A. brunneus, from above. 
 
 13. Ommatidium of A. brunneus , from a teased and depigmented prepa- 
 
 ration. 
 
 14. 15, 16. Transverse sections through ommatidium of A. furcatus at 
 
 different levels. 
 
 tn] 
 
P. z . s 
 
 . 1830 , PI . XXXI 
 
Ban und Entwicklung des Auges der 
 zehnfussigen Crustaceen und der 
 Arachnoiden. 
 
 Von 
 
 J. Carrier© ill Strabburg i. E. 
 
 Sonderabdruck aus dem „Biologischeu Centralblatt". 
 Band IX. Nr. 8, ausgegeben am 15. Juni 1889. 
 
 Erlangen. 
 
 Verlag von Eduard Besold. 
 
Ban und Entwicklung des Auges tier zehnfttBigen Crusta- 
 ceen und der Arachnoiden. 
 
 Es erscheint rair angezeigt, die Besprechung neuerer Unter- 
 suchungen liber die Entwicklung und den Bau des Crustaceenauges 
 an das Werk von Beic ben bach anzukniipfen, welches schon ein- 
 mal, aber ohne nakere Beriihrung dieses Kapitels, im VIII. Bande 
 dieser Zeitscbrift kurz besprochen wurde. Wie ich zunachst kervor- 
 keben muss, gibt Reich en bach richtig an, dass in dem Auge des 
 Flusskvebses die Corneazellen und die Krystallkegelzellen zwei ge- 
 sonderte Schichten bilden, und auch bei dem erwacbsenen Tiere sind 
 in der entsprechenden Anordnung beide vorlianden und die zuge- 
 horigen Kerne nachzuweisen. (Wahrend meine frlihern Praparate die 
 Kerne der Krystallzellen auch heute noch nicht mit einiger Sicherheit 
 erkennen lassen, sind sie in einer spatern Serie deutlich zu sehen. 
 Es scheint, dass ich damals einem Reagens zu groBes Yertrauen 
 schenkte und dieses grade den Teil der Krystallzellen, in welchem 
 die Kerne liegen, so verdunkelte bezw. fixierte, dass die Kerne nicht 
 zu erkennen waren); ich bitte somit in den „Sehorganen“ S. 168 
 Fig. 130 bei der Zeichnungserklarung zu andern: „2 Corneazellen, 
 3 auBerer Teil des Krystallkegels, der Rest der Krystallzellen mit 
 den Kernen. w 
 
 Das Auge von Astacus entwickelt sich nacli Reichenbach aus 
 einer Einstiilpung des Ektoderms, indem eine solide Zellmasse 
 (Augenfalte) sich von einer Grube her unter die vor der Grube ge- 
 legene Ektodermstelle schieht. Letztere, die Epidermislage, zuerst 
 ein-, bald aber vier- bis ftlnfschichtig, soli die Cornea- und die 
 
 1) Dr. Heinrich Reichenbach, Studien zur Entwicklungsgeschichte 
 des Flusskrebses. Abhandl. der Senckenb. naturf. Gesellsch. Frankfurt 1886. 
 
226 Carriere, Auge der zelmfiiBigen Crustaceen und Arachnoiden. 
 
 Krystallkegelzellen liefern, wahrend aus der Augenfalte durch Spal- 
 tung in.zwei Ballen einerseits die Retina , anderseits eine „Innen- 
 wand“ hervorgehen, von denen letztere bald mit der AuBenwand 
 (Retina) und dem Sehganglion in innigen Zusammenhang tritt. 
 Zwischen Krystallkegellamelle und Retina wandern Mesodermzellen 
 ein, welche sick pigmentieren. 
 
 Das Ganglion opticum entsteht als Ektodermverdickung im Augen- 
 segment, in unmittelbarer Beriihrung mit der Einstulpung und dem 
 Gebirn. Die Entwicklung vollzieht sich im Naupliusstadium und den 
 darauffolgenden Stadien. Die Deutung, welche Reic hen bach den 
 einzelnen Teilen bei der Embryonalentwicklung des Auges gibt, lasst 
 sich aber mit den Verhiiltnissen, die das Auge des erwachsenen Tieres 
 bietet, nicht vereinigen. Wenn eine Mesodermschicht zwischen die 
 Ektodermanlage der „Krystallkegelschicht u und die Retina eindrange, 
 miisste zunachst aus der Basalmembran der betreffenden Epithelstelle 
 eine scharfe Grenze, eventuell eine praretinale Membran hervorgehen. 
 Dann mltssten aber auch die eingewanderten Elemente in entspre- 
 chender Lage und Zahl sich im Auge vorfinden. Beides ist nicht der 
 Fall. Was zunachst die Abbildungen Reichenbach’s betrifft, so 
 ist das, was Reichenbach in Figur 225 als Mesodermschicht (Pg. m.) 
 bezeichnet, gar nicht dasselbe, wie in Figur224, wie die Vergleichung 
 beider Abbildungen zeigt. Eine Grenzlinie, die sclion bei schwacherer 
 VergroBerung so deutlich ist wie die in Fig. 224 zwischen „Krystall- 
 kegel M - und Mesodermschicht befindliche, konnte bei starkerer Ver- 
 groBerung nicht spurlos verschwinden ; ebenso wenig wie die sehr 
 deutlichen schwarzen Fasern, welche in Fig. 224 als Verlangerung 
 der „Krystallkegel u durch die Mesodermschicht ziehen. 
 
 Yergleicht man die altern Embryonalstadien mit dem erwachsenen 
 Auge, so stellt sich zunachst keraus, dass die „Mesodermschicht“ 
 vollkommen zu Recht besteht und an dem entsprechenden Platz liegt. 
 Nur schiebt sie sich nicht zwischen „Krystallkegelschicht“ und „Re- 
 tina“ ein, sondern zwischen Retina und den auBersten Abschnitt des 
 Ganglion opticum. Die schwarzen Fasern sind auch jetzt noch darin 
 vorhanden, es sind die pigmentierten, nach innen ausgewachsenen 
 Enden der Retinulazellen (oder die an diese antretenden Nervenfasern, 
 wenn man will). Die deutliche Grenze, welche zwischen der Meso- 
 dermschicht und der „Krystallkegelschicht“ hinzieht, ist die Basal- 
 membran des Epithelbezirks, aus welchem das Auge hervorgeht, und 
 findet sich in dem erwachsenen Auge entsprechend als Basalmembran 
 der Hypodermis wieder, welche kontinuierlich von alien Seiten her 
 unter der Retina hinzieht. 
 
 Es ware also die Deutung der verschiedenen Teile dahin ab- 
 zuandern, dass aus der Epithelverdickung K. K. nicht nur Krystall- 
 kegel und Pigment, sondern auch Retina hervorgehen, aus der AuBen- 
 wand der Augenfalte dagegen nicht Retina, sondern der iiuBerste 
 
Carriere, Auge der zehnfufiigen Crustaceen und Arachnoiden. 227 
 
 Abschnitt des Ganglion opticum. Damit ware dann allerdings die 
 Entwicklung des Facherauges wieder daliin vereinfacht, dass diop- 
 triscker Apparat und Retina nicht an verschiedenen Stellen, sondern 
 von einer Anlage her entstanden, dass die Einfaltung Reichen- 
 bach’s somit in keiner direkten Beziebung zum Auge stehen konnte. 
 Und doch gibt eine neuere Untersuchung mit aller Bestimmtheit eine 
 deutliche Einsttilpung als erste Anlage des Krebsauges an — aber 
 der Wert der einzelnen Falten erscheint bier ein anderer als Reichen- 
 bach annahm. (Ich hatte obige Betrachtungen schon niedergeschrie- 
 ben, ebe ich Kingsley’s Abhandlung erhielt). 
 
 Kingsley l ) lindet bei Crcingon vulgaris in den friihesten Stadien 
 ganz wie Reichenbach zuerst eine Einsenkung, dann Einsttilpung 
 der Augenanlage. Die Einsttilpung ist aber bier ho hi und ibr Lumen 
 bleibt lange Zeit kenntlich; die Augenblase ist schrag nach innen 
 gerichtet und legt sich unmittelbar unter das Ektoderm, so dass ihre 
 AuBenwand dessen Innenwand dicbt anliegt. Wir haben also drei 
 iibereinander liegende Zellschichten, deren mittlere und innerste fast 
 in ihrer ganzen Ausdehnung getrennt sind; die Hoblung zwischen 
 ilinen verflacht sich, aber verschwindet nie ganz. Die auBerste Zell- 
 schicht, das Ektoderm , wird zur Cornea, die mittlere (AuBenwand 
 der Blase, Retinogen) zur Retina, (bildet also die ganzen Omma- 
 tidien, dioptrischen Apparat und Retinula), die innerste (Innenwand 
 der Blase, Gangliogen) zu der Ganglien* und Nervenkette im Stiele 
 des erwachsenen Auges. Wahrend der ersten Differenzierungs- 
 erscbeinungen im Gangliogen und Retinogen schieben sich zwischen 
 beide in die Hohlung der Augenblase Mesodermzellen ein, welche 
 bis zum Ausschliipfen des Embryo ibren Charakter als einsckichtige 
 Lamelle bewabren, dann vermutlicb die pigmentierte Zellmasse bilden, 
 welche die Nervenfasern zwischen den Ommatidien und dem auBersten 
 Ganglion umgibt. 
 
 Wahrend die Retinawand einheitlich bleibt und in ibr Krystall- 
 kegel und Retinula entstehen, spaltet sich die Ganglienwand in zwei 
 Ganglien, deren weitere Entwicklung nicht in den Rahmen dieser 
 Besprechung gehort. 
 
 Gleichzeitig mit der Gruppierung der Retinazellen zu Omma- 
 tidien treten die Kerne der Ektodermlamellen entsprechend zu je vier 
 zusammen und erzeugen die zugehorige Cornea. 
 
 Das Pigment tritt zuerst an der distalen Seite der Retinawand 
 auf. Wahrend Reichenbach die mogliche Umkehrung der Retina- 
 zellen nicht bertthrt, welche nach seiner Darstellung auch nicht un- 
 bedingt notig ware, gibt Kingsley folgerichtig an, dass bei Crangon 
 die Retinazellen infolge der Einsttilpung umgekehrt sein mtissten. 
 Von der histologischen Differenzierung der Retina an und iiber 
 
 1) J. S. Kingsley Sc. D., The developement of the Compound Eye of 
 Crangon. Journal of Morphology, Vol. I, Nr. 1, November 1887, Boston. 
 
228 Carriere, Auge der zehnfiiBigen Crustaeeen und Arachnoiden. 
 
 clieselbe sind die Angaben Kingsley’s unsicher und zum teil un- 
 brauchbar. Den Grund davon gibt er selbst an in der Bemerkung, 
 dass — vermutlieh infolge der Hartungsmethode — die Zellgrenzen 
 auf den Schnitten nicht genligend zu erkennen waren, um sie abbilden 
 zu konnen, so dass er nur nacli Lage, Richtung etc. der Kerne ur- 
 teilen konnte. Es ist somit zu bedauern, dass Kingsley ohne feste 
 Grundlage, und obwolil er Patten’s Hypothesen in wesentlichen 
 Punkten nicht bestatigen konnte, in zu groBem Vertrauen auf 
 des letztern Glaubwiirdigkeit ein Schema des jungen Omrnatidiums 
 aufstellt, welches eigentlich nur eine Kopie nach Patten darstellt. 
 
 Bei aller Klarheit, welche die Darstellung von Kingsley aus- 
 zeichnet, und der Uebereinstimmung, welche hier zwischen der Em- 
 bryonalentwicklung und dem ausgebildeten Auge besteht, bleibt doch 
 ein sehr wichtiger Punkt, die Umwendung der Retina, welche mit der 
 Stellung der Ommatidien im ausgebildeten Auge und mit ihrer 
 Innervierung noch nicht in Einklang zu bringen ist. 
 
 Da ich bald an anderer Stelle die wichtigen Untersuchungen von 
 Bertkau 1 ), M a r k 2 ) und P a r k e r 3 ) ausfiihrlich wiirdigen zu konnen 
 lioffe, will ich hier nur erwahnen, dass die in der Entwicklung des 
 Crustaceen- Auges noch vorhandene Llicke von Parker bei einer 
 andern Arthropodengruppe ausgefiillt wurde. Die beiden Mittelaugen 
 (oder, wie man jetzt richtiger sagen miisste „das Doppelauge u ) des 
 Skorpion gehen aus einer zunaehst unpaaren medianen Einstiilpung 
 hervor, welche taschenformig mit deutlichem Lumen von vorn nach 
 hinten gerichtet ist. Das dariiberliegende Ektoderm erzeugt die 
 Linsen, die mittlere Lamelle wird zur Retina, und der Antritt des 
 Nerven (noch vor der Differenzierung der Zellen zu den Bestand- 
 teilen des definitiven Auges) erfolgt an dem ur sprlinglich 
 innern (jetzt nach auBen gerichtet en) Ende dieser Zellen. 
 Bei den Wirbeltieren treten bekanntlich die Sehzellen in dieser Page, 
 mit Ausbildung des Stabchens am urspriinglich auBern Ende in 
 Thiitigkeit ; in den Sehzellen des Skorpion dagegen findet nach dieser 
 ersten Lageveranderung der ganzen Retinalamelle eine zweite Um- 
 walzung im Innern einer jeden einzelnen Sehzelle statt, indem der 
 Kern aus dem urspriinglich innern Ende der Zelle in das jetzt nach 
 innen gerichtete Ende riickt und gleichzeitig die Nervenfasern, ihre 
 Ansatzstelle verandernd, dieselbe Wanderung an das urspriinglich 
 aufiere Ende vollfiihren und dieses in jeder Beziehung auch noch 
 dadurch zu einem definitiv innern sich umgestaltet, dass die Rhab- 
 
 1) Bertkau Ph., Beitrage zur Kenntnis der Sinnesorgane der Spinnen: 
 1. Die Augen der Spinnen. Arch. f. mikr. Anat., Bd. XXVII, 1886. 
 
 2) Mark E. L., Simple Eyes in Arthropods. Bull, of the Museum of Com- 
 parative Zoology at Harvard College, vol. XIII, Nr. 3, Cambridge Mass. 1887. 
 
 3) Parker (J. H. , The Eyes in Scorpions. Bull, of the Mus. of Comp. 
 Zool. at Harvard College, vol. XIII, Nr. 6, Cambridge Mass. 1887. 
 
Carriere, Auge der zelmfufiigen Crustaceen und Arachnoiden. 229 
 
 domere an dem jetzt nach auhen gerichteten Ende zur Ausbildung 
 gelangen. 
 
 Diese „innere Umkehrung“ ist allerdings nicht beobaclitet, son- 
 dern aus den durchgreifenden Verscbiedenlieiten in der Lagerung der 
 einzelnen Teile des embryonalen wie des ausgebildeten Auges er- 
 scblossen worden. Sind die Untersnchungen R eichenb a cb- Kings- 
 ley’s ricktig, dann muss die gleiche innere Umwandlung auch an 
 den Retinazellen des Flusskrebses stattfinden, und da es sich hiemit 
 nicht nur um eine Umwalzung der Zellen, sondern auch unserer An- 
 schauungen liber das Wesen derselben handelt, ist eine Bestatigung 
 und Durchfuhrung dieser Untersuchung hier wie dort, auch iiber die 
 jiingsten Stadien hinaus an Material , welches uns die Zellkorper in 
 vorziiglichem Erhaltungszustande zeigt, unumganglich notig. 
 
 Mit Recht schliefit Parker aus dem urspriinglichen Ansatz der 
 Nervenfasern, dass die Retina schon vor der Einstiilpung funktioniert 
 haben, also ein einschichtiges Auge als Yorganger des dreischichtigen 
 angenommen werden miisste. 
 
 Die in den Sehzellen des Skorpion - Mittelauges hinter den Kernen 
 liegenden eigentumlichen Korper werden von Mark als Rudiment der 
 urspriinglichen (vor der Umkehrung am distalen Ende entwickelten 
 Rhabdomere) aufgefasst, und Parker’s Beobachtung, dass diese 
 „Pkaospkaren“ in den einschichtigen Seitenaugen von Centrums 
 fehlen, wiirde zu gunsten dieser Hypothese sprechen. Da meine 
 Untersnchungen aber die Angaben Ray-La nkester’s liber das 
 Yorkommen der Phaospharen in den Seitenaugen von Euscorpius 
 italicus bezw. carpathicus , sowie deren Lage bald vor, bald hinter 
 den Kernen vollkommen bestatigen, fallt die Bedeutung dieser Korper 
 fur die Mark’sche Hypothese hinweg; da ferner sowohl die Stelle 
 ihres Yorkommens bei einer Species als dieses selbst bei verschiedenen 
 Gattungen schwankt und unabhangig von ihnen immer Rhabdomere 
 vorhanden sind, diirfte diesen Gebilden tiberhaupt kein besonderer 
 Wert beizulegen sein. 
 
 Auf die Untersuchung Parker’ s 7 dem ich in alien wesentlichen 
 Punkten (nur nicht in dem, dass „Phaospharen nicht immer sehr ver- 
 schieden im Aussehen von Kernen seien a ) nach meinen an einer 
 andern Gattung gemachten Beobachtungen beistimmen kann, weiter 
 einzugehen ist hier nicht der Ort. Als wichtig will ich hier nur noch 
 liervorheben , dass durch sie zwar die Yermutung Patten’s uber 
 die Dreischichtigkeit des Spinnen- und Skorpion -Ocells zur Gewiss- 
 heit erhoben, zugleich aber die Vorstellung, welche letzterer liber die 
 Entstehung und den Aufbau dieser Ocellen sicli konstruierte, als falscli 
 hingestellt wurde. 
 
 Die Klarheit, mit welcher die Einstiilpung und dreischichtige 
 Anlage des Skorpionauges durch Parker geschildert und abgebildet 
 ist ; entzieht den Einwanden ; welche ich frtther gegen Locy’s Dar- 
 
230 Carriere, Auge der zehnfiiUigen Crustaceen und Arachnoiden. 
 
 stellung der Entwicklung des Spinnenauges zu richten genotigt war, 
 zum groBten Teil den Boden. Was meinen Einspruch veranlasste, 
 war hauptsachlich die Verallgemeinerung, welche die meisten Forscker 
 auf diesem Gebiete ihren Resultaten gaben und der ich mich auch 
 heute noch widersetzen muss. Ich habe, wie das in meiner „Ent- 
 wicklung der Ocellen und Seitenaugen der Insekten" des nahern 
 gezeigt werden wird, zwei Arten von Entstehung des Auges zu unter- 
 scheiden, entsprechend den beiden Hauptgruppen, in welche die Arthro- 
 poden nach Entwicklungsgeschichte und sonstigen verwandtschaft- 
 lichen Bezieliungen zerfallen. Es sind das auf der einen Seite die 
 Insekten, bei welchen sich Augen und Ocellen ohne Einstiilpung 
 bezw. nur unter Einsenkung entwickeln und die ursprtinglich distalen 
 Enden der Retinazellen dauernd nach auBen gerichtet bleiben, auf 
 der andern Seite die Arachnoideen (Spinnen und Skorpione) und 
 Crustaceen, deren Augen durch Einstiilpung und Abschniirung 
 unter Umkehrung der Retina entstehen; fur die Arachnoideen scheint 
 mir das sicher zu sein, bei den Crustaceen noch genauerer Unter- 
 suchung zu bediirfen; den Insekten wiirde auBerdem ein besonderes 
 Cornea -bildendes Epithel fehlen, den Crustaceen zukommen. Ein 
 durchaus nicht unwesentlicher Unterschied innerhalb der zweiten 
 Gruppe zeigt sich bis jetzt darin, dass bei den Arachnoideen aus der 
 innersten (dritten) Schicht der Augenanlage die Augenkapsel, bei 
 den Crustaceen dagegen die Augen ga u glien hervorgehen sollen. 
 
 Mogen Crustaceen und Arachnoideen durch fernere entwicklungs- 
 geschichtliche Untersuchungen einander noch naher kommen oder 
 wieder weiter getrennt werden, immer bleibt die Kluft zwischen 
 ihnen und den Insekten so groB, dass es unstatthaft erscheint, bei 
 den einen gemachte Beobachtungen ohne weiteres auf die andern zu 
 iibertragen oder umgekehrt. 
 
 Wie ich oben erwahnte, liegt die Hauptschwierigkeit bei der 
 Entwicklung des Crustaceenauges durch Einstiilpung und Abschniirung 
 eines blaschenformigen Gebildes in der Umkehrung der Retina, welche 
 gefordert werden muss, falls sich dieselbe — wie Kingsley und 
 Reich enbach angeben — aus der auBern Wand jener Blase ent- 
 wickelt, von welcher aber bei dem ausgebildeten Auge keine An- 
 deutung vorhanden ist. Patten (in den „Eyes of Molluscs and 
 Arthropods", Mitt. zool. Stat. Neapel Bd. 6, 1886; P. hatte gleich- 
 zeitig mit Reichenbach die besondere Corneazellenschicht im Deka- 
 podenauge gefunden) ging bckanntlich der ganzen Schwierigkeit aus 
 dem Wege, indem er die Sache nur theoretisch betrachtete, das 
 Ommateum (Retina mit Ausnahme der Cornea) aus der innern Wand 
 der Blase hervorgehen und die unbequeme mittlere Schicht (AuBen- 
 wand der Blase) einfach verschwinden lieB. Fiir die beiden jiingsten 
 Bearbeiter dieses Gebiets besteht diese Schwierigkeit gleichfalls nicht, 
 aber aus anderem Grunde. Obschon erst vorlaufige Mitteilungen vor- 
 
Carriere, Auge der zehnfiiftigen Crustaceen und Arachnoiden. 231 
 
 liegen, will ich die Resultate, zu welchen Parker 1 ) und Herrick 2 ) 
 gekommen sind, bei der Wichtigkeit, welche sie besitzen, bier anfugen. 
 
 Hack Parker zeigt sick bei dem Hummer das erste Auftreten 
 des optisclien Apparats in einem Paar von Ektodermverdickungen am 
 Vorderende des Embryo, wobei der oberflacbliche Teil dieser Yer- 
 diekungen die Retina, der untere das Ganglion des Auges entsteben 
 lasst; beide Teile werden spater durcb eine Basalmembran getrennt. 
 An gewissen Stellen wird der Zusammenbang beider Schichten durcb 
 diese Membran nicbt aufgeboben — diese Yerbindungen bleiben er- 
 halten und bilden die Nervenfaserstrange des erwacbsenen Auges 3 ). 
 Das Ommatidium des ausgebildeten Auges besteht aus mindestens 
 16 Zellen, zwei Corneabildungszellen unter jeder Fassette, und 4 Retino- 
 pboren (Krystallzellen), welche mit fadenfbrmigen Auslaufern an der 
 Rhabdomspindel vorbei und zwiscben den Retinulazellen bindurcb bis 
 zur Basalmembran reichen, wo sie an der unter jedem Ommatid be- 
 findlichen Yerdickung derselben endigen. Dann 10 Pigmentzellen, 
 zwei distale, welche den Krystallkegel umgeben und nach der Basal- 
 membran zu fadenformig verlangert sind, und acbt proximale (eine 
 von ibnen ist pigmentlos), welche der Rhabdomspindel eng anliegen 
 und nach auBen nicht weit liber dieselbe vorragen. Die sieben pig- 
 mentierten (Retinula-) Zellen gehen in dicke, die Basalmembran durcli- 
 setzende Nervenfasern liber; die zwei distalen Pigmentzellen sollen 
 gleichfalls in feinen Fasern durch die Basalmembran treten, moglichen- 
 falls dtirften sich diese scklankern Fasern, deren je zwei zu einem 
 Ommatid gehoren, als BlutgefaBe erweisen, denn auch fur solche 
 miissen regelmaBige Oeffnungen in der Basalmembran ebenso erwartet 
 werden, wie sie bei den Insekten flir die Tracheenschlauche vor- 
 handen sind. 
 
 Da Parker (von dem Standpunkte der Patten’schen Hypothese 
 aus) seinen Befund — dass namlich die Retinula- und nicht die 
 Krystallzellen (Retinophoren) mit den Nerven zusammenhangen — 
 als sehr auffallend bezeichnet, diirfen wir wohl annehmen, dass er 
 
 1) Parker G. H., Preliminary account of the developement and histology 
 of the Eyes in the Lobster. Oktober 1888. Proceedings of the American 
 Academy 1888. 
 
 2) Herrick F. H., The developement of the compound Eye of Alpheus. 
 Zoolog. Anzeiger, Bd. XII, Nr. 308, 1889. 
 
 3) Es ist vielleicht nicht iiberfliissig daran zu erinnern, dass nach neuern 
 Untersuchungen auch bei den Wirbeltieren die. Verbin dung des Zentralorganes 
 mit den Sinnesorganen nicht durch von ersterem auswachsenden Nervenfasern, 
 die mit einem merkwiirdigen Gliick immer grade ein solches Organ treffen, 
 hergestellt wird, sondern dadurch, dass sich zwischen beiden gelegene Zellen 
 — seien es nun solche der urspriingliclien gemeinsamen Anlage beider Teile 
 oder zwischen diese tretende indifferente Zellen (Bildungszellen) — in Nerven 
 umwandeln. 
 
232 Carriere, Auge der zelinfuBigen Crustaceen and Arachnoiden. 
 
 diesem Punkte besondere Aufmerksamkeit gewidmet hat und somit 
 wertvolles Material fur die frUkere und gegen die Patten’sche Auf- 
 fassung beibringt. 
 
 Gleicherweise durch Delamination , durch selbstthatige Trennung 
 eines Zellenhaufens in verschiedene Schichten lasst Herrick das 
 Auge sich bilden, welcher die Entstelmng des Auges von dem ersten 
 Auftreten der Augenscheiben (Kopflappen) bis zum Larvenstadium 
 auf Schnittserien verfolgte. 
 
 Bei einer Garnele, Alpheus, besteht der Embryo zur Zeit der 
 ersten Anlage der Augen aus drei Flecken (Haufen) von Zellen, der 
 Ventralplatte und den beiden Augenscheiben; aus letztern entwickelt 
 sich das ganze Auge mit seinem Ganglion. Wenn die Augenscheiben 
 sich mit der Bauchplatte vereinigt haben, beginnt die Yerdickung 
 der erstern, und zwar einmal durch Vermehrung der Ektodermzellen 
 durch Teilung so wohl in zur Oberflache senkrechter als dieser 
 paralleler Riclitung (Emigration und Delamination), als auch wahr- 
 scheinlich durch Anlagerung „indifferenter Zellen u aus dem Dotter. 
 Bei einem Alter des Embryo von einer Woclie ( Eunauplius ) bilden 
 die Koptlappen eiformige, dichte Ektodermmassen , welche uber die 
 Oberflache vorgewolbt sind, nacli dem Auftreten von 7 Paar von 
 Kdrperanhangen beginnt die Sonderung einer auBern Schicht groBer, 
 korniger Zellen, des Retina kei ms (Retinogen) von der darunter 
 liegenden Zellmasse, dem Ganglienkeim (Gangliogen) , zwischen 
 denen bei dem Stadium mit 10 Korperanhangspaaren eine Membran 
 (Basalmembran des Retinakeims) aufgetreten ist. Der Retinakeim ist 
 zu dieser Zeit nur noch an den Randern einschichtig, bald beginnt 
 die Sonderung seiner Zellen in die Ommatidien, indem sich einerseits 
 auf der Basalmembran aufsitzende Zellstrange in radiarer Richtung 
 zu einem im Ganglienkeim gelegenen Zentrum anordnen, die Reti- 
 nulae (Herrick verwendet die neuen Ausdrucke von Pattens, 
 aber mit der alten Bedeutung, so dass ich der Verstandlichkeit halber 
 die bekannten Bezeichnungen benutze), anderseits auBen sich eine 
 Zellscliicht abtrennt, die Corn easchicht, deren Zellen sich paar- 
 weise gruppieren, so dass je zwei uber eine Retinula zu liegen 
 kommen. Unmittelbar unter dieser Schicht kommt die der Krystall- 
 zellen (retinophoral layer) zur Ausbildung, welche zu je vieren an- 
 geordnet die Krystallkegel bilden. Die Retinulazellen, zu sieben in 
 rbhrenformige Btindel gestellt (spater eine solide Saule bildend) be- 
 gegnen sich mit den Gruppen der Krystallzellen und nelimen das 
 spitzausgezogene innere Ende derselben in ihr auBeres auf. Die 
 Krystallzellen erstrecken sich nicht weiter nacli der Basalmembran 
 zu, sondern endigen im auBern Abschnitt der Retinula. An der Basal- 
 membran und an der Grenze von Retinula und Krystallzellen ent- 
 wickelt sich ein Netzwerk von Chitin, zwischen den Ommatidien 
 stelien zahlreiche unveranderte Ektodermzellen. 
 
Carriere, Auge der zehnfiiBigen Crustaceen und Arachnoiden. 
 
 233 
 
 Bei der Larve findet sich kein spindel - oder saulenformiges 
 Rhabdom, wenn man nicht den innersten soliden Kern der Retinula, 
 welcher ein Produkt dieser selbst ist, als solches betracbten will, 
 und in dem Krystallkegel sind keine Nervenfasern zu finden. 
 
 Weder eine Einstiilpung nock irgend eine Art yon Aushoklung 
 konnte wahrend der Entwicklung dieses Auges beobachtet werden; 
 wenn nun bei andern Garnelen eine Einsenkung den ersten Schritt 
 zur Entwicklung des Auges bilde , so konne dem, wie das Beispiel 
 von Alpheus zeige, nur sehr geringe Bedeutung zugemessen werden, 
 und nur in der Beziebung, dass es sick dabei einfacb darum handle, 
 schnell eine grofie Anzahl von Ektodermzellen unter die Oberflache 
 zu bringen. Die Anordnung, welcbe die Zellen dabei erlialten, mass 
 (wenn icb den Autor ricbtig verstebe) als voriibergehend und fur die 
 weitere Entwicklung des Organs gleichgiltig betracbtet werden. 
 
 Ein Urteil iiber diese Mitteilungen und die Erorterung der Be- 
 ziehungen, in welcbe sie mit den Ergebnissen Reichenbach’s und 
 Kingsley’s zu bringen waren, erscbeint mil* ebenso wie ein Ver- 
 gleich mit den iibrigen hier besprochenen Abbandlungen vor dem 
 Erscbeinen der ausfiibrlichen Untersuchungen ausgescblossen. Ich 
 begniige micb, darauf hinzuweisen, dass ein prinzipieller Unterschied 
 zwischen Einstiilpung und Abblatterung nur dann besteht, wenn, wie 
 das ja in einer Anzabl von Fallen sick findet, den Zellen des unter 
 die Oberflache gelangten Blaschens bereits ein bestimmter morpho- 
 logiscber und pbysiologischer Wert zukommt, mit andern Worten, 
 wenn ein urspriinglich oberfiacklich gelegenes Organ in die Tiefe 
 riickt (z. B. Augen der Gastropoden, der Wirbeltiere, der Skorpioue?). 
 Wir warden diese Art der Einstiilpung von einer zweiten zu trennen 
 haben, welcbe nur das robe Material zum Aufbau eines Organs in 
 die Tiefe schafft, und welcbe, wie gesagt, von der Delamination nicbt 
 scbarf abzugrenzen ist; hier ist die Lage der einzelnen Zellen bei 
 dem Uebergang in die Tiefe bedeutungslos fur ihre spatere Ver- 
 wendung und Lage bei dem Aufbau des Organs, und hierher waren 
 beispielsweise alle sog. soliden Einstiilpungen und Einwucherungen, 
 sowie die Anlage des Seitenauges der Crustaceen zu stellen. Was 
 die Angaben der beiden letzten Beobacliter iiber den innern Bau der 
 Ommatidien betrifft, so sind die Unterschiede dabei von geringem 
 Werte gegeniiber den Punkten, in welchen sie ubereinstimmen, indem 
 sie die Krystallzellen ihrer friibern Bestimmung wiedergeben und des 
 Ranges von Sehzellen, welcher ibnen ja verliehen worden war, ent- 
 kleiden. Wenn nach der Ansicht des einen Autors diese Zellen in 
 der Hoke der Kegelspitze endigen, nacb der des andern gleich den 
 aufiern Pigmentzellen sick mit feinen Auslaufern bis zur Basalmembran 
 erstrecken, so wiirde letzterer Fall, wenn er Bestatigung fande, eine 
 scbon von Patten ausgesprochene Modifikation unserer bisherigen 
 Ansicht iiber den Bau des Artbropodenauges (nach welcher nur die 
 
234 Carriere, Auge der zehnfiiBigen Crustaceen unci Arachnoiden. 
 
 zwischen den Ommatidien stehenden, unveranderten Zellen der Augen- 
 anlage auch im ausgebildeten Auge von der Cornea bis zur Basal- 
 merabran reichen) bilden. Welches von beiden fur die Dekapoden 
 Geltung hat, wird sich vielleicbt schon nach dem Erscheinen der 
 vollstandigen Abhandlungen entscheiden lassen; die Frage, wie sich 
 das Auge der Insekten in diesem Punkte verhalt, hoffe ich in meiner 
 schon erwahnten Untersuchung mit Bestimmtheit erledigen zu konnen. 
 
 J. Carriere (Strafiburg i. E.). 
 
 Druck von Junge & Sohn in Erlangen. 
 
(Separat-Abdruck aus dem »Zoologischen Anzeiger« No. 40t. 1892.) 
 
 liber die Entwicklung des Imagoauges von Vanessa. 
 
 (Vorlaufige Mittheilung.) 
 
 Von H. Johansen, Mag. zool. 
 
 Als Ergebnis einer an Vanessa urticae angestellten Untersuchung 
 iiber die Entwicklung des zusammengesetzten Auges ist Folgendes her- 
 vorzuheben. Das Facettenauge gebt aus der einschicbtigen Epidermis 
 der Raupehervor, obne daB in derselben eine Einstiilpung nacbzu- 
 weisen ware, wie eine solcbe yon Patten * an der Wespe bescbrieben 
 wird. Das innigere Zusammentreten der Epidermiszellen zur Bildung 
 der Ommatidien findet bald nach der Ablbsung der Raupenaugen von 
 der Epidermis statt , wobei die Larvenaugen, dem Ganglion opticum 
 der Raupe aufsitzend durch die bei der Verpuppung gebildete Kopf- 
 blasenhohle von der Epidermis getrennt werden. Der Kopfblasen- 
 raum entbalt Mengen von Leucocyten auBer den Zertallproducten der 
 larvalen Organe. An der Bildung eines Ommatidium sind urspriing- 
 licb 13 Zellen betheiligt und zwar vier Zellen, deren Kerne von 
 Clap are de 1 2 die »Semper J schen« benannt sind, zwei Pigmentzellen 
 erster Ordnung und sieben Retinulazellen, zu denen noch erne Gan- 
 glienzelle und secbs Pigmentzellen zweiter Ordnung binzukommen; 
 letztere geboren zugleich aber aucb benachbarten Ommatidien an. 
 Die Kerne der Pigmentzellen erster Ordnung liegen urspriinglich iiber 
 den ))Semper J scheme Kernen, d. h. distal von denselben. Bald findet 
 aber eine Yerlagerung statt. Die den sSemper'schenee Kernen ange- 
 liorenden Zellen sebeiden die Cornealinsen und die Krystall- 
 k egelsegme nte aus und bilden Ftftutu ngsbar cben, die die 1 up- 
 penbiille von der sicb zu Ommatidien umbildenden Epidermis zu 
 lockern baben. Die auf der Oberflacbe des Auges unregelmaBig zwi- 
 seben Gruppen von Facetten stebenden C uticularbaare sind Aus- 
 scbeidungsproducte von Zellen , die sicb an der Ommatidienbildung 
 niebt betbeiligen. Diese Haarzellen konnen als letzte Reste der ur- 
 spriinglich in groBerer Anzahl zwischen den Ommatidien stebenden 
 gewobnlicben Epidermiszellen aufgefaBt werden und weisen darauf 
 bin, daB die phylogenetische Entwicklung des Facetten- 
 auges der Tracbeaten auf eine Anhaufung von Einzelaugen 
 zuriickzufiihren ist. Diese Anhaufung gieng im Laut der phylogene- 
 tiseben Entwicklung in die Bildung eines scheinbar einheitlicben 
 
 1 W. Patten, Development of the Eyes of Vespa. Journ. of Morphol. Vol. 1. 
 
 No. 1. 1887. ... 
 
 2 E. Claparede, Zur Morphologie des zusammengesetzten Auges bei den 
 
 Arthropoden. Zeitschr. f. wiss. Zool. 10. Bd. 1860. 
 
Organs fiber, je mehr die die Einzelaugen oder Gruppen derselben 
 von einander trennenden Epidermiszellen nicht mehr zur Anlage 
 kamen. 
 
 Das Auge scheint in ganzer Ausdehnung einschichtig zu bleiben, 
 nur macht sich eine Yerkiirzung der distalen Abschnitte der Retinula- 
 zellen geltend. Die Krystallkegelgenese ist keine auBere Aus- 
 scheidung im Sinne von Claparede (1. c.), sondern eine inn ere 
 Aus schei d ung : innerhalb jeder der vier Zellen tritt proximal vom 
 » Semper J schen« Kern ein Kliimpchen Kegelsnbstanz auf. Das 
 Rhabdom ist keine Ausscheidung der Retinulazellen , sondern eine 
 lebende Modification des Rrotoplasma derselben. Das Rhabdom ist 
 somit kein Fortsatz des Krystallkegels , wie es Patten angiebt (1. c.). 
 An der Ausscheidung der Cornealinse ist kein besonderes, von den die 
 Krystallkegel ausscheidenden Zellen verschiedenes Zelllager be- 
 theiligt, wie es Patten angiebt. 
 
 Pigment tritt in sammtlichen Zellen des Facettenauges auf. mit 
 Ausnahme der distalen Enden der Krystallkegelzellen und der Haar- 
 zellen. Die sich zu Ommatidien gruppierenden Epidermiszellen er- 
 halten von den Raupenaugen her nicht unbedeutende Mengen von 
 Pigment, das ihnen durch die Leucocyten, die den Raupenaugen 
 gegeniiber als Phagocyten auftreten, durch den Kopfblasenraum zu- 
 geflihrt und iibergeben wild. Das Pigment der Raupenaugen wird in 
 dem Plasma der Epidermiszellen zu dem bleibenden Pigment umge- 
 wandelt. 
 
 Das optische Ganglion des Imago geht aus dem Ganglion 
 opticum der Raupe hervor und ist keine absolute Neubildung, wie der 
 epidermale Theil der Augen. Die Nervenbiindelschicht (Ber- 
 ger 3 ; allein erscheint als Neubildung, indem dieselbe aus zwei primi- 
 tiven Nervenbiindeln , einem dorsalen und einem ventralen, ihren 
 Ursprung nimmt, welche schon in den ersten Stadien anzutreffen sind 
 und durch centripetal vor sich gehende Spaltung eine gauze Menge 
 Nervenbfindel hervorgehen lassen. In den iibrigen Schichten des 
 Ganglion opticum finden bloB Wachsthuinsprocesse statt, denen zu- 
 folge sich das ganze Ganglion bedeutend vergroBert und den Kopf- 
 blasenraum anfiillt. 
 
 Die ausffihrliche Schilderung der angedeuteten Verhiiltnisse soli 
 mit den Zeiclmungen demnachst erscheinen und zugleich Betrachtun- 
 gen theoretischer Natur bringen. 
 
 Tarassowka bei Moscau, den 1. Juli 1892. 
 
 3 Berger, Untersuclmngen iiber den Bau des Gehirns und der Retina der 
 Artbropoden. Arbeit, zool. Instit. Wien, 1. Bd. 1878. 
 
a sur le developpement de l’ceil COMPOSE DE VANESSA, 
 
 par H. Johansen, Mag-d. zool. 
 
 LPexamen du developpement de Poeil compose de Vanessa 
 nrticae nous a donne le resultat suivant: 
 
 L’oeil sort de Pepiderme a simple couclie de la larve, sans 
 qu’on puisse y trouver une invagination, comrne Patten *) 
 pretend Pavoir trouvee chez la guepe. Les cellules de Pepi- 
 derme se reunissent pour former les ommatidies, bientdt 
 apres que les yeux de la larve se sont detaches de Pepiderme, 
 au moment oil les yeux de la larve, disposes sur le gan- 
 glion optique, se separent de Pepiderme par la vessie de tetc 
 (Kopf blase) formee pendant la perimorphose. 
 
 II y a dans la vessie de tete, outre les produits de dis- 
 solution des organes de larve, de nombreux leucocytes, 
 13 cellules qui ont part a la formation d’un ommatidium. 
 Parmi ces 13 cellues, il y en a 4, dont les noyaux sont nommes 
 par Glaparede**) „les noyaux de Semper w , puis 2 cellules 
 pigmentaires du premier ordre, 7 cellules de retinale, aux- 
 quelles se joignent encore deux cellules ganglionnaires. Cliaque 
 ommatidium est entoure par 6 cellules pigmentaires du se- 
 cond ordre, mais chacune d'elles appartient en meme temps 
 aux ommatidies voisines. 
 
 *) W. Patten , Studies on the eyes oi‘ Arthropods. Development of the 
 Eyes of Vespa. Journ. of Morphol. Vol. I ( JVs 1. 1887. 
 
 **) JE. Glaparede , Zur Morphologic des zusammengesetzteTi Augcs bei den 
 Arthropoden. Z. fur wise. Zool. Bd, X. 1860. 
 
9 
 
 Les noyaux des cellules pigmentaires du premier ordre 
 sont d’abord poses a une certaine distance au-dessus des 
 noyaux de Semper. Plus tard, les noyaux de Semper avan- 
 cent de maniere que les noyaux des cellules pigmentaires 
 du premier ordre se trouvent places a cote. 
 
 Je propose de donner aux cellules le meme nom de Sem- 
 per , que les noyaux seuls ont porte jusqu’a present. Ces 
 cellules de S&rhper trient les facettes de la cor nee et les 
 cones cristallins et foment des poils de depouillement 
 protoplasmatiques, dont la fonction est de reparer la peau 
 cliitineuse de la nymphe de repiderme, oil Ton peut constater 
 la formation des ommatidies. Les poils cuticulaires, qui 
 se trouvent a la surface des yeux irregulierement places 
 parmi des groupes de facettes, sont tries par des cellules 
 epidermales qui ne prennent pas part a la formation des 
 ommatidies. 
 
 Je regarde ces cellules de poils comme les derniers rudi- 
 ments des cellules epidermales, qui se trouvaient dans Porigine 
 en plus grand nombre parmi les ommatidies, et ces cellules 
 nous montrent que le developpement phylogenetique de Poeil 
 facette des Arthropodes traclieates doit etre considere comme 
 un assemblage d’yeux simples (Einzelaugen). Cet assemblage 
 formait, pendant le developpement phylogenetique, un organe 
 apparent uni, tant que les cellules epidermales, qui sepa- 
 raient les yeux simples ou leurs groupes, iPetaient pas 
 employees. Les cellules coinposant rominatidium occupent 
 toute, la largeur de l’epidenne, mais on apergoit une abr.e- 
 viation des bouts distaux des cellules de retinule. La for- 
 mation des cones cristallins iPest pas une excretion exterieure, 
 comme le pense Claparede , mais c’est une excretion inte- 
 rieure: au dedans de chaque cellule de Semper parait une 
 petite masse de substance du cone cristallin. Le ,,rhabdoin“ 
 ( Grenadier *) iPest pas une excretion des cellules de retinule, 
 (Pest une modification vivante du. protoplasma de ces cellules. 
 
 *) Grenadier. Untersiiclnnigen liber das Sehorgan der Artliropoden. Got- 
 tingen. 1879. 
 
3 
 
 Ce n’est pas unappenclice du cone cristallin, comme ledit Pat- 
 ten. Les facettes de la cornee ne sont pas formees par une 
 couche de cellules differente des cellules qui donnent les co- 
 nes cristallins. Les bouts distaux seuls des cellules de Sem- 
 per et les cellules de poils restent sans pigment. Le 
 pigment des yeux de la larve ne se perd pas; les leucocy- 
 tes, qui jouent ici le role des phagocytes, le transmettent 
 aux cellules epider males, qui se convertissent en ommatidies 
 et transforment le pigment des yeux de la larve en pigment 
 des yeux du papillon. 
 
 Le ganglion optique de l’imago sort du ganglion optique 
 de la larve et ne presente pas une nouvelle formation, comme 
 la partie epidermale des yeux. La couche des paquets ner- 
 veux (Nervenbundelschicht, Berger *) seule apparait comme 
 une formation toute nouvelle, tirant son origine des deux 
 paquets nerveux primitifs: d’un paquet dorsal et d’un paquet 
 ventral, ce qu’on peut voir dans les premiers stades, ct qui 
 par une fenderie centripetale laissent sortir une quantite de 
 paquets nerveux. Dans les autres couches du ganglion opti- 
 que,, des proces de croissance ont lieu, a la suite desquels 
 le ganglion optique s’accroit et remplit toute la vessie de 
 la tete. 
 
 *) Berger. Untersuclmngen iiber den Bail des Gehirns und der Retina 
 der Arthropoden. Arbeiteri Zool. Inst. Wien. Bd. I. 1878. 
 
IJeiaraHo no uocraHOBJi. CoBiia IlMiiepaiop. OGinecTBa J106. Ecrecr., Ampon. n ;->THorpatin. 
 Thiio-.Iht. T «a .laiuKCBH'ib. oHhmchckIh h K n . MocKna. 'Ihctwc iipyAM, JVs 199. 
 

 
 Ueberreicht vom Verfasser. 
 
 Die Entwicklung des Imagoauges von Vanessa 
 
 urticae L. 
 
 Yon 
 
 Hermann Johansen, 
 
 Magd. zool. 
 
 (Aus dem zool. Institut der Universitat Dorpat.) 
 
 Mit 2 lithographischen Tafeln. 
 
 ^ ^,7 
 
 Abdruck 
 aus den 
 
 Zoologischen Jahrbiichern. 
 
 Abtheilung fur Anatomie und Ontogenie der Thiere. 
 Herausgegeben von Professor Dr. J. W. Spengel in Giessen. 
 Seehster Band. 
 
 Verlag von Gustav Fischer in Jena. 
 
J\[ iA ia/il ut i £ a * il y~c u - asHufc 
 
 2 r r)T a n n U c k et n.s en 
 
 A\a&d. zool. 
 
Nachdruck verboten. 
 Uebersetzungsrecht vorbehalten. 
 
 
 Die Entwickluug des Imagoauges von Vanessa urticae L. t 
 
 Yon 
 
 Hermann Johansen, 
 
 Magd. zool. 
 
 Hierzu Tafel 23 und 24. 
 
 (Aus dem zool. Institut der Universitat Dorpat.) 
 
 Durch eine von der physico - mathematischen Facultat der Uni- 
 versitat Dorpat fur das Jahr 1888 gestellte Preisaufgabe wurde ich 
 zur Untersuchung der Entwicklung des zusammengesetzten Auges eines 
 tracheaten Arthropoden angeregt. Da Vanessa urticae L. sich als 
 leicht in grossern Mengen zu beschatfendes Material erweist und die 
 Entwicklungsdauer dieses Tracheaten eine kurze ist (9 — 14 Tage je 
 nach den Temperaturverhaltnissen) , wurden die Untersuchungen an 
 diesem Object angestellt. 
 
 Indem ich hiermit die Resultate meiner preisgekronten, im Laufe 
 der Zeit bedeutend vervollstandigten Untersuchung der Oeffentlichkeit 
 iibergebe, ist es mir eine angenehme Pflicht, meinem hochverehrten 
 Lehrer, Herrn Prof. Dr. J. v. Kennel meinen warmsten Dank auszu- 
 sprechen fur die vielfache, wahrend meiner Studienzeit und bei der 
 Vollendung meiner Untersuchung mir zu Theil gewordene Anregung 
 und Belehrung und fur das mir in liebenswiirdiger Weise zur Ver- 
 fiigung gestellte Praparat des epidermalen Theils des Auges einer 
 j ungen Puppe von Sphinx euphorbiae. Ein Theil der Untersuchungen 
 wurde im Laufe des Sommers 1891 im Privat - Laboratorium des 
 Herrn N. Abrikossow in der Nahe von Moskau ausgefiihrt, und ich 
 bitte denselben meinen Dank annehmen zu wollen fur die Liebens- 
 
 Zool. Jahrb, VI. Abth. f. Morph. OQ 
 
446 
 
 HERMANN JOHANSEN, 
 
 wiirdigkeit, mit welcher er mir sein wohlausgestattetes Laboratorium 
 zu meinen Untersuch ungen tiberliess. Herrn N. Goronowitsch in 
 Puschkino bei Moskau bin ich flir die Benutzung seiner Bibliothek 
 zu Dank verpflicktet. 
 
 Die als Untersuchungsobject dienenden Raupen und Puppen von 
 Vanessa urticae wurden in den verschiedensten Stadien der Ent- 
 wicklung durch Uebergiessen mit kochendem destillirten Wasser ge- 
 todtet und darauf in Alkohol von verschiedenen Concentrationsgraden 
 in iiblicher Weise gehartet. Bei einigen jungen lebenden Puppen 
 wurde mit einer scharfen Scheere die Kopfgegend abgeschnitten und 
 das auf diese Weise erhaltene Stuck durch einen Medianschnitt halbirt 
 und darauf moglichst schnell in Osmiumsaurelosung resp. in die sog. 
 „FLEMMiNo’sche Losung“ auf 3 Stunden gebracht. Bei altera Puppen 
 konnte am lebenden Object die Puppenhiille in der Kopfgegend ent- 
 fernt und der Kopf darauf mit den erwahnten Reagentien behandelt 
 werden. Beim plotzlichen Abkochen der Puppe ist die Coagulation 
 der Gewebe eine momentane. Daraus resultirt, dass das histologische 
 Bild, das man beim Untersuchen so behandelter Objecte erhalt, selir 
 genau die Verhaltnisse zeigt, wie sie im Leben vorliegen, d. h. wir 
 erhalten undeutliche Zell- und Kerngrenzen. Urn die Coagulation der 
 Gewebe zu verlangsamen und deutlichere Abgrenzungen zu erhalten, 
 wurde, besonders bei jiingeren Stadien, die Anwendung der erwahnten 
 Reagentien der Abkochungsmethode vorgezogen. 
 
 Die auf die genannten Weisen fixirten Gewebe wurden nun in 
 folgender Art gefarbt. Alkoholisches Boraxcarmin ergab bei altera 
 Stadien ziemlich brauchbare Bilder, doch musste der Vorzug dem be- 
 kannten GRENACHER’schen Alauncarmin gegeben werden, das siclr zur 
 Untersuchung von Arthropodengeweben ganz besonders eignet. Nach 
 dem Alauncarmin folgt als Farbemittel das Hamatoxylin, und zwar 
 wurden sowohl Totalfarbungen der Objecte vorgenommen als auch 
 Schnittfarbungen. Als Aufhellungsmittel dienten Terpentin, Creosot, 
 Toluol und Xylol, als Einbettungsmasse Paraffin. Zur Benutzung 
 kamen sowohl Schlittenmikrotome als auch das „ Rocking microtome 4 * 
 der „Cambridge scientific instrument company 44 . Letzteres leistet vor- 
 zugliche Dienste. Mittels der sog. ^ScHALLiBAUM’schen Mischung 44 
 (Collodium und Nelkenol 1 : 2) wurden die Schnitte auf dem Object- 
 trager befestigt und in Iiblicher Weise das Paraffin entfernt. 
 
 Bei Untersuchung alterer Stadien des epidermalen Theils der 
 Augen ist es nicht nothig, einzelne Kopfe bloss zur Herstellung von 
 Langsschnitten durch die Ommatidien, andere Exemplare wieder zur 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 447 
 
 Herstellung von Querschnitten durch dieselben zu verwenden, sondern 
 die Gruppirung der genannten Elemente im Auge erleichtert die Unter- 
 suchung insofern, als wir in Folge der radiaren Anordnung der Retina- 
 elemente in dem eine Halbkugel darstellenden Auge in einer Serie 
 durch dasselbe immer Schnitte in den gewunschten zwei Richtungen 
 und in den diese verbindenden Uebergangsebenen vorfinden, so dass 
 die Natur hier der Untersuchung in sehr liebenswurdiger Weise ent- 
 gegenkommt. 
 
 Indem ich nun zur Beschreibung der an Vanessa urticae ge- 
 wonnenen Resultate iibergehe, erscheint es mir zweckmassig, um die 
 Uebersicht zu erleichtern, die Hauptbestandtheile des zusammen^e- 
 setzten Auges, den Epidermistheil und das optische Ganglion, gesondert 
 zu besprechen, woran sich dann ein Schlusscapitel mit dem Resume 
 der Resultate und Betrachtungen allgemeinerer Natur anreihen soli. 
 
 Der epidermale Theil der Augenanlage. 
 
 In der Literatur der letzten drei Decennien linden wir iiber die 
 erste Anlage des zusammengesetzten Auges recht verschiedenartige 
 Angaben, die sich aber im Grunde auf zwei Haupttypen zuruckfiihren 
 lassen. Das Tracheatenauge hat eine recht sparliche Berticksichtigung 
 gefunden, wir ziehen aber auch die branchiaten Arthropoden, die 
 Crustaceen, in den Kreis unserer Betrachtungen, da der Bau der 
 Augen bei beiden Stammen der Arthropoden bekanntlich ein so un- 
 gemein ubereinstimmender ist, dass es sich erwarten lasst, dass wir 
 auch im Entwicklungsmodus desselben Uebereinstimmungen linden. 
 
 Der Unterschied in den beiden Haupttypen der Entwicklung be- 
 steht darin, dass ein Theil der Autoren, wir nennen Dohrn (5), 
 Reichenbach (12 u. 17), Kingsley (20), Patten (19), als erste An- 
 lagen des zusammengesetzten Auges in der Epidermis auftretende 
 Einstiilpungen mit damit verbundenen Zellenwucherungsprocessen be- 
 schreiben, die von ihnen als „halbmondformiger Spalt“ (Dohrn), „Augen- 
 falte“ (Reichenbach), „optic invagination“ (Kingsley) und „ganglionic 
 fold“ (Patten) bezeichnet werden. Die tibrigen, auch die erste Anlage 
 des uns beschaftigenden Organs mehr oder weniger beriicksichtigenden 
 Autoren erwahnen eines derartigen Bildungsmodus nicht. Dazu ge- 
 horen: Bobretzky (6), Grenacher (11), Carriere (15), Lebedinsky 
 (24), Herrick (25), Parker (27), Nusbaum (22), Claus (16). 
 
 Von den letztgenannten Autoren wird die Entwicklung des Epi- 
 dermistheils des Auges auf Verdickungen in der „Hypodermis“ zuriick- 
 
 29* 
 
448 
 
 HERMANN JOHANSEN. 
 
 geffihrt, wobei (lurch Umlagerungen und Umgestaltungen der die „Hypo- 
 dermis“ zusammensetzenden Zellen die definitive Form der Augen- 
 elemente hervorgeht, ohne dass eine Einstiilpung oder etwas dem 
 Aehnliches zu bemerken ware. Eine weitere Differenz unter den An- 
 gaben der Autoren besteht darin, dass ein Theil derselben die Augen- 
 epidermis in ihrer Anlage einschichtig sein lasst (Patten), wahrend 
 nach Anschauung der andern, meist altera Forscher die Zellen nicht 
 die ganze Breite des epidermalen Theils der Augen durchziehen, 
 sondern sich in zwei fiber einander gelagerte Zellenschichten trennen, 
 somit das Auge als aus zwei Zellenlagern zusammengesetzt zu betrachten 
 ist (Grenacher). 
 
 Meine Beobachtungen an Vanessa urticae ergeben nun folgende 
 Einzelheiten, wobei ich betonen mochte, dass ich die Epidermis des 
 Kopfabschnittes durch alle Stadien, mit der gewohnlichen Raupe be- 
 ginnend, durch den ganzen Process der Yerpuppung hindurch, bis 
 zum Ausschlfipfen der Imago verfolgt habe, weil es sich als noth- 
 wendig erwies, auch den sich zur Verpuppung anschickenden, schon 
 hangenden Raupen voile Aufmerksamkeit zuzuwenden. Vergleichen 
 wir die Epidermis einer ausgewachsenen , sich kurz vor der An- 
 schickung zur Verpuppung befindenden Raupe mit der einer jungen 
 Puppe, welche eben ihre Raupenhfille abgestreift hat, so finden wir 
 im Kopfabschnitt derselben nicht unwesentliche Differenzen. Die Dauer 
 des Hangens der Raupe bis zum Moment des Abstreifens der Raupen- 
 htille betragt ungefahr 40 Stunden, und in dieser Zeit findet die Bildung 
 der Kopfblase der Imago statt. Im ersteren Stadium finden wir die 
 Raupenaugen noch innerhalb der Epidermis, nach einer Hangezeit. 
 von 6 Stunden sind dieselben jedocb schon aus dem Verbande der 
 Epidermiselemente geschieden und rticken zum optischen Ganglion 
 hin. Das Herausrficken der Raupenaugen aus dem Complex der Epi- 
 dermiszellen findet ganz allmahlich statt, und die die Raupenaugen 
 umgebenden und von einander trennenden Epidermiszellen schliesscn 
 sich schon sofort an einander, so dass das Herausrficken der Raupen- 
 augen gleichzeitig mit dem sich Aneinanderschliessen der Epidermis- 
 zellen vor sich geht. Die Epidermis bleibt wahrend des Processes 
 eine einheitliche und weist keinerlei Lficken in Folge des Heraus- 
 rfickens der Raupenaugen auf (Fig. 1). Was nun die Epidermis selbst 
 anlangt, so sind deren Elemente bei der Raupe sehr dicht an 
 einander gedrangt , in Folge lebhafter Zellvermehrungsprocesse an 
 beiden Kopfseiten, so dass wir die Einschichtigkeit derselben nur auf 
 dfinnen Schnitten nachweisen konneu. Die Zellkerne befinden sich in 
 
Die Entwieklung des Inaagoauges vou Vanessa urticae L. 
 
 449 
 
 verschiedenen Hohen und tauschen so bei dem allgemeinen Zusammen- 
 gedrangtsein der Epidermiselemente eine Mehrschichtigkeit derselben 
 yor. Diese zusammengedrangten Epidermiszellen stellen nicht in ihrer 
 ganzen Ausdehnung die Anlage des epidermalen Theils der Augen dar, 
 sondern nur ein Theil derselben wird zum Aufbau der Augenelemente 
 der Imago verwandt. Wir haben es hier mit Processen zu thun, die, 
 der Hautung vorangebend, zur Folge haben, dass die Epidermis uber- 
 haupt, nicht die der Augenanlage allein, sich nach dem Abstreifen 
 der Raupenhiille vergrossert. Die Wand der nach der Verpuppung 
 entstandenen Kopfblase der Imago hat eine viel grossere Oberflache 
 als die der Raupe. Die Epidermiszellen miissen daher in viel grosserer 
 Anzahl vorhanden sein, um in derselben Weise wie bei der Raupe 
 die Wand als einschichtiges Cylinderepithel auszukleiden. Nachdem 
 die Raupenhiille abgestreift ist und auch schon in alteren Stadien der 
 hangenden Raupe sehen wir in je einem centralen Theil der an beiden 
 Seiten des Kopfes stark verdickten Epidermis eine Lockerung der 
 Elemente eingetreten (Taf. 23, Fig. 2 und Taf. 24, Fig. 15), die 
 immer mehr zunimmt und uns die Einschichtigkeit der Epidermis auf 
 das Schonste zeigt. Die Lockerung erscheint auf den ersten Pdick 
 durch eine fliissige Ausscheidung von Seiten der Epidermiszellen be- 
 dingt, die die Zellen zum Auseinanderweichen bringen konnte. Da 
 aber an den Zellen selbst keine Verminderung ihres Yolumens be- 
 merkbar ist, sondern die Zellen im Laufe der Entwieklung wachsen, 
 so liegt der Gedanke naher, dass aus dem machtigen Kopfblasenraum 
 fliissige Theile durch die diinne , protoplasmatische Basalmembran 
 hindurchfiltriren und so eine Lockerung der Epidermiselemente herbei- 
 fiihren. Damit stimmt auch die von jetzt ab stetig im Laufe der 
 Entwieklung vor sich gehende Reduction des Kopfblasenraumes iiberein. 
 Diese hindurchfiltrirten Fliissigkeiten werden jedenfalls yon den Epi- 
 dermiszellen zum Wachsthum verwandt; ihr Wachsthum geht eben auf 
 Kosten dieser Nahrfliissigkeit vor sich. 
 
 In diesen centralen Theilen der sich immer mehr lockernden Epi- 
 dermis haben wir die Anlage des epidermalen Theils der 
 Augen zu sehen, die nach den Seiten von einem verdickten Theil 
 der Epidermis umschlossen ist, welcher die die Augen umgebenden Epi- 
 dermistheile aus sich hervorgehen lasst. Was nun die Form dieser 
 die erste Anlage des zusammengesetzten Auges bildenden Elemente 
 der Epidermis betrifft, so haben wir es mit stark in die Lange ge- 
 zogenen Zellen zu thun (Fig. 3), die mit einem proximalen, im All- 
 gemeinen sich verjiingenden Ende zum Kopfblasenraum gekehrt sind, 
 
450 
 
 HERMANN JOHANSEN, 
 
 einen grossen ovalen Kern besitzen, der sich mehr oder weniger im 
 Centrum der Zelle befindet, und deren distale Enden sich verbreitern, 
 um sich mit denen der benachbarten Zellen an einander zu legen, 
 theilweise sogar zu verschmelzen, obwohl in den meisten Fallen eine 
 Grenze der sich an einander legenden distalen Abschnitte zu erkennen 
 ist. Proximal vom Kern beginnt die Verjungung dieser Zellen, die 
 in diinne Protoplasmafaden auslaufen, welche in sich braune Pigment- 
 kornchen aufweisen, auf deren Bedeutung weiter unten eingegangen 
 werden soil. Die proximalen Enden dieser Zellen nahern sich mehr 
 oder weniger unter einander, und wir sehen dieselben durch eine fein- 
 kornige Protoplasmamasse mit einander in Verbindung stehen, offenbar 
 Fortsatze dieser Zellen, die sich zu einer die Epidermis gegen den 
 Kopfblasenraum abgrenzenden Protoplasmamembran vereinigt haben, 
 deren Aufgabe in der Ausscheidung der Basalmembran besteht. Was 
 nun die Bildung der fur das zusammengesetzte Auge so charakte- 
 ristischen Ommatidien betriflt, so konnte ich auf dem Stadium einer 
 jungen Puppe, die ca. 15 — 18 Stunden nach der Yerpuppung conservirt 
 war, abgesehen von der Einschicbtigkeit der Epidermiselemente, den 
 Beginn einer Differenzirung insofern constatiren, als die sie zusammen- 
 setzenden Zellen die Kerne auf verschiedenen Hohen gelagert aufweisen, 
 mit einer gewissen sich wiederholenden Regelmassigkeit, so dass man 
 schon ungefahr die die verschiedenen Theile der Einzelaugen bildenden 
 Elemente zu erkennen glaubt, obgleich noch kein innigeres Zusammen- 
 treten der Zellen zur Bildung der Retinulae stattgefunden hat (Fig. 4). 
 Auf diesem Stadium sieht man auch die Basalmembran in Gestalt 
 einer hellen, scharf begrenzten Linie als Randsaum der Epidermis 
 gegen den Kopfblasenraum auftreten. Auf Stadien von l 3 / 2 Tagen 
 ist das Zusammentreten der Augenelemente zur Bildung der Omma- 
 tidien erfolgt, doch ist dabei die ganze Anlage schon einschichtig. 
 Das ganze Gefiige der Epidermis ist ein innigeres geworden (Fig. 5), 
 die zwischen den einzelnen Epidermiszellen auf friiheren Stadien sicht- 
 baren, mit Nahrfliissigkeit erfiillten Liicken sind verschwunden. 
 
 In diesen und nicht weit entfernt stehenden Stadien kann man 
 beziiglich der Anordnung der Zellkerne in den verschiedenen Hohen 
 der Epidermis drei Regionen oder Zonen unterscheiden, die ich nach 
 ilirem Abstande von dem Ganglion opticum als distale, mittlere und 
 proximale Kernzone bezeichnen will. Die beiden ersten sind durch 
 eine grossere kernlose Zone von einander getrennt, wahrend die beiden 
 letzten nicht so scharf von einander geschieden sind, indem vereinzelte 
 Kerne sich zwischen beide Zonen einschieben. Die distale Kernzone 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 451 
 
 bestekt nur aus einer Schicht von Kernen. Es sind das die Kerne 
 derjenigen Zellen, die sich im Yerbande mit den iibrigen Zellen der 
 Augenepidermis an der Ausscheidung der die Augen uberziehenden 
 Puppenhiille betheiligt haben und deren Function im Imagoauge, wie 
 es sich erwarten lasst, wenn man vorn Bau des ausgebildeten Auges 
 ausgeht, darin bestehen miisste, die Cornealinsen und die Krystall- 
 kegel zu bilden. Mit andern Worten, die Kerne mussten nach Cla- 
 parede (2) als die „ SEMPER’schen “ bezeicknet werden. Ich mochte 
 der Einfachkeit und der bequemeren Ausdrucksweise wegen nicht bloss 
 die Kerne allein die ,, SEMPER’schen u nennen, sondern diese Bezeicknung 
 auck auf die diesen Kern entkaltenden Zellen ausgedeknt wissen und 
 verstehe unter den „ SEMPER’schen Zellen“ diejenigen Zellen im 
 zusammengesetzten Auge, die, bei den meisten Arthropoden einen Com- 
 plex von vier Zellen in jedem Ommatidium bildend, einerseits die 
 Cornea, andrerseits die Krystallkegelsegmente ausscheiden. 
 
 Wie sehr man sick aber bei einer derartigen, vom Bau des aus- 
 gebildeten Organs ausgehenden Beurtheilungsweise irren kann, lehrt 
 das Verbal ten der Kerne der distalen Kernzone im Laufe der weitern 
 Entwicklung. Anstatt nun auck weiter in ihrer Lage an der Ober- 
 flacke der Augen zu verkarren, treten in deutlicke Beziehungen zur 
 Oberflacke des Auges Zellen, deren Kerne der mittlern Kernzone an- 
 gehoren, die aber im Laufe der Entwicklung vollstandig in die distale 
 Zone iibergehen, wahrend andrerseits die primar in der distalen Zone 
 befindlicken Kerne hinunterriicken und sick zwiscken die eben er- 
 wahnten, secundar distal gelagerten Kerne und die Kerne der mittlern 
 Zone lagern. 
 
 Die mittlere Zone bestekt aus mehreren Reihen von Kernen (2 — 4), 
 die ziemlick dickt an einander gedrangt sind. Die letzte Kernzone, 
 an der Basalmembran gelegen, ist am unregelmafcsigsten zusammen- 
 gesetzt, indem hier bald mekr, bald weniger Zellkerne vorkommen oder 
 auck stellenweise feklen und auck in der Form und Grosse der Kerne 
 recht bedeutende Untersckiede zu verzeicknen sind. 
 
 Die Beziehungen der eckten „ SEMPER’schen Zellen “ zur Oberflache 
 des Auges macken sick in friiken Stadien bemerkbar, so lange die 
 grosste Masse der „ SEMPER’schen Zellen “ nock innerkalb des Om- 
 matidiums liegt und ihr Kern der mittlern Kernzone angehort. Diese 
 Beziehungen wurden mir zuerst verstandlich an einem Praparat, das 
 Herr Prof. v. Kennel die Giite hatte, mir bei der Vollendung dieser 
 Untersuchung zur Yerfiigung zu stellen. Es sind sekr gelungene Scknitte 
 durch die Augenepidermis einer jungen Puppe von Sphinx euphorbiae, 
 
452 
 
 HERMANN JOHANSEN, 
 
 die sich durch die Grosse der das Auge zusammensetzenden Elemente 
 auszeichnet. Taf. 23, Fig. 6 giebt die Zusammensetzung eines Orn- 
 matidiums dieser Retina wieder, die die namliche wie bei Vanessa 
 urticae ist. 
 
 Was ich an diesem Praparat aber mit besonderer Deutlichkeit 
 beobachten kann, besteht darin, dass von jeder Zelle, die hinter ihrem 
 Kern ein kleines Kliimpchen Krystallkegelsubstanz aufweist, distal- 
 warts ein Fortsatz ausgebt, der sich zwischen den Zelleibern der 
 nachst hohern Kerne hindurchschiebt und sich iiber die Oberflache 
 des Ommatidiums in Gestalt eines kleinen stumpfen Protoplasma- 
 fortsatzes erhebt, der von festerer Consistenz zu sein scheint als das 
 iibrige Protoplasma dieser Zellen. Diese Zellen sind als die „Semper- 
 schen“ aufzufassen, was erstens daraus hervorgeht, dass sie in ihrem 
 Protoplasma die Anlage der Krystallkegel erkennen lassen; ferner 
 spricht dafiir die vollkommene Uebereinstimmung der Verhaltnisse mit 
 den Befunden von Vanessa , wo aus diesen Zellen die direct unterhalb 
 der Cornea ihre Kerne aufweisenden „ SEMPER’schen Zellen“ hervor- 
 gehen. Als ich mich nach entsprechenden Stadien bei Vanessa umsah, 
 konnte ich bei einer 2 Tage und 1 Stunde alten Puppe die geschil- 
 derten Verhaltnisse in vollkommen iibereinstimmender Weise constatiren 
 (Taf. 23, Fig. 7); nur sind die Zellen bei Vanessa urticae bedeutend 
 kleiner, so dass ich die uns hier interessirenden Protoplasmafortsatze 
 nur bei starken Vergrosserungen auffinden konnte. Dieses Stadium 
 von Vanessa lasst in ganzer Ausdehnung des epidermalen Theils der 
 Augenanlage die eingetretene Bildung der Retinulae erkennen, und in 
 Folge dieser Vereinigung treten in der Epidermis zwischen den ein- 
 zelnen Ommatidien, an der Ommatidienbildung nicht unmittelbar theil- 
 nehmend, die Pigmentzellen 2. Ordnung scharfer hervor, als es bis 
 jetzt der Fall war*, in denen von Pigment keine Spur nachzuweisen 
 ist. Ausserdem bemerken wir noch eine andere Art von Zellen, die 
 dieselbe Lage wie die Pigmentzellen 2. Ordnung einnehmen, aber nicht 
 in so regelmassiger Weise zwischen den Ommatidien angeordnet sind, 
 die Haarzellen, deren Aufgabe in der Ausscheidung der zwischen den 
 Corneafacetten zerstreut stehenden Cuticularhaare besteht (Fig. 7). 
 Am distalen Ende der Einzelaugen dieses Stadiums fehlt noch der 
 Anfang einer Corneabildung. Die Pigmentzellen 2. Ordnung durch- 
 ziehen die ganze Breite der Epidermis als lange, in ihrem proximalen 
 Verlauf etwas gewundene Zellen, die bei Combination von Langs- und 
 Querschnitten schon eine mehr oder weniger prismatiscbe Form er- 
 kennen lassen. Die Querschnittsbilder lehren die Zusammensetzung 
 
Die Eutwicklung des Imagoauges von Vanessa urtieae L. 
 
 453 
 
 der Retinula aus je sieben Zellen in einem Ommatidium, von denen 
 eine central gelegen ist, auf das Deutlichste. 
 
 Die Aufgabe der distalwarts sich iiber die Oberflache der Orn- 
 inatidien erhebenden Fortsatze der SEMPER’schen Zellen besteht darin, 
 die Chitinhtille der Puppe von der sich bald darauf mit neuer Cuticular- 
 substanz — im Bereich der Augen mit den Corneafacetten — bedecken- 
 den Epidermis zu lockern. Diese Fortsatze erweisen sich als proto- 
 plasmatische Hautungsharchen, die, nachdem sie ihre Aufgabe erfiillt, 
 eine Lockerung zwischen Puppenhiille und Epidermis herbeigefuhrt 
 haben, wieder in den Leib ihrer Zellen eingezogen werden. Es ist 
 nicht ohne Interesse, an der schon zu Ommatidien umgebildeten Epi- 
 dermis der Augenanlage noch eine derartige Betheiligung an dem wohl 
 alien tibrigen Epidermiszellen der jungen Puppe zukommenden Geschaft 
 zu bemerken. Die in der nachsten Umgebung der Augen befindlichen 
 gewohnlichen Epidermiszellen lassen deutlich ahnliche Hautungsharchen 
 erkennen. Auffallend ist ferner der Umstand, dass diese Hautungs- 
 harchen nicht in den Zellen (Fig. 6 u. 7) ihren Ursprung nehmen, 
 die von alien das Ommatidium zusammensetzenden Zellen mit grosster 
 Oberflache frei nach aussen liegen, sondern Zellen angehoren, die die 
 Krystallkegel auszuscheiden haben, und die, wie wir sehen werden, 
 insofern ihren Beziehungen zur Oberflache des Ommatidiums treu 
 bleiben, als sie wieder die die Corneafacetten ausscheidenden Zellen sind. 
 
 Wenden wir uns nun zu den distal von den SEMPER’schen Kernen 
 gelagerten Kernen, so finden wir, dass dieselben Zellen angehoren, 
 die bei Vanessa im Vergleich zu Sphinx euphorhiae eine Verschieden- 
 heit bedingen, indem sie jedes Ommatidium an seinem distalen Elide 
 sich iiber die umgebenden Pigmentzellen 2. Ordnung vorwolben lassen. 
 Auf dem Gipfel der Vorwolbung kommen die Hautungsharchen zu 
 stehen (Fig. 7). Spater rucken diese Kerne in tiefere Schichten, es 
 findet die erwahnte Verlagerung statt und die Zellen werden zu den 
 Hauptpigmentzellen oder Pigmentzellen 1. Ordnung. 
 
 Was nun die Lagerung der das Ommatidium zusammensetzenden 
 Zellen gegen einander betrifft, so kann ich nicht mit derselben Be- 
 stimmtheit wie Patten und Kingsley den Verlauf der diinnen Proto- 
 plasmafaden der das Ommatidium zusammensetzenden Zellen angeben. 
 Dass die meisten dieser Zellen die ganze Breite der Epidermis durch- 
 ziehen, steht fur mich fest. Nur zwingen mich rneine Beobachtungen 
 an Vanessa zu der Anschauung, und das Sphingidenpraparat bestarkt 
 mich in derselben, dass die Retinulazellen sich in ihren distalen Enden 
 bedeutend verkurzen, um bier den mit den verschiedenen Ausscheidungen 
 
454 
 
 HERMANN JOHANSEN, 
 
 beschaftigten SEMPER’schen Zellen Platz zu macben. Die Zusammen- 
 setzung der Ommatidien babe icb durcb die scbematiscben Zeichnungen 
 Taf. 23, Fig. 8 u. 9 wiederzugeben versucht, und zwar stellt Fig. 8 
 ein junges Stadium dar, wahrend Fig. 9 den Bau eines ausgebildeten 
 Ommatidiums erlautert. Der Vergleicb zwiscben beiden Zeichnungen 
 lebrt deutlicb die die Kerne betreffenden Verlagerungen in den Semper- 
 schen Zellen und in den Pigmentzellen 1. Ordnung. Beziiglich der 
 Stellung zur Frage nacb der Einscbicbtigkeit des Ommatidiums hat 
 sicb bei mir eine zwischen Grenacher und Patten vermittelnde An- 
 schauung entwickelt, indem bloss die das Rhabdom bildenden Retinula- 
 zellen der P ATTEN’schen Ansicbt widersprechen, wahrend die tibrigen 
 Zellen die ganze Breite der Epidermis durchziehen. Wir haben also 
 das zusammengesetzte Auge aus einer einschicbtigen Epidermis hervor- 
 gegangen zu betrachten, die in einem grossen Tbeil der Zellen (den 
 SEMPER’schen , den Hauptpigmentzellen , den Pigmentzellen zweiter 
 Ordnung und den Iiaarzellen) aucb einscbicbtig bleibt, wabrend nur 
 im Centrum eines jeden Ommatidiums eine Verkummerung der distalen 
 Abschnitte in je sieben Zellen stattfindet, die ein besonderes Organ, 
 die Retinula, zusammensetzen, so dass hier eine scbeinbare Mehr- 
 scbicbtigkeit gebildet wird. Eine wirkliche Doppelschicbtigkeit konnte 
 nur dann im Ommatidium erblickt werden, wenn die SEMPER’schen 
 Zellen in ibren proximalen Abscbnitten verkummerten und keine an 
 den Seiten der Retinulazellen zur Basalmembran ziehenden Fortsatze 
 lieferten. 
 
 In die scbematischen Figuren habe ich alle in dem zusammen- 
 gesetzten Auge yorkommenden Elemente mit ibren Ausscheiduugs- 
 und Umwandlungsproducten aufgenommen und zwar so, dass Fig. 8, 
 als Schema eines Entwickluugsstadiums, nur die Krystallkegelbildung 
 zeigt, entsprecbend dem auf Taf. 23, Fig. 7 abgebildeten Stadium, 
 wabrend Fig. 9 alle im Auge auftretenden Elemente aufweist, so weit 
 es moglich ist, dieselben auf einem idealen Scbnitt zur Darstellung 
 zu bringen. 
 
 Bei der Beschreibung der weitern Entwicklungsstadien werde icb 
 nicht mehr alle Theile der Ommatidien einer jedesmaligen Besprecbung 
 unterwerfen, sondern, um Wiederholungen zu vermeiden und die Ueber- 
 sichtlichkeit zu erleicbtern, nur die einzelnen wesentlicben Verande- 
 rungen hervorheben. 
 
 Die jetzt zu besprechenden Veranderungen beziehen sich aucb 
 auf die SEMPER’schen Zellen, deren weitere Function in der Aus- 
 scheidung der Krystallkegel und der Cornealinsen bestebt. Beide 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 455 
 
 Vorgange konnte ich von ihren ersten Anfangen an verfolgen, unci 
 zwar tritt die Bildung der Krystallkegel vor dem Beginn eines ersten 
 Sicktbarwerdens der Cornea auf. Die Zellkorper der genannten Zellen 
 zeigen, nachdem die Hautungsharchen verschwunden sind, an ihren 
 distalen Enden, dass sie durch Furchen von einander getrennt sind; 
 das freie Ende jeder Zelle ist namlich in der Mitte etwas nach vorn 
 vorgewolbt. Die Zellgrenzen sind deutlich erkennbar, was fiir die Be- 
 urtheilung der Krystallkegelgenese von Wichtigkeit ist. Die ovalen 
 Kerne liegen in der Mitte der ziemlich langgestreckten, im Laufe der 
 Entwicklung cylindrische Form annehmenden Abschnitte der Semper- 
 schen Zellen, deren proximale Fortsatze uns hier weniger zu be- 
 schaftigen brauchen. Hinter den Kernen dieser Zellen, also proximal 
 von denselben, konnen wir gegen Anfang des 3. Puppentages im Zell- 
 protoplasma ein kleines Kliimpchen einer das Licht starker als das 
 Protoplasma brechenden Substanz, die erste Anlage eines der Krystall- 
 kegelsegmente, bemerken (Taf. 23, Fig. 7). Wahrend die Kerne der 
 SEMPER’schen Zellen sich alle in gleicher Hohe befinden und regel- 
 massig angeordnet sind, zeigen die Abstande der Krystallkegelkliimp- 
 chen von den Kernen Versckiedenheiten. Im Laufe der weitern Ent- 
 wicklung werden nun diese Kliimpchen grosser, und nach 4 — 5-tagiger 
 Puppendauer konnen wir die Krystallkegel als aus vier separaten, noch 
 durch Protoplasma von einander getrennten Segmenten bestehend er- 
 kennen, wahrend in einzelnen Ommatidien dieselben schon zu einem 
 Klumpen verschmolzen sind, an dem man aber noch die Zusammen- 
 setzung aus den vier Theilstiicken nachweisen kann. Die Kegel- 
 kliimpchen nehmen eine ziemlich regelmassige Gestalt an, indem zwei 
 senkrecht auf einander stehende plane Flachen sie von zwei Seiten 
 begrenzen, wahrend die dritte Flache gewolbt ist (Taf. 23, Fig. 10). 
 Mit den planen Flachen sind sie zu einander gekehrt, doch beriihren 
 sich diese Flachen noch nicht, und wir erblicken in Folge dessen auf 
 Querschnitten durch die Kegel in der Mitte derselben ein Kreuz, den 
 optischen Ausdruck fiir die noch nicht erfolgte Aneinanderlagerung 
 der vier Theilstiicke. 
 
 An dem Complex der Kegelsegmente kann man einen aussern 
 homogenen, hellen Abschnitt von einem centralen kornigen unter- 
 scheiden. Letzterer wird durch das Kreuz in 4 Theilstiicke getrennt. 
 Die Trennungslinien werden immer undeutlicher, doch geht die Ver- 
 schmelzung der Theilstiicke nicht gleichzeitig an den vier Beriihrungs- 
 flachen vor sich, sondern man kann haufig Stadien mit einer den Quer- 
 schnitt des Kegels genau halbirenden Trennungslinie nachweisen, bis 
 
456 
 
 HERMANN JOHANSEN, 
 
 sckliesslich in den letzten Puppenstadien die Kegel als vollkomraen 
 einheitliche Gebilde erscheinen. 
 
 Beziiglich der Entstehung der Krystallkegel kann ick mich Cla- 
 parede’s Auffassung (2) nicht anschliessen, die in Weismann (3), 
 Grenacher (11) und andern Autoren Anhanger gefunden hat. Cla- 
 parede halt es fur das Wahrscheinlichere, dass die vier dicht an ein- 
 ander liegenden Zellen in der Mitte aus einander weichen und dass in 
 dem auf diese Weise gebildeten mittlern In ter cellular raum die Krystall- 
 korper als aussere Ausscheidungen entstehen. Wie aus obigen Aus- 
 fiibrungen ersicbtlicb, findet bei Vanessa und Sphinx kein Ausein- 
 anderweicben der SEMPER’schen Zellen statt. Die Krystallkegelgenese 
 ist daher als eine Art innerer Ausscbeidung resp. Umbildung 
 eines Theils des Protoplasmas der SEMPER’scken Zellen aufzufassen. 
 Das im zool. Jahresberickt fur 1879 enthaltene Referat liber die 
 Arbeit von Lowne (8) lasst erkennen, dass Lowne die Krystallkegel 
 (soli wohl beissen: Bildungsmaterial fur die Krystallkegel) aus acbt 
 Zellen zusammengesetzt sein lasst, namlich aus vier vordern oder den 
 SEMPER’schen und aus vier hintern, welche die eben so vielen Kegel- 
 segmente ausscheiden. Zu noch mehr abweichenden Ansicbten iiber 
 die Krystallkegelgenese kommen Kingsley (20) und Patten (19), be- 
 sonders letzterer gemass seiner Auffassung der Einheitlicbkeit des 
 Krystallkegels und des Rhabdoms. Icb beschranke mich darauf, hier 
 auf die in der Literatur anzutreffenden Notizen bloss binzuweisen. 
 Die Autoren haben andere Untersuchungsobjecte gebabt, so dass obne 
 Nacbuntersucbungen an densclben Objecten die sehr widerspreckenden 
 Angaben kaum verwertbet werden konnen. Die Angaben sind so 
 sparlich und dabei so widersprecbend, dass erneute Untersuchungen 
 dringend notbwendig erscheinen. 
 
 Die Entstehung der C o r n e a 1 i n s e n ist auf dieselben vier Zellen 
 zuriickzufubren , docb ist es bier von Anfang an eine einheitliche 
 aussere Ausscheidung, und zwar ist der ganze Complex dieser Zellen 
 an der Ausscbeidung betbeiligt. In der distalen Yertiefung zwiscben 
 den vorgewolbten Enden dieser Zellen siebt man als erstes Stadium 
 eine kleine Menge einer glashellen Substanz ausgeschieden, die nock 
 nicht ausreicht, um die Gruppe der vier Zellen zu bedecken (Fig. 11 
 und 12). Die Ausscbeidung der Linsensubstanz nimmt zu, so dass 
 der Complex der vier Zellen von derselben bedeckt wird, docb stossen 
 die Linsen benachbarter Ommatidien nicht an einander, was erst spater 
 erfolgt, so dass wir dann die Cornealinsen als diinne Cuticularmem- 
 bran den Epidermistheil der Augen iiberziehen seben. Die die erste 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 457 
 
 Anlage der Linse entkaltende Vertiefung auf dem SEMPER’schen Zellen- 
 complex verschwindet dabei immer mehr und geht allmahlich in eine 
 Vorwolbung iiber. Die Kerne der SEMPER’schen Zellen riicken wahrend 
 der Ausscheidung der Cornea und wahrend der Bildung der Kegel 
 immer mehr an die Cornea heran, offenbar in Folge eines bedeutenden 
 Protoplasmaverlustes , den die Zellen wahrend der doppelten Aus- 
 scheidung erleiden. Die Form der Linse geht allmahlich aus der 
 einer fast gleichmassig diinnen Cuticularmembran bei fortschreitender 
 Vorwolbung der sie ausscheidenden Zellen in eine convexconcave iiber, 
 wobei die Convexitat nach aussen gerichtet ist. Sie lasst bald eine 
 Schichtung erkennen, und ihre der Retina zugekehrte Seite wird bei 
 fortschreitender Ablagerung immer ebener, so dass schliesslich eine 
 planconvexe Bildung zu Stande kommt. Die in der Literatur anzu- 
 trelfenden Angaben iiber die Bildung der Linse sind im Allgemeinen 
 iibereinstimmend i ). Es wird die Bildung auf Ausscheidung von Seiten 
 der vier SEMPER’schen Zellen zuriickgefiihrt. Bei den Crustaceen haben 
 wir es aber insofern mit einem abweichenden Bildungsmodus zu thun, 
 als bier nach Angabe der Autoren vier Zellen die Cornealinse aus- 
 scheiden, wahrend vier an der e Zellen mit der Ausscheidung der 
 Krystallkegel beschaftigt sind. Patten (19) fiihrt die Corneabildung 
 bei Vespa auf ein besonderes Zellenlager zuriick, das er sich an der 
 Kegelbildung nicht betheiligen lasst. In Betreff der SEMPER’schen 
 Kerne muss ich noch erwahnen, dass ich deren zuweilen fiinf auf 
 Querschnitten uuter einer Facette zahlte. Ob mit dieser Anomalie 
 auch eine Zusammensetzung der Krystallkegel in den betreffenden 
 Ommatidien aus fiinf Theilstucken verbunden ist, konnte nicht ver- 
 folgt werden. 
 
 Den SEMPER’schen Zellen kann hier die Betrachtung der Haar- 
 zellen angeschlossen werden, da sie, wie jene, an der Bildung der 
 die aussere Oberflache des Auges bedeckenden Cuticularsubstanzen 
 betheiligt sind. Semper (1) hat zuerst die Bildung der Haare und 
 der sonstigen Cuticularbildungen der Lepidopteren erklart und 
 W. Breitenbach (10) gewisse Cuticularhaare naher beschrieben. In 
 jungen Puppen von 1 — 2 Tagen linden wir in der proximalen Kern- 
 
 1) Bloss Weismann (3) ist der Ansicht, dass die SEMPER’schen Kerne 
 nicht vier Zellen angehoren, sondern nur einer; dass der Theilung des 
 Kerns die Theilung des Zelleibes nicht gefolgt sei. Grenacher (1. c. p. 92) 
 erklart Weismann’s Widerspruch, indem er darauf kinweist, dass bei den 
 Musciden ganz ausnahmsweise die Zellgrenzen fehlen. 
 
458 
 
 HERMANN JOHANSEN, 
 
 zone, der Basalmembran anliegend, grosse birnformige Zellen mit im 
 Yerhaltniss zu den der iibrigen Epidermiszellen sehr grossen Kernen, 
 die zwischen den iibrigen Zellen einen Protoplasmafortsatz distalwarts 
 entsenden, der die ganze Breite der Epidermis durchzieht (Fig. 4, 5, 
 
 7, 8, 9, 13, 14). 
 
 Meist sind diese Zellen nicht haufig auf einem Schnitt zu finden 
 und recht unregelmassig zwischen den sich zu den Einzelaugen 
 gruppirenden Zellen vertheilt. Im Laufe der Entwicklung sehen wir 
 nun , dass die Kerne dieser grossen Haarzellen ihre Lage an der 
 Basalmembran aufgeben und im Protoplasma ihrer Zellen distalwarts 
 wandern, wobei das proximale Ende dieser Zellen immer unansehn- 
 licher wird. Gegen Ende der Entwicklung konnen wir die Kerne der- 
 selben in der Hohe der mit ihrer Langsaxe schrag zur Axe des 
 Ommatidiums (Fig. 12) gelagerten Kerne der Hauptpigmentzellen an- 
 treffen; aber schon in fruheren Stadien hat sich der distal zwischen 
 den Ommatidien hindurchtretende Protoplasmafortsatz derselben be- 
 merkbar gemacht, der als feiner Faden tiber die Oberflache der Cornea- 
 facetten , zwischen denselben hindurchtretend , hervorragt. Dieser 
 Protoplasmafaden scheidet nun an seiner Oberflache die Cuticular- 
 substanz des Haares aus, das als gerade, ungefiederte Nadel im Innern 
 einen Canal aufweist, in den sich das Protoplasma der Haarzelle 
 hineinzieht (Fig. 9 und 13). Das Haar wachst durch Neuablagerung 
 von Cuticularsubstanz an seiner Basis. Das Haar ist ein vollkommen 
 einheitliches Gebilde, an dessen Basis man keine Verdickung in Ge- 
 stalt eines Cylinders sehen kann, wie solche von Breitenbach an den 
 auf dem Russel des Lepidopteren stehenden Haaren beschrieben werden 
 und an andern Stellen von Vanessa auch zur Beobachtung kamen. 
 Die Oberflache des Haares ist glatt. 
 
 Die R e t i n u 1 a haben wir auf einem Stadium verlassen, auf dem 
 sie schon die Zusammensetzung aus sieben Zellen aufwies, von denen 
 eine central gelegen ist (cf. Fig. 12 Z). Sie ist also in diesem Stadium 
 ein solider Zellenklumpen, der, wie oben erwahnt, nicht die ganze Breite# 
 der Epidermis durchzieht. Bald wird aber die Gruppirung der Zellen 
 eine andere, indem die sechs peripheren Retinulazellen die centrale 
 Zelle in ihren Yerband aufnehmen, so dass wir nun sieben peripher 
 um eine helle Axe angeordnete Zellen erhalten, welche als erstes 
 Entwicklungsstadium des Rhabdoms aufzufassen ist (Fig. 12). Die 
 Gestalt der Retinula ist im Allgemeinen eine cylindrische, doch ist 
 das distale und proximale Ende derselben verjungt und letzteres zieht 
 sich in altern Stadien in eine diinne Spitze aus, die von der Basal- 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 459 
 
 membran begrenzt wird und mit den unter derselben stehenden Gan- 
 glienzellen durch eine Oeffnung in der Basalmembran in Verbindung 
 tritt (Fig. 13). In der Hoke der Kerne weist die Retinula eine 
 geringe Verdickung auf, die distal- und proximalwarts allmahlich ab- 
 nimmt. Die Kerne der Retinulazellen sind nicht in gleicher Hoke 
 gruppirt, wie aus Fig. 12 und 13 ersicktlick, so dass wir in altera 
 Stadien nie alle Kerne auf einem Querscknitt der Retinula erblicken 
 konnen. In jungeren Stadien ist dieses Yerkalten der Kerne eker zu 
 beobackten, und kier setzten sie kauptsaclilick die mittlere Kernzone 
 der Epidermis zusammen. Im Laufe der weitern Entwicklung riicken 
 nun ein oder zwei Kerne proximalwarts, um an dem basalen Ende 
 der Retinula zu verbleiben und kier eine kleine Ansckwellung der 
 Retinula zu verursacken (Fig. 7, 9 und 13). Ob die diese Kerne 
 entkaltenden Zellen in besonderer Beziehung zu den mit der Retinula 
 verbundenen zwei Ganglienzellen steken, lasst sick sckwer beantworten. 
 Die Zellen betkeiligen sick an der Bildung des Rkabdoms wie die 
 iibrigen Retinulazellen. Auffallend ist jedenfalls die constante Lage- 
 rung von mindestens einem Kern in der Nake dieser Ganglienzellen, 
 und es durfte vielleickt moglick sein, dass die dazu gekorenden Zellen 
 die Yermittlung der Retinula mit den Ganglienzellen ubernommen 
 kaben. Das Hinunterriicken dieser Kerne findet um dieselbe Zeit statt, 
 in welcker jedes Ommatidium an der Basalmembran mit zwei Ganglien- 
 zellen in Yerbindung tritt. 
 
 Riicksicktlick der in der Literatur iiber die Bildung der Retinulae 
 vorkommenden Notizen ist zu erwahnen, dass Grenacher iiber den 
 Bau der Retinula an den Tagsckmetterlingen zu keinem bestimmten 
 Resultat gelangt ist (1. c. p. 98 f.). Lowne (8) fiihrt fur die Be- 
 zeicknung „Retinula“ den uberfliissigen Namen „Facellus“ ein und 
 bekauptet, dass derselbe aus sieben Zellen zusammen gesetzt sei Be- 
 ziiglick der Entstekung des Rkabdoms ist Grenacher zu dem Resultat 
 gekommen, dass dasselbe ein Ausscheidungsproduct der Retinula ist, 
 und zwar „verschmelzen die Stabckensaume sammtlicker Retinulazellen 
 zu einem axialen , sckeinbar einfacken Stab , dem Rkabdom 1 ) 
 
 1) Ick kabe mir erlaubt, Grenacher’s Definition des Rhabdoms 
 wortlick anzufiihren, weil sick in der Literatur eine Unkenntniss dessen 
 bemerkbar gemacht hat, was unter „Rhabdom“ zu verstehen ist. Die 
 Arbeit von Lebedinsky (24) bringt fiber das Facettenauge die interessante 
 Mittkeilung, dass „die ganze Sckickt der Ganglienzellen 14 , 
 die mit den Ommatidien in Verbindung treten, als „rabdom w bezeicknet 
 werde. Wie der Verfasser zu einer derartig abweickenden Auffassung 
 gekommen, ist rathselhaft. 
 
460 
 
 HERMANN JOHANSEN, 
 
 (1. c. p. 77), an dem naan auf Querschnitten zuweilen noch Spuren 
 der Trennungslinien nachweisen kann“. Patten (1. c. p. 200) halt 
 das Rhabdom fur einen Fortsatz des Krystallkegels. 
 
 . Meine Reobachtungen liber das Auftreten des Rhabdoms sind 
 folgende. Nachdem sammtliche Retinulazellen sich peripher angeordnet 
 haben, tritt zwischen denselben eine hellere Plasmamasse auf, die als 
 solche langere Zeit unverandert bestehen bleibt. Gegen Ende der 
 Puppenzeit macht sich an diesem Plasma eine Veranderung bemerkbar, 
 indem es starker lichtbrechend wird und sich gegen das Protoplasma 
 der Retinulazellen scharfer absetzt. Eine Ausscheidung von Seiten 
 der Retinulazellen ist nicht zu bemerken, auch habe ich mit den 
 starksten mir zur Yerfiigung stehenden Systemen keine Trennungs- 
 linien an dem Rhabdom wahrnehmen konnen. Das Rhabdom ist ein 
 einfacher Stab, der an dem der Basalmembran zugekehrten Ende 
 sich plotzlich zu einem kleinen Cylinder erweitert (Fig. 9), wie ein 
 solcher auch bei andern Arthropodenaugen nachgewiesen ist. Eine 
 Fortsetzung des Krystallkegels ist das Rhabdom nicht ; beide sind voll- 
 kommen verschiedene, von einander absolut unabhangige Bildungen. 
 Ich glaube riicksichtlich des Rhabdoms auf Grund meiner Unter- 
 suchungen der Auffassung beistimmen zu mtissen, die von v. Kennel 
 (31, p. 20) auf die Sehstabchen der verschiedensten Augen ausgedehnt 
 ist : dass dieselben keine Cuticularsubstanz sind, sondern ein integriren- 
 der, lebender Theil der Retinulazellen, der durch das Licht in Thatig- 
 keit versetzt wird. „Sie sind zu einseitiger physiologischer Function 
 metamorphosirtes Protoplasma, gerade wie die Muskelsubstanz einer 
 Zelle auch; diese hat in exquisitem Maasse die Fahigkeit der Con- 
 tractilitat, jene die der specifischen Sensibilitat erhalten.“ Da ich 
 das Rhabdom nicht auflosen konnte, bin ich geneigt anzunehmen, dass 
 das Sehplasma der Retinulazellen zu einem einheitlichen Organ, einem 
 Sehplasmastab, zusammentritt, obgleich es immerhin moglich ist, 
 dass bessere Untersuchungsmethoden auch hier sieben einzelne Seh- 
 stabe erkennen lassen werden. Der Krystallkegel ist nicht der licht- 
 empfindliche Apparat, wie es Patten (18) angiebt, sondern hat nur 
 die Lichtstrahlen zu brechen. Die Lage des Pigments am proximalen 
 Ende der Krystallkegel, in besonders starker Anhaufung in den Pigment- 
 zellen 1. Ordnung, weist jedenfalls darauf bin, dass hier der das Om- 
 matidium tretlcnde Lichtstrahl durch das so gebildete Diaphragma 
 auf das Rhabdom zu wirken hat. Die am starksten pigmentirte 
 Stelle im Ommatidium befindet sich am spitzen Ende des Krystall- 
 kegels. 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 461 
 
 Die Mehrzahl der Autoren lasst das Pigment, das mehr oder 
 weniger zum Schluss der Entwicklung in den Augen auftritt, in Ecto- 
 dermzellen sich bilden, also in Zellen, die direct aus der Epidermis 
 hervorgegangen sind. Dieser Anschauung sind Grenacher, Claus, 
 Patten und Kingsley. Sie beschreiben, mit einziger Ausnahme von 
 Claus, ausser den Retinulazellen als pigmentfuhrend noch ganz be- 
 sondere Zellen, die als Pigmentzellen erster (oder Hauptpigmentzellen) 
 und zweiter Ordnung (oder Nebenpigmentzellen) unterschieden werden. 
 Claus konnte (1. c. p. 53) uberhaupt keine besondern Pigmentzellen 
 nachweisen und sieht sich daher gezwungen, anzunehmen, dass die 
 den Retinulae zugehorige Pigmentmasse im Protoplasma der „Stab- 
 zellen u selbst als Molecularsubstanz ausgeschieden wird. Nach An- 
 schauung der meisten Forscher ist das auf diese Weise auftretende 
 Pigment auch zugleich das bleibende, allein Patten stellt (1. c. p. 200) 
 die Behauptung auf, dass das Pigment zuerst als voriibergehende Bil- 
 dung in Gestalt paariger Flecken in der Nahe der Anfangs paarigen 
 „Retinophoren u auftritt und so lange erhalten wird, bis vier Retino- 
 phoren da sind. Er sieht in dieser Bildung Anklange an die Verhalt- 
 nisse der „Ocelli“ der Insecten auf Grundlage des „biogenetischen 
 Grundgesetzes u . Bobretzky (6) und Reichenbach (17) leiten einen 
 Theil der Pigmentzellen vom Mesoderm her. 
 
 Meine eigenen Beobachtungen fiber das Auftreten des Pigments 
 lehren mich Folgendes. In den letzten Tagen vor dem Ausschlupfen 
 der Imago sieht man in der Umgebung des Krystallkegels braunes, 
 korniges Pigment auftreten, und zwar auf der Oberflache der Pigment- 
 zellen 1. Ordnung. Die Retinulazellen lassen auf Querschnitten an 
 der Peripherie schwarze Pigmentkorner erkennen, die Querschnitte 
 von Pigmentfaden, die dem Protoplasma der Retinulae aufliegen, in 
 mehr proximalen Hohen dagegen allmahlich zur Axe des ganzen Ge- 
 bildes riicken und das Rhabdom wie ein schwarzer Ring umgreifen. 
 Im Laufe der weitern Entwicklung tritt nun das Pigment in alien 
 dasselbe fiihrenden Zellen in grosserm Maasse auf. Eine Pigment- 
 bildung unterbleibt bloss in den distalen Enden der SEMPER’schen 
 Zellen, deren Protoplasma durch die Ausscheidung der Krystallkegel 
 und der machtigen Corneafacetten auf ein Minimum reducirt ist, und 
 in den Haarzellen, die vollig unpigmentirt, ihre durch Grosse auf- 
 fallenden Kerne noch in den spatesten Stadien deutlich erkennen lassen. 
 Die Kerne der Pigmentzellen 1. Ordnung sind proximalwarts gewandert, 
 ihre zur Axe des Ommatidiums schrage Lage bewahrend (Fig. 11, 12, 
 13 und 14), und jede Zelle umfasst die Halfte des Krystallkegels. 
 
 Zool. Jahrb. VI. Abth f. Morph. Q() 
 
462 
 
 HERMANN JOHANSEN, 
 
 Auffallig stark pigmentirt sich der am spitzen Ende des Krystallkegels 
 gelegene Theil derselben. Die Pigmentzellen 2. Ordnung lassen ihre 
 Kerne distalwarts wandern, wo man sie in der Hohe der Retinula- 
 zellkerne antrifft (Fig. 12 und 13). Die Form dieser Zellen ist eine 
 mehr oder weniger prismatische ; die Querschnitte derselben sind in 
 den am meisten distal gelegenen Theilen annahernd dreieckig (Taf. 23, 
 Fig. 12), doch geht diese Form in der Mitte und noch mehr im proxi- 
 malen Theil im Laufe der Entwicklung in eine runde iiber. In den 
 Pigmentzellen 2. Ordnung und in den Retinulazellen tritt ein gelblich- 
 brauues Pigment auf, das aber nicht so dunkle Nuancen annimmt 
 wie in den Hauptpigmentzellen und in den distalen Theilen der 
 Pigmentzellen 2. Ordnung dichter ist. Wenn wir uns die Frage vor- 
 legen, woher dieses Pigment stammt, so muss es naturlicher Weise 
 als Ausscheidungsproduct der dasselbe fuhrenden Zellen aufgefasst 
 werden, und damit konnte die Frage nach dem Ursprung des Pigments 
 als erledigt angesehen werden, wenn nicht Erscheinungen in andern 
 Gebieten der pupalen Gewebe, die durch Ganin (7) entdeckt und deren 
 Bedeutung von Metschnikow (14) und Kowalewsky (21) in vollem 
 Maasse erkannt wurde, unsere Aufmerksamkeit auf das Schicksal der 
 Raupenaugen und des in denselben enthaltenen Pigments lenkten. 
 Wir haben gesehen, wie sich die Larvenaugen von der Epidermis 
 losten, und jetzt kann die Betrachtung der Riickbildungen derselben 
 folgen. 
 
 Die Untersuchungen von Pankrath (28) haben unsere Kenntnisse 
 iiber den Bau der Raupenaugen wesentlich gefordert, und die folgenden 
 Bezeichnungen der einzelnen Theile derselben sind diesem Autor ent- 
 lehnt. Am Raupenauge haben wir unter den Zellen und deren Aus- 
 scheidungsproducten folgende Theile zu unterscheiden : die Cornea, 
 den aus drei grossen Zellen zusammengesetzten „Umhullungskorper w , 
 den Krystallkorper und die aus sieben Zellen bestehende Retinula. 
 Nachdem die Raupenaugen sich als ganzer Complex von den Epidermis- 
 zellen getrennt haben, linden wir sie in ihrer Form nicht wesentlich 
 verandert, aber ohne Cornea, die im Zusammenhang mit der iibrigen 
 Chitinbekleidung der Raupe bleibt, dicht unter der Epidermis im 
 Kopfblasenraum , mit ihren basalen Theilen dem Ganglion opticum 
 aufliegend und dorsal und ventral von zwei dem optischen Ganglion 
 entspringenden und zur Epidermis ziehenden Nervenbiindeln begrenzt 
 (Taf. 23, Fig. 1). Die Raupenaugen lassen ihre Bestandtheile mehr 
 oder weniger deutlich erkennen, es ist in ihnen noch keine Veranderung 
 vor sich gegangen. Bald macht sich innerhalb eines jeden Ocellus eine 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 463 
 
 Trennung in zwei Abschnitte bemerkbar. Es tritt ein Spalt auf, dem 
 zu Folge der distale Abscbnitt der Raupenaugen, der aus den fast 
 gleichartig plasmatischen Enden der grossen Zellen des „Umhullungs- 
 korpers“ besteht, mit einem grossen Theil des Pigments dieser Zellen 
 von den tibrigen Theilen abgetrennt wird. Die Trennungslinie geht 
 quer durck den Ocellus, etwas distal von dem Krystallkorper , der 
 durch einen feinen Protoplasmasaum gegen den Spalt abgegrenzt er- 
 scheint. Dieser Spalt wird nun iramer grosser, je mehr sich die dem 
 optischen Ganglion aufsitzenden Theile der Augen mit demselben von 
 der Epidermis bei der Kopfblasenbildung entfernen. Die Ablosung der 
 distalen Theile der Raupenaugen von den proximalen ist als erstes 
 Stadium des Zerfalls der Raupenaugen zu bezeichnen. Das zweite 
 Stadium besteht darin, dass die abgetrennten distalen Theile derselben 
 sich zu rundlichen, grossen Klumpen einer homogenen Grundsubstanz 
 zusammenballen, in welcher betrachtliche Mengen von Pigment ange- 
 hauft sind. Diese protoplasmatischen Pigmentballen bezeichnen bis 
 zum Beginn der Verpuppung den Weg, den die ubrigen Theile der 
 Raupenaugen auf ihrer W anderung durch den Kopfblasenraum zuriick- 
 gelegt haben (Taf. 24, Fig. 15). Die dem Ganglion opticum auf- 
 sitzenden Theile der Raupenaugen lassen wahrend des Processes der 
 Verpuppung keine besondern Veranderungen wahrnehmen. Ihr Ab- 
 stand von der Epidermis wird grosser ; ihre Form bleibt aber dieselbe, 
 mehr oder weniger cylindrische, langgestreckte. Bald nachdem die 
 Raupenhiille abgestreift ist, verschwindet die cylindrische Form des 
 Raupenaugencomplexes immer mehr. Die Augen scheinen in sich 
 selbst zusammenzufallen ; sie erscheinen nicht mehr als ein dem 
 optischen Ganglion aufsitzendes langgestrecktes Gebilde, sondern platten 
 sich immer mehr ab und stellen nur rundliche Vorspriinge auf dem- 
 selben dar, bis schliesslich (Stadium einer Puppe von l 1 / 2 Tagen, 
 Fig. 17 und 22) auch diese Form verloren geht und dieselben gar 
 keine Erhebungen an den Umrissen des Ganglions mehr bilden. Diese 
 Erscheinung hangt einerseits mit den an den Raupenaugen vor sich 
 gehenden Veranderungen zusammen, andrerseits werden die durch die 
 Raupenaugen gebildeten Vorspriinge durch Wachsthumsprocesse im 
 Ganglion selbst und dadurch bedingte Formveranderung ausgeglichen. 
 Die Ansatzstelle der Raupenaugen am Ganglion unterliegt auch mit 
 der Zeit, durch Wachsthumsvorgange in demselben bedingt, einer Ver- 
 lagerung, indem das ganze Ganglion sich immer mehr in seinen vordern 
 Theilen entfaltet, so dass die Raupenaugen reste scheinbar auf dem 
 Ganglion nacli hinten wandern. Die geschilderten Riickbildungen der 
 
 30* 
 
464 
 
 HERMANN JOHANSEN, 
 
 Raupenaugen werden durch die Leukocyten hervorgebracht, die 
 hier als Phagocyten wirken. Sie sind bei der Kopfblasenbildung 
 in grossen Mengen aus der Leibeshohle in dieselbe geratben, und ihnen 
 kommt nacb den Untersuchungen der genannten russischen Forscher 
 eine hohe Bedeutung durch die Zerstorung der larvalen Organe zu. 
 Wir sehen nun zuerst in auffalliger Weise die Thatigkeit der Phago- 
 cyten an den aus den vordern Teilen der Raupenaugen hervorge- 
 gangenen pigmentfuhrenden Protoplasmaballen (Taf. 24, Fig. 16), die 
 nach vollendeter Verpuppung schon von den Phagocyten in kleine 
 Theile zerstiickelt sind und nun im Leibe derselben in verschiedenen 
 Theilen des Kopfblasenraums, meist in der Nahe der Epidermis, anzu- 
 treffen sind. 
 
 Aber nicht nur diese Theile der Raupenaugen werden von den 
 Phagocyten aufgenommen und unterliegen in deren Protoplasma Um- 
 wandlungsprocessen, in Folge deren das Anfangs tiefschwarze Pigment 
 allmahlich in ein braunliches verwandelt wird, sondern auch fur die 
 iibrigen Theile der Raugenaugen gilt die Zerstorung von Seiten der 
 Phagocyten, wenn auch nicht in so auffallender Weise, wie es bei den 
 ganz ungeschiitzten , nach alien Seiten freien Zutritt gewahrenden 
 Protoplasmaballen der Fall ist. Die dem Ganglion opticum aufsitzenden 
 Theile der Raupenaugen sind dorsal und ventral durch die ihnen sich 
 anschmiegenden, dicken Nervenbiindel, die als Imaginaltheile keiner 
 Zerstorung von Seiten der Phagocyten unterliegen, gegen Angriffe 
 von Seiten derselben geschtitzt (Taf. 24, Fig. 15). Mit ihrer Basis 
 sitzen sie unmittelbar dem Ganglion auf, und an ihrer zur Epidermis 
 gewandten Seite sind die Krystallkorper wieder die den Phagocyten 
 die Thatigkeit erschwerenden Momente, so dass es vollkommen ver- 
 standlich erscheint, wenn die dem Ganglion ansitzenden Theile der 
 Raupenaugen eine relativ langere Zeit den Phagocyten Widerstand 
 leisten. Dass die Krystallkorper der Raupenaugen von den Phago- 
 cyten nicht aufgenommen und resorbirt werden, beweist die Thatsache, 
 dass ich dieselben noch im letzten Stadium, im Rindenbeleg des Gan- 
 glions, von einer geringen Menge Pigment umgeben, als letzten Rest 
 der Raupenaugen auffinden konnte. Die Deutung der einzelnen Stadien 
 der Phagocytenthatigkeit fallt nicht schwer, sieht man sie doch un- 
 mittelbar dem zu zerstorenden Gewebstheile , besonders dem uns 
 hier interessirenden Pigment (Taf. 24, Fig. 16 und 17) anliegend, mit 
 pseudopodienartigen Fortsatzen in dasselbe eindringen und darauf mit 
 Theilen desselben in verschiedener Weise beladen im Raum der Kopf- 
 blase herumwandern. Zuweilen nehmen die Phagocyten einen fast 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 465 
 
 ihren ganzen Leib anfullenden Pigraentballen in sich auf, der Anfangs 
 vollkommen einheitlich erscheint, im Laufe der Wanderung zur Epi- 
 dermis aber einen offenbar durch die Thatigkeit der Phagocyte be- 
 wirkten Zerfall in kleinere Pigmentmassen erkennen lasst. Hervor- 
 zuheben ist, dass das Anfangs tiefschwarze Pigment nach dem Zerfall 
 im Innern der Wanderzelle einen immer hellern Farbenton annimmt, 
 bis es vollig mit dem in den proximalen Theilen der Epidermis voruber- 
 gehend auftretenden Pigment iibereinstimmt, so dass schon in Folge 
 dieses Uebereinstimmens in der Farbe des Pigments sich die Ver- 
 muthung aufdrangt, dass das Pigment in beiden Theilen das gleiche 
 ist In dieser Yermuthung werden wir urn so mehr bestarkt, als wir 
 an der Epidermis die mit ganz feinen Kornchen des gelbbraun ge- 
 wordenen Pigments in grosser Menge beladenen Phagocyten in un- 
 mittelbarer Beriihrung mit der Epidermis treten sehen. Ich mochte 
 die Vermuthung aussprechen, dass die Phagocyte nicht bloss das 
 Pigment der Epidermiszelle ubergiebt, sondern selbst von der letztern 
 aufgenommen wird. Die Epidermiszellen vergrossern sich im Laufe 
 der Entwicklung, und es muss ihnen reichliche Nahrung zu diesem 
 Zweck zugefiihrt werden. Ich bin der Ansicht, dass das Wachsthum 
 der Epidermiszellen ausser auf Kosten der aus der Kopfblasenhohle 
 filtrirten Nahrfliissigkeit, die bald resorbirt wird, noch auf Kosten der 
 mit den Trummern der verschiedensten larvalen Organe erfullten 
 Phagocyten vor sich geht, muss dabei aber betonen, dass die pigment- 
 fuhrenden Phagocyten nicht etwa selbst zu Pigmentzellen 1. und 
 2. Ordnung werden. Sie dienen eben nur als Nahrung fur die Epi- 
 dermiszellen, die mit diesen Zellen das in ihnen enthaltene Pigment 
 aufnehmen und allmahlich in das Pigment der Imago uberfiihren. 
 Bei dieser Umbildung wird das Pigment fur einige Zeit, wahrschein- 
 lich in Folge chemischer Umwandlungen in der Substanz des Pigments, 
 unsichtbar, um darauf in den Epidermiszellen der Augen als bleiben- 
 des Pigment ausgeschieden zu werden. Zu bemerken ist, dass das 
 Pigment nicht bloss den sich zum Auge umbildenden Zellen iibergeben 
 wird, sondern der gesammten Epidermis, so dass erstere nicht als 
 ein gesonderter Theil der Epidermis hinsichtlich des Pigments zu be- 
 trachten sind. In beiden Theilen kommt es zur Verwendung, indem 
 es in dem einen Fall zum Pigment der Augen wird, im andern Fall 
 dagegen in das in den gewohnlichen Epidermiszellen der Imago anzu- 
 treffende Pigment umgewandelt wird. Zwischen den Epidermiszellen 
 habe ich im Ganzen recht selten Leukocyten angetroffen. Patten 
 beriicksichtigt in seiner Darstellung der Entwicklungsvorgange der 
 
466 
 
 HERMANN JOHANSEN, 
 
 Wespe die Thatigkeit der Pkagocyten nicht. Eine dem von ihm auf- 
 gefundenen voriibergehenden Pigment entsprechende Bildung habe icb 
 nicht constatiren konnen. 
 
 Das optische Ganglion. 
 
 Die Besprecbung der Entwicklungsvorgange am Ganglion opticum 
 wird uns nicht in so ausfuhrlicher Weise zu beschaftigen haben, wie 
 das bei dem epidermalen Tbeil der Augenanlage der Fall war, weil 
 wir es bei diesem Tbeil der Augen nicht mit einer absoluten Neu- 
 bildung zu tbun haben. Das Centralnerven system der Imago geht be- 
 kanntlich unmittelbar aus dem der Larve hervor, wahrend die Om- 
 matidien eine vollstandige Neubildung sind. Die Eutstehung des 
 Centralnervensystems und somit auch des uns hier beschaftigenden 
 optischen Ganglions fallt in den Bereich der Embryonalentwicklung 
 und kann daher auch gar nicht im Plan dieser Untersuchung liegen, 
 welche es bloss mit den wahrend der Verpuppung vor sich gehenden 
 Veranderungen an den Organen der Larve zu thun hat. Die in der 
 Literatur vorhandenen Angaben iiber das optische Ganglion beziehen 
 sich meist auf seine erste Anlage und haben daher, als die Embryonal- 
 entwicklung beschreibend, kein besonderes Interesse fur uns. 
 
 Bei der Schilderung der Umbildungen, die das Ganglion opticum 
 erfahrt, bediene ich mich zur Bezeichnung der verschiedenen Theile 
 desselben der von Berger (9) eingefiihrten Benennungen, denen zu 
 Folge wir am Ganglion zwei Haupttheile zu unterscheiden haben, „von 
 denen der eine in directer, unzertrennlicher Beziehung zu dem Fa- 
 cettenauge steht und mit der Sehstabschicht desselben zusammen die 
 Retina des Facettenauges bildet, wahrend der andere Theil sich mehr 
 an das Gehirn anschliesst u (p. 36). An der Retina (Berger) konnen 
 wir bei Vanessa urticae dieselben fiinf Schichten unterscheiden, welche 
 Berger bei den verschiedenen Vertretern sowohl tracheater als 
 branchiater Arthropoden constatirt hat, die aber in mannig- 
 faltigster Weise angeordnet sein konnen und nach Berger’s Unter- 
 suchungen nicht einmal bei den Lepidopteren vollstandig iiberein- 
 stimmende Verkaltnisse aufweisen. 
 
 Schon bei der sich zur Verpuppung anschickenden Raupe finden 
 wir die ersten Spuren der Nervenbundelschicht in Gestalt eines 
 dorsal und ventral von den Augen befindlichen und denselben dicht 
 anliegenden Biindels von Nervenfasern, die, aus den peripheren Schichten 
 des optischen Ganglions hervortretend, zu der in nachster Nahe be- 
 findlichen Epidermis zieheu (Fig. 1). Mit der Ablosung der Raupen- 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 467 
 
 augen von der Epidermis sehen wir diese zwei Nervenbiindel frei durch 
 den nun entstandenen machtigen Kopfblasenraum ziehen und an zwei 
 Stellen, einer dors ale n und einer ventralen, mit der Epidermis 
 durch Ganglienzellen in Verbindung treten (Taf. 24, Fig. 15). Zwischen 
 diesen zwei Verbindungsstellen der Epidermis mit dem Ganglion opticum 
 ist dieselbe vollstandig frei von Ansatzstellen der Nervenfasern. Die 
 Ursprungsstellen dieser zwei primitiven Nervenbiindel liegen jederseits 
 dicht neben den auf dem Ganglion sitzenden Raupenaugencomplexen 
 in der Kornerschicht des ganglionaren Theils der Retina, und so lange 
 die Raupenaugencomplexe noch als frei in den Kopfblasenraum vor- 
 springende cylindrische Gebilde zu erkennen sind, liegen die beiden 
 Nervenbiindel in ihrem ersten Yerlauf denselben dicht an, urn erst 
 weiter distal, zu je einem Biindel verbunden bleibend, durch den Kopf- 
 blasenraum zu ziehen und sich in der Nahe der Epidermis in zwei 
 Bezirken, einem dorsalen und einem ventralen, dendritisch zu verzweigen. 
 Nach vorn von der Basis der Raupenaugen gehen die Ursprungsstellen 
 dieser zwei Nervenbiindel in einander iiber, wahrend nach hinten eine 
 derartige Vereinigung derselben nicht wahrzunehmen ist, so dass die 
 Raupenaugen an ihrer Basis von der Austrittsstelle der Nervenbiindel 
 in einem nach hinten offenen Halbkreise umschlossen werden (Taf. 24, 
 Fig. 18). 
 
 Im Laufe der zwei ersten Tage nach der Verpuppung sehen wir 
 im Bereich des Nervenbundellagers die Hauptveranderungen vor sich 
 gehen, als deren Resultat die Anfiillung des Kopfblasenraumes mit den 
 reich verzweigten Theilen der Nervenbiindelschicht erscheint (Taf. 24, 
 Fig. 19). Diese Yeranderungen spielen sich im vordern Theil des 
 optischen Ganglions ab, so dass die Raupenaugen scheinbar immer 
 mehr an den hintern Rand des Ganglions rticken. Gleich vor der Ver- 
 bindungsstelle des ersten Nervenbiindelpaares sehen wir zu Anfang des 
 zweiten Puppentages, dem ersten dicht anliegend, ein zweites Nerven- 
 biindelpaar, aufgetreten, dessen basale Theile gleichfalls mit einander 
 und auch mit den Anfangstheilen des ersten Nervenbiindelpaares in 
 Verbindung stehen, d. h. alle vier Nervenbiindel nehmen ihren Ursprung 
 dicht neben einander im Ganglion opticum, ohne dass dieselben durch 
 dem Ganglion angehorende Zellen von einander geschieden waren. 
 Das zweite Paar besteht gleichfalls aus einem dorsalen und einem 
 ventralen Stamm, die sich an der Epidermis in entsprechenden Regionen 
 reichlich verzweigen *). Diesen ersten Paaren von Nervenbtindeln folgen 
 
 1) Die Erkenntniss dieser Verhaltnisse verdanke ich der Recon- 
 struction von Serien, die ich mit Hlilfe des in der Einleitung erwahnten 
 
468 
 
 HERMANN JOHANSEN, 
 
 nun bald mehrere, und wir sehen dieselben mit ihren Verzweigungen, 
 von denen jede mit der Epidermis in Beriihrung stebt, in kurzer Zeit 
 den Kopfblasenraum erfiillen, wobei zugleich die Umrisse des Ganglion 
 opticum auf seiner Kornerschicht eine Reihe von Vorspriingen auf- 
 weisen, die die Austrittsstellen der Nervenbiindel aus dem „ganglionaren 
 Theil der Retina u sind (Fig. 20). Im vordern Theil des Ganglions 
 liegen die Austrittsstellen der einander entsprechenden dorsalen und 
 ventralen Nervenbiindel ziemlich genahert, wahrend sie nach hinten zu 
 immer mehr aus einander weichen und in den hintersten Regionen des 
 Ganglions, etwa in der Mitte zwischen den dorsalen und ventralen 
 Theilen, den Raupenaugencomplex erkennen lassen. Bis zum Auf- 
 treten von 12 Nervenbiindeln lassen sich diese Vorgange sehr gut 
 verfolgen, wahrend spater die Verhaltnisse weniger deutlich wahrzu- 
 nehmen sind, weil die Verzweigungen in grosser Menge auftreten und 
 auch der Kopfblasenraum durch das grosser werdende Ganglion ein- 
 geengt wird. Die Verzweigung der Nervenbiindel und das Auftreten 
 neuer Paare derselben ist auf eine Spaltung der urspriinglichen zwei 
 ersten Nervenbiindel zuriickzufiihren, und zwar erfolgt die Spaltung 
 in centripetaler Richtung (Taf. 24, Fig. 19), so dass wir an den 
 distalen Theilen der Nervenbiindelschicht immer die meisten Ver- 
 zweigungen antreffen, wahrend die Stamme zum ganglionaren Theil 
 der Retina hin sich immer mehr vereinigen. Wir sehen weder ein 
 Hervorwachsen neuer Nervenbiindel aus der Substanz der Korner- 
 schicht, noch eine centrifugale Bildung von Seitenzweigen an den altera 
 Nervenbiindeln, sondern die ganze complicirte Verzweigung kommt 
 in der Weise zu Stande, dass von der Peripherie zum Centrum hin die 
 urspriinglich verbundenen Nervenbiindel von einander getrennt werden. 
 Indem die dieselben mit den Ommatidien verbindenden Ganglien- 
 zellen sich vermehren, erhalt jede derselben eine Nervenfaser, und von 
 hier aus geht der Zerfall der Nervenbiindel in centripetaler Richtung 
 immer weiter. Auf diese Weise entstehen auch die auf dem Ganglion 
 von einander getrennten Ursprungsstellen der Nervenbiindel, indem 
 die Kornerschicht des Ganglions, wenn eine Zertheilung des Nerven- 
 biindels bis an die Peripherie der Kornerschicht gelangt ist, durch 
 Vermehrung ihrer Elemente die Vereinigungsstelle der Nervenbiindel 
 
 ,, rocking microtome “ angefertigt habe. Mit diesem Instrument allein 
 war es mir moglich, liickenlose Serien durch halbirte Puppenkopfe zu 
 erhalten, die mit der Chitinhiille geschnitten wurden. Eine Entfernung 
 des Chitins in jungen Stadien ist ohne Beschadigung der darunter 
 liegenden Gewebe kaum moglich. 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 469 
 
 aus einander schiebt, so dass dieselben scheinbar einen gesonderten Ur- 
 sprung im Ganglion erhalten, der aber nur durch das eingescbaltete 
 Wacbsthum der Elemente der Kornerschicht bedingt ist. Hier finden 
 wir in mittlern Stadien haufig karyokinetische Figuren. An der Ober- 
 flache der Kornerschicht des Ganglions kann man auf Querschnitten 
 mittlerer Stadien stets eine Zone ohne Austrittsstellen von Nerven- 
 bundeln auffinden, die die Trennung zwischen dorsalen und ventralen 
 Nervenbiindeln angiebt. Diese Grenze hort erst in den letzten Stadien 
 auf, wo die Verzweigung derartig zugenommen hat, dass die Nerven- 
 btindel, dicht an einander gedrangt, aus der Kornerschicht austreten. 
 Die Auflosung der urspriinglich recht starken Nervenbiindel in zahl- 
 reiche von geringerm Durchmesser scheint durch die Phagocyten be- 
 wirkt zu werden, die man an den Verzweigungsstellen haufig erblicken 
 kann (Fig. 21 und 23). Sie resorbiren einerseits das die Nerven- 
 biindel einhiillende Gewebe, das einen dunnen, mit spindelformigen 
 Kernen versehenen Ueberzug derselben bildet, und scheinen andrerseits 
 wieder selbst zu Bindegewebszellen zu werden, indem sie sich ab- 
 platten und die aus der Theilung hervorgegangenen dunnen Nerven 
 umgeben. Dieses durch Phagocyten bewirkte Lostrennen der Biindel 
 von einander wird durch eine Vermehrung der Bindegewebszellen 
 innerhalb eines Biindels vorbereitet, die, in centripetaler Richtung sich 
 vermehrend, in die dicksten Nervenbiindel hineinwandern. 
 
 Patten (19) ist der einzige Autor, der die Entstehung des Nerven- 
 biindellagers aus einem dorsalen und einem ventralen Stamm constatirt 
 hat und dessen Angaben ich bestatigen kann; auf eine ausfiihrlichere 
 Beschreibung der Wachsthumsvorgange des Nervenbiindellagers geht 
 er aber nicht ein. Die dunnen distalen Enden der Nervenbiindel bilden 
 Ganglienzellen, von denen gegen Ende der Entwicklung je zwei 1 ) 
 dicht neben einander unter einer Retinula zu stehen kommen und mit 
 derselben durch feine Fortsatze in Verbindung treten. Auf mittlern 
 Entwicklungsstadien sind diese Ganglienzellen mehr oder weniger un- 
 regelmassig unter der Basalmembran angeordnet. Spater wird aber 
 die Lagerung eine ausserst regelmassige, und die beiden Ganglien- 
 zellen lagern sich derartig dicht an einander, dass sie den Eindruck 
 eines einheitlichen Apparates machen , welcher von dem von der 
 Retinula stammenden Fortsatz wie durchbohrt erscheint (Fig. 23). 
 
 Kurz vor dem Ausschliipfen der Imago ist die Nervenbiindel- 
 schicht durch eine durchlocherte Chitinmembran gegen die Korner- 
 
 1) In meiner vorlaufigen Mittheilung (32) befindet sich einFehler; 
 auch dort muss es so heissen: zwei Ganglienzellen. 
 
470 
 
 HERMANN JOHANSEN, 
 
 schickt des ganglionaren Theils der Retina abgegrenzt. Diese Membran 
 setzt sich in continuo in die das ganze Ganglion opticum und das Gehirn 
 bekleidende chitinige Hiille fort und auf etwas jtingern Stadien auch 
 auf die basalen Theile der Nervenbtindel. Die Nerven treten nicht 
 bloss in feinen Fasern durch diese Membran aus der Kornerschicht 
 hinaus, sondern auch in grossern Biindeln. Die Chitinmembran er- 
 scheint dann in grosserer Ausdehnung unterbrochen. Berger macht 
 darauf aufmerksam, dass bei Pieris brassicae sammtliche Schichten 
 der Retina, mit Ausnahme der Molecularschicht, ein schwarzes Pigment 
 enthalten. Ick konnte an Puppen von Vanessa, die kurz vor dem 
 Ausschliipfen sich befanden, bloss in der Nervenbtindelschicht Pigment 
 nachweisen, wahrend die Korner- und Ganglienzellenschicht ganzlich 
 unpigmentirt sind, so dass ich an Vanessa zu demselben Resultat ge- 
 langt bin, zu dem Berger an Cossus ligniperda und Macroglossa 
 stellatarum kam. Das in der Nervenbundelschicht auftretende Pigment 
 ist genau von derselben Beschaffenheit wie das der Retinulazellen. 
 
 Beziiglich der auf die Nervenbundelschicht folgenden Theile des 
 optischen Ganglions ist zu bemerken, dass dieselben schon in den 
 jiingsten Stadien zu erkennen sind. Das sind die nach Berger als 
 „ein in seinem Baue modificirter Theil des Rindenbelegs“ aufzufassen- 
 den Korner- und Ganglienzellenschichten, die durch die 
 Molecularschicht von einander getrennt sind und zusammen das 
 bildeu, was Berger mit „ganglionarer Theil der Re tin a“ 
 bezeichnet (Fig. 20, 22 und 23). Die Molecularschicht oder das 
 aus sere Marklager besteht bloss aus Nervenfasern ohne einge- 
 lagerte Ganglienzellen, wahrend in der die Hauptmasse des optischen 
 Ganglions bildenden Marksubstanz oder dem innern Marklager 
 Ziige von Ganglienzellen anzutreffen sind (Fig. 22). 
 
 Unter den Kernen der Kornerschicht und der Ganglienzellen- 
 schicht erweisen sich bei genauerer Betrachtung gewisse Kerne als 
 vollig iibereinstimmend mit den Bindegewebskernen, die auf friihern 
 Stadien im Ueberzug der Nervenbtindel anzutreffen waren, als die- 
 selben noch durch den Kopfblasenraum zogen. Solche Kerne findet 
 man noch in den letzten Entwicklungsstadien an den aus der 
 Kornerschicht tretenden Nervenbtindeln (Fig. 23). Das Auffallende an 
 diesen Kernen der Korner- und Ganglienzellenschicht besteht in der 
 abweichenden Lagerung derselben im Vergleich zu den tibrigen Ele- 
 menten dieser Schichten ; sie sind namlich streng in der Richtung des 
 Verlaufs der durch diese Schichten ziehenden Fasern der Nervenbtindel 
 orientirt und begrenzen die Nervenbtindel gegen die Gruppen der 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 471 
 
 Ganglienzellen. Dieses Hineingerathen von Bindegewebszellen in die 
 Elemente des Ganglions erklare ich mir auf die Weise, dass das 
 optische Ganglion bei seinem Wachsthum die Nervenbtindel in sich 
 aufnimmt und sich an den Nervenbtindeln zur Epidermis hin aus- 
 breitet. Dafiir spricht auch die Gruppirung der Zellen in der Korner- 
 schicht und noch mehr in der Ganglienzellenschicbt. Die diese Schichten 
 zusammensetzenden Ganglienzellen vermehren sich in der Richtung 
 zur Peripherie. Die Fasern des Nervenbiindellagers kommen auf diese 
 Weise ins Innere des Ganglions, und nur die denselben ansitzenden 
 Bindegewebszellen weisen noch auf ihr friiheres, freies Verweilen im 
 Kopfblasenraum hin. Die ganze mehr oder weniger radiare Anordnung 
 der Zellen in dem zu den Ommatidien gekehrten Theil des Rinden- 
 belegs des optischen Ganglions ist bedingt durch den Verlauf der 
 Nervenbundel, die diese Schichten durchziehen. Die Zellen dieser 
 Schichten konnen sich nur zwischen den Nervenbtindeln vermehren, 
 und fasst man die Lage der Nervenbtindel als primar auf, so ergiebt 
 sich die Anordnung der Zellen im Rindenbeleg in sehr einfacher Weise. 
 Vanessa urticae weist rticksichtlich des Baues und der gegenseitigen 
 Lagerung der Theile des Ganglion opticum Verhaltnisse auf, die am 
 meisten mit den von Berger an Pieris brassicae constatirten tiberein- 
 stimmen. Auch hier „fallt die Ganglienzellenschicht schon innerhalb 
 der Begrenzungslinie des Rindenbelegs“ (Berger, 1. c. p. 23). 
 
 Nachdem wir nun die am meisten auffallenden Veranderungen am 
 Ganglion opticum, die in den Bereich der pupalen Entwicklungsperiode 
 fallen, besprochen haben, bleibt uns nur tibrig, darauf hinzuweisen, 
 dass die Veran derung in den tibrigen, nach Berger mehr zum Gehirn 
 gehorenden Theilen des optischen Ganglions ziemlich geringftigiger 
 Natur sind. Diese Theile, die als ausseres und inneres Marklager, 
 keilformiges Ganglion etc. bezeichnet werden, konnen schon in frtihen 
 Stadien erkannt werden und lenken die Aufmerksamkeit in keiner 
 Weise auf sich, da dieselben sich nur zu vergrossern scheinen, indem 
 die sie zusammensetzenden Elemente sich vermehren. 
 
 Zum Schluss bin ich die Mittheilung schuldig, dass ich die An- 
 ordnung der Tracheen, die sich im Kopfblasenraum befinden und 
 von dort sowohl in das Gehirn als auch in das Ganglion opticum ein- 
 dringen und auch in Beziehung zu dem epidermalen Theil der Augen 
 treten, nicht verfolgt habe, weil die Untersuchungsmethode nur die 
 starksten Stamme derselben erkennen Hess, wahrend die tibrigen Ver- 
 zweigungen derselben sich vollstandig der Beobachtung entzogen. 
 
472 
 
 HERMANN JOHANSEN, 
 
 Schluss. 
 
 Die Zusammenfassung der Resultate meiner Untersuchung ergiebt 
 Folgendes : 
 
 1) Die Ommatidien gehen aus der einschichtigen 
 Epidermis der Larve hervor. In der Epidermis kommt 
 keine Einstiilpung vor, wie eine solche von Patten an 
 der Wespe beschrieben wird. 
 
 2) Die das Ommatidium zusammensetzenden Zel- 
 len sind: 4 „SEMPER’sche“ Zellen, 7 Retinul azellen, 
 2 Hauptpigmentzellen, 2 Gan glienzellen. Jedes Om- 
 matidium wird von 6 Pigm en tzellen 2. Ordnung um- 
 geben, doch gehoren dieselben zugleich auch benach- 
 barten Ommatidien an. 
 
 3) Die „SEMPEE’schen“ Kerne liegen urspriinglich 
 proximal von den Kernen der Hauptpigmentzellen. 
 
 4) Die „SEMPER’schen w Zellen bilden Hautungshar- 
 chen. 
 
 5) Die Cuticularhaare sind Ausscheidungen beson- 
 derer, zwischen den Ommatidien zerstreut stehender 
 Zellen. 
 
 6) Es lasst sich kein Zellenlager nachweisen, das 
 getrennt von den Kry stallkegelz ellen die Ausscheidung 
 der Cornealinsen ubernimmt. 
 
 7) Die das Ommatidium zusammensetzenden Zellen 
 durchziehen die ganze Lange desselben. Eine Aus- 
 nabme machen die Retinulazellen. 
 
 8) Die Krystallkegel sind urspriinglich innere Aus- 
 scheidungen der „Semper’s ch en“ Zellen. 
 
 9) Das Pigment der Raupenaugen wird durch Wan- 
 derzellen (Phagocyten) den Epidermiszellen iibergeben. 
 
 10) Das Ganglion opticum wachst hauptsachlich in 
 seinen peripheren Theilen, der Nervenbiin del schicht 
 und dem ganglionaren Theil der Retina. 
 
 11) Die Nervenbiindelschicht geht aus zwei pri- 
 mitiven N erv enbiindel n , einem dorsalen und einem 
 ventralen, hervor. 
 
 Indem wir nun die Resultate dieser Arbeit zu allgemeinern Be- 
 trachtungen verwerthen, ist vor allem darauf hinzuweisen, dass die 
 beiden Gruppen der Arthropoden, die branchiaten und die tra- 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 473 
 
 cheaten, sowohl Differenzen im Bau und in der Entwicklung des 
 facettirten Auges erkennen lassen, als andrerseits auch der Bau der 
 Augen im Allgemeinen ein so iibereinstimmender ist, dass es schwer 
 fallt, zu sagen, ob ein Schnitt durch ein ausgebildetes Auge einem 
 Krebs oder einem Insect angehort. Obgleich es in Folge vergleichend- 
 anatomischer und entwicklungsgeschichtlicher Grtinde unmoglich er- 
 scheint, die Tr a cheaten von den Branchiaten abzuleiten, so ist 
 doch andrerseits das Vorkommen von facettirten Augen bei beiden 
 Stammen der Arthropoden eine zum mindesten sehr auffallige Er- 
 scheinung. Wie aber vor kurzem J. v. Kennel betont hat (30), bleibt 
 nichts anderes ubrig, als anzunehmen, dass das zusammengesetzte 
 Auge zweimal in verschiedenen Thierstammen zur Ausbildung ge- 
 kommen ist. Ich glaube nun, dass aus den Befunden, zu denen ich 
 an Vanessa urticae gelangt bin, im Vergleich mit den Angaben der 
 Autoren sich Schliisse ziehen lassen, die die Unabhangigkeit des Auf- 
 tretens der Facettenaugen in beiden Stammen der Arthropoden wahr- 
 scheinlich machen. Vor allem ist darauf hinzuweisen, dass bei den 
 Crustaceen meist ein besonderes Zellenlager auftritt, das die Cornea- 
 linsen ausscheidet, wahrend bei den Tracheaten etwas derartiges nicht 
 vorzukommen scheint und ein und derselbe Zellencomplex, die Semper- 
 schen Zellen, die Ausscheidung der Cornealinsen und der Krystallkegel 
 besorgt. Eine Ausnahme machen die PATTEN’schen Befunde an Vespa , 
 die aber noch einer Bestatigung zu bedlirfen scheinen. 
 
 Ein weiterer Hauptunterschied bezieht sich auf die von verschie- 
 dener Seite constatirten Einstiilpungen, die in dem Stamm der Crusta- 
 ceen ausser der Bildung des optischen Ganglions noch mit der Ent- 
 wicklung des epidermalen Theils der Augen zu thun haben, wahrend 
 bei den Tracheaten derartige Einstiilpungen nur auf die Entwicklung 
 des Ganglions in Embryonalstadien bezogen werden konnen. Die 
 Wespe macht nach Patten’s Untersuchungen hier wiederum eine Aus- 
 nahme, indem eine Faltenbildung innerhalb der Epidermis vor sich 
 gehen soil. Ob dieselbe aber zu einem Verschluss kommt, ist nicht 
 einmal von Patten selbst beobachtet worden, so dass diese Frage 
 noch als offen zu betrachten ist. 
 
 Da die am meisten iibereinstimmenden Formen des zusammen- 
 gesetzten Auges bei den hohern Krebsen und den hohern Insecten 
 vorkommen, wahrend die niedern Formen unter denselben recht ver- 
 schiedenartige Bildungen aufweisen, so ist fur die Ableitung der zu- 
 sammengesetzten Augen zum mindesten ein doppelter Ursprung anzu- 
 nehmen. Das zusammengesetzte Auge der Tracheaten erscheint als 
 
474 
 
 HERMANN JOHANSEN, 
 
 eine Anhaufung von Ocellen, und die Untersuchungen von Pankrath 
 liber die Augen der Raupen- und Phryganidenlarven fiihren diesen 
 Forscber zu derselben Folgerung, indem sich nacb ihm aus den Augen 
 der Raupen leicht die Augen der Phryganidenlarven und aus letztern 
 leicbt das Facettenauge ableiten lassen. Gegen diese Auffassung muss 
 aber der Einwand erhoben werden, dass das Facettenauge als imaginales 
 Organ nicht direct von Larvenaugen abgeleitet werden kann, denn 
 letztere sind selbstandige Bildungen, als Ersatz fur welche die Imago 
 das Facettenauge erhalt. Zur Beurtheilung der pbylogenetischen Ent- 
 wicklung des Facettenauges der Tracheaten konnen nur imaginale 
 Augen in Betrackt kommen, und wir haben in den Myriapoden jeden- 
 falls Formen, die riicksichtlich der Augen wohl am meisten Verhalt- 
 nisse aufweisen, die denen der Vorfahren der hohern Tracheaten am 
 meisten ahnlich sind. Es ist zu bedauern, dass unsere Kenntnisse liber 
 den Bau und die Entwicklung dieser „gehauften Ocellen u noch so un- 
 gemein liickenhafte sind. Jedenfalls erscheint aber soviel als sicher, 
 dass wir hier die Bildung eines immer mehr einheitlich werdenden 
 Organs durch Anhaufung von Einzelaugen als erwiesen betrachten 
 konnen. Das Einzelauge der Vorfahren der hohern Tracheaten denke 
 ich mir dem Auge der bekannten GRENACHER’schen Dytiscus - Larve 
 ahnlich gebaut, nur mit dem Unterschiede, dass die Sehstabchen des- 
 selben, die sog. „Rhabdomer en“, nicht am distalen Ende der Reti- 
 nulazellen standen, sondern schon, wie bei Scorpio , seitliche Um- 
 bildungen des Protoplasmas der Retinulazellen wurden. Aus einer 
 Anhaufung solcher Ocellen ging das Facettenauge hervor, wobei mit 
 der Vermehruug der das zusammengesetzte Auge bildenden Einzel- 
 augen auch zugleich eine Verminderung der das Ommatidium bilden- 
 den Zellen Hand in Hand ging. So blieben nur sieben Retinulazellen 
 nacli, und auch in den Glaskorperzellen machte sich eine Reduction 
 geltend. Aus ihnen gingen die SEMPER’schen Zellen und die Pigment- 
 zellen 1. Ordnung hervor. Die Pigmentzellen 2. Ordnung konnen bei 
 dieser Auffassung sowohl aus Glaskorperzellen ihren Ursprung erhalten 
 haben, als auch einfache Epidermiszellen darstellen. Die auf der 
 Oberflache des facettirten Auges unregelmassig zwischen Gruppen der 
 Facetten (Fig. 24) stehenden Cuticularhaare sind Ausscheidungen von 
 Zellen, die an der Ommatidienbildung nicht tkeilnehmen. Diese Haar- 
 zellen konnen als letzte Reste der urspriinglich in grosserer Anzahl 
 zwischen den Ommatidien stehenden gewoknlichen Epidermiszellen auf- 
 gefasst werden und bestatigen die Anschauung, dass die phylogenetische 
 Entwicklung des Facettenauges der tracheaten Arthropoden auf eine 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 475 
 
 Anhaufung von Einzelaugen zuruckzufuhren ist. Diese Anhaufung ging 
 im Laufe der phylogenetischen Entwicklung in die Bildung eines schein- 
 bar einheitlichen Organs iiber, je mehr die die Einzelaugen oder aus 
 der Umbildung eines Einzelauges hervorgegangene Gruppen derselben 
 von einander trennenden Epidermiszellen nicht mehr zur Anlage kamen. 
 Bei den Crustaceen muss das Facettenauge auch auf einfachere 
 Bildungen zuruckgefuhrt werden ; es tritt hier aber nicht als ein Er- 
 satz fur Larvenorgane erst beim ausgebildeten Thiere auf und erscheint 
 mir nicht aus einer Anhaufung schon vorhandener gleichartiger Augen 
 hervorgegangen, wie das fur das Auge der Tracheaten angenommen 
 werden muss, sondern aus der Umgestaltung und Umbildung eines 
 von Anfang an einheitlichen Organs, in welchem zum Zwecke hoherer 
 Leistungsfahigkeit eine Sonderung in eine Anzahl gleichwerthiger Theile, 
 die Ommatidien, vor sich ging. Um die Bildung dieser Ommatidien 
 schneller zu Stande kommen zu lassen, erscheint besonders bei den 
 hohern Formen der Branchiaten die Bildung der Invagination eiuge- 
 treten zu sein, die ja immer der Ausdruck einer abgekurzten Ent- 
 wicklungsweise ist, wie schon die Gastrula durch Invagination als 
 Abkiirzung der Gastrula durch polare Einwucherung anzusehen ist. 
 Bei den Tracheaten erscheint eine Invagination innerhalb der Epi- 
 dermis vollkommen iiberfliissig zu sein, wenn man die Entstehung des 
 Facettenauges derselben aus der Anhaufung von Einzelaugen annimmt. 
 
 Das Resultat beider Bildungsweisen sind Organe, die im ausge- 
 bildeten Zustand so iibereinstimmende Verhaltnisse zeigen, dass sie 
 als das schonste Beispiel eines convergenten Entwicklungsganges 
 betrachtet werden miissen. 
 
 den 11. Juli 1892. 
 
 Nachtrag bei der Correctur. 
 
 Als das Manuscript schon abgesandt war, erhielt ich den sehr 
 verspateten Zoolog. Jahresbericht fur 1890, der die mir bis 
 dahin entgangene Mittheilung brachte, dass Patten in der Schrift: 
 „Is the ommatidium a hair-bearing sense bud?“ (Anatomischer Anzeiger, 
 5. Jahrg.) verschiedene seiner friiher gemachten An- 
 gaben zuriicknimmt, so besonders die ausserst wichtige, dass 
 Krystallkegel und Rhabdom von denselben Zellen ausgeschieden werden. 
 Ausserdem ist zu verzeichnen, dass Patten „haarartige Vorspriinge“ an 
 den Krystallkegel zellen gefunden hat, die mit den an Vanessa und Sphinx 
 von mir beschriebenen Hautungsharchen ubereinstimmen diirften. 
 
476 
 
 HERMANN JOHANSEN, 
 
 Literatur. 
 
 1) Semper, C., Ueber die Bildung der Flfigel, Scbuppen und Haare 
 bei den Lepidopteren, in: Zeitschr. f. wiss. Zool., Bd. 8, 1857. 
 
 2) ClaparEde, E., Zur Morphologie des znsammengesetzten Auges bei 
 den Artbropoden, in: Zeitschr. f. wiss. Zool., Bd. 10, 1860. 
 
 3) Weismann, A., Die nachembryonale Entwicklung der Musciden, in : 
 Zeitschr. f. wiss. Zool., Bd. 14, 1864. 
 
 4) — — Entwicklung der Dipteren, ibid. Bd. 16, 1866. 
 
 5) Dohrn, A., Untersuchungen fiber Bau und Entwicklung der Arthro- 
 poden, in: Zeitschr. f. wiss. Zool., Bd. 20, 1870. 
 
 6) Bobretzky, N., Entwicklung von Palaemon und Astacus (russisch), 
 Kiew 1873. 
 
 7) Danin, Beitrage zur postembryonalen Entwicklung der Insecten 
 (russisch), Warschau 1876. 
 
 8) Lowne, On the modifications of the simple and compound eyes of 
 insects, London 1878. 
 
 9) Berger, Untersuchungen fiber den Bau des Gehirns und der Retina 
 der Arthropoden, in : Arbeiten Zool. Instit. Wien, Bd. 1, 1878. 
 
 10) Breitenbach, W., Untersuchungen an Schmetterlingsrfisseln, in : 
 Arch. f. Microsc. Anat., Bd. 15, 1878. 
 
 11) Grenacher, H., Untersuchungen fiber das Sehorgan der Arthro- 
 poden, Gottingen 1879. 
 
 12) Reichenbach, H., Die Embry onalanlage und erste Entwicklung des 
 Flusskrebses, in: Zeitschr. f. wiss. Zool., Bd. 29, 1879. 
 
 13) Balfour, Handbuch der vergleichenden Embry ologie, Jena 1881. 
 
 14) Metschnikoff, E., Untersuchungen fiber die intracellulare Verdauung 
 bei wirbellosen Thieren, in: Arb. Zool. Instit. Wien, Bd. 5, 1884. 
 
 15) CarriEre, J., Die Sehorgane der Thiere, Mfinchen und Leipzig 1885. 
 
 16) Claus, C., Untersuchungen fiber die Organisation von Branchipus 
 und Artemia, in: Arb. Zool. Instit. Wien, Bd. 7, 1886. 
 
 17) Reichenbach, H., Studien zur Entwicklungsgeschichte des Fluss- 
 krebses, in: Abhandlungen der Senckenbergischen Naturforschenden 
 Gesellschaft, Bd. 14, Frankfurt a/M. 1886. 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 477 
 
 W8) Patten, W., Eyes of Molluscs and Arthropods, in: Mittheilungen 
 Zool. Stat. Neapel, 1886. 
 
 19) — — Studies on the eyes of Arthropods. Development of the 
 eyes of Vespa, in: Journ. of Morphol., Vol. 1, No. 1, Sept. 1887. 
 
 20) Kingsley, J. S., The development of the compound eye of Crangon, 
 in: Journ. of Morphol., Vol. 1, No. 1, 1887. 
 
 21) Kowalewsky, A., Beitrage zur Kenntniss der nachembryonalen Ent- 
 wicklung der Musciden, in : Zeitschr. f. wiss. Zool., Bd. 45, 1887. 
 
 22) Nusbaum, L’ embryologie de Mysis chamaeleo, in : Arch. Zool. Exper., 
 T. 5, 1887. 
 
 23) Lang, A., Lehrbuch der vergleichenden Anatomie, Jena 1889. 
 
 24) Lebedinsky, J., Untersuchungen iiber die Entwicklungsgeschichte 
 einer Seekrabbe (Eryphia spinifrons), in : Mem. Soc. Natural. Nouv. 
 Buss., Tom. 14, P. 2 (russisch), 1889. 
 
 25) Herrick, E. H., The development of the compound eye of Alpheus, 
 in : Zool. Anzeiger, 1889. 
 
 26) Watase, S., On the migration of the retinal area and its relation 
 to the morphology of the simple ocelli and the compound eyes of 
 Arthropods, in: John Hopkins’ Univer. Circul., Vol. 9, No. 80, 1890. 
 
 27) Parker, Gr. H., The history and development of the eye in the 
 Lobster, in: Bull. Mus. Comp. Zool. Cambridge, 1890. 
 
 28) Pankrath, 0., Das Auge der Baupen und Phryganidenlarven, in: 
 Zeitschr. f. wiss. Zool., Bd. 49, 1890. 
 
 29) Korschelt, E., und Heider, K., Lehrbuch der vergleichenden Ent- 
 wicklungsgeschichte der wirbellosen Thiere, Jena 1891. 
 
 30) v. Kennel, J., Die Verwandtschaftsverhaltnisse der Arthropoden, 
 in: Schriften d. Naturf. Gesellschaft Dorpat, 1891. 
 
 31) — — Die Ableitung der Vertebratenaugen von den Augen der 
 Anneliden, Dorpat 1891. 
 
 32) Johansen, H., Ueber die Entwicklung des Imagoauges von Vanessa, 
 in: Zool. Anzeiger, 15. Jahrg., No. 401, 1892. 
 
 ^ 
 
 Zool. Jahrb. VI. Abth. f. Morph. 
 
 31 
 
478 
 
 Hermann johansen. 
 
 ErklSrung der AbMldungen. 
 
 Fur alle Figuren giiltige Bezeichnungen: 
 
 bm Basalmembran. 
 
 Sz SEMPER’sche Zellen. 
 pz. I Pigmentzellen 1. Ordnung. 
 pz. II Pigmentzellen 2. Ordnung. 
 hz Haarzellen. 
 
 khk Krystallkegelklumpchen. 
 kk Krystallkegel. 
 
 ,ep Epidermis. 
 
 nb Nervenbundel. 
 go Ganglion opticum. 
 rp Kaupenaugen. 
 kbr Kopfblasenraum. 
 rz Petinulazellen. 
 ch Cuticularhaar. 
 ph Phagocyten. 
 
 Tafel 23. 
 
 Fig. 1. Sicb zur Verpuppung anscbickende Raupe, sofort nacb 
 Beginn des Hangens fixirt. Schnitt durch die Epidermis, aus welcher 
 sich ein Baupenauge lost. 88/1. 
 
 Fig. 2. Raupe. Hangezeit 25 Stunden. Schnitt durch die sich 
 in einem centralen Theil lockernde Epidermis der Augenanlage. 210/1. 
 Das Pigment ist etwas zu rothlich wiedergegeben ! 
 
 Fig. 3. Puppe, 15 Stunden alt. Schnitt durch die Augenepi- 
 dermis. 443/1. 
 
 Fig. 4. Puppe, 15 — 18 Stunden alt. Schnitt durch die Augen- 
 epidermis. 270/1. 
 
 Fig. 5. Puppe, 1 Tag 12 Stunden alt. Schnitt durch die Augen- 
 epidermis. 264/1. Beginn der Ommatidienbildung. 
 
 Fig. 6. Sphinx euphorbiae. Junge Puppe. Ein Ommatidium im 
 Langsschnitt. 
 
 Fig. 7. Puppe, 2 Tage 1 Stunde alt. Langsschnitte der Om- 
 matidien mit Hautungsharchen. 640/1. hh Hautungsharchen. 
 
 Fig. 8. Schema eines Ommatidiums in Bildung. hh Hautungs- 
 harchen. 
 
 Fig. 9. Schema eines altera Ommatidiums mit seinen Aus- 
 scheidungsproducten. c Cornealinse ; g Ganglienzelle ; rh Rhabdom. 
 
Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 479 
 
 Pig. 10. Puppe, 6 Tage 13 Stunden alt. Querschnitte der distalen 
 Enden von 5 Ommatidien. 407/1. Zwei Corneafacetten, durch welche 
 die SEMPER’schen Zellen durchschimmern, und drei Ommatidien in der 
 Hohe der Krystallkegelsegmente. 
 
 Fig. 11. Puppe, 3 Tage 12 Stunden alt. Langsschnitte der 
 distalen Enden von drei Ommatidien. 640/1. Bildung der Cornea- 
 linsen c. 
 
 Fig. 12. Puppe, 3 Tage 12 Stunden alt. Querschnitt durch alle 
 Hohen der Ommatidien. 550/1. Cornealinsen c. Eine Retinula zeigt 
 noch die centrale Lagerung eines Kerns z. 
 
 Tafel 24. 
 
 Fig. 13. Puppe, alteres Stadium, noch ohne Pigment. Langs- 
 schnitt durch Ommatidien. 220/1. gz Ganglienzellen. 
 
 Fig. 14. Puppe, 10 Tage alt. Querschnitte durch Ommatidien in 
 der Hohe der proximalen Krystallkegelenden. 400/1. 
 
 Fig. 15. Raupe. Hangezeit 14 Stunden. Schnitt durch die Epi- 
 dermis, das Ganglion opticum, die Raupenaugen und Nervenbiindel. 88/1. 
 An zwei Raupenaugen sind die Krystallkorper sichtbar. pb Pigment- 
 ballen. 
 
 Fig. 16. Raupe. Hangezeit 30 Stunden. Schnitt durch zwei im 
 Kopfblasenraum befindliche Pigmentmassen. 407/1. a Pigmentballen, 
 an welchem die Thatigkeit der Phagocyten eben begonnen ; b distal von 
 a befindliche Pigmentmasse, von den Phagocyten schon bedeutend zer- 
 stuckelt. 
 
 Fig. 17. Puppe, l x / 2 Tage alt. Schnitt durch die dem Ganglion 
 opticum aufsitzenden Raupenaugenreste. Thatigkeit der Phagocyten. 
 400/1. 
 
 Fig. 18. Puppe, 1. Tag. Ansicht des Ganglion opticum mit den 
 zwei ersten Nervenbiindeln, dem dorsalen ( d ) und dem ventralen (v). 
 a vorn; b hinten. Reconstruction. Zeichnung von A. v. Stieren. 
 
 Fig. 19. Puppe, 2 Tage 7 Stunden alt. Dicker Schnitt zur De- 
 monstration der Verzweigung eines Nervenbiindels. 88/1. x ein Zweig 
 vereinigt sich mit dem Hauptstamm nicht in der Ebene des Schnittes. 
 
 Fig. 20. Puppe, 3V 2 Tage alt. Schnitt durch das Ganglion 
 opticum. 88/1. „ Ganglionarer Theil der Retina w (Berger), nb basale 
 
 Theile der Nervenbiindelschicht; hs Kornerschicht ; ms Molecularschicht ; 
 gs Ganglienzellenschicht ; rb Rindenbeleg des Ganglion opticum ; mb Mark- 
 substanz desselben. 
 
 Fig. 21. Puppe, 2 Tage 16 Stunden alt. Schnitt durch eine Ver- 
 zweigungsstelle eines Nervenbiindels. 264/1. Bei x ist das Nerven- 
 biindel schrag angeschnitten ; sb spindelformige Bindegewebskerne. 
 
 Fig. 22. Puppe, l 1 / 2 Tage alt. Querschnitt des Gehirns und 
 des Ganglion opticum in der Gegend der Raupenaugenreste. 88/1. 
 
 31 * 
 
480 HERMANN JOHANSEN, Die Entwicklung des Imagoauges von Vanessa urticae L. 
 
 oe Oesophagus der Imago, umgeben von Zerfallproducten des larvalen 
 Oesophagus und Phagocyten; cm Commissur; gh Ganglienzellen und 
 Fasersuhstanz des Gehirns ; rg angeschnittene Kerne von Riesenganglien- 
 zellen ; mk Marksubstanz des optischen Ganglions ; rb Rindenbeleg des- 
 selben ; ks Kornerschicht ; ms Molecularschicht oder ausseres Marklager; 
 gs Ganglienzellenschicht ; ZZ angeschnittene Theile des Rindenbelegs. 
 
 Fig. 23. Puppe, 6 Tage 13 Stunden alt. Schnitt durch den 
 ganglionaren Theil der Retina mit austretendem und sich mit den Om- 
 matidien in Verbindung setzendem Nervenbiindel. 264/1. Bezeichnung 
 der Theile wie in Fig. 22 und 21. gz paarweise unter den Ommatidien 
 stehende Ganglienzellen. 
 
 Fig. 24. Puppe kurz vor dem Ausschlupfen der Imago. Cornea- 
 facetten mit den Querschnitten der basalen Theile der Cuticularhaare. 
 264/1. Rand des Auges. cs gewohnliche Chitinhulle des Kopfes. 
 

 the library 
 
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Zoolog. Jahrbucker Bd 6 Abth. f ' . Morph . 
 
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THE LIBRARY 
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THE LIBRARY 
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 A 
 
 
 
 
 \ 
 
862 
 
 The Arthropod Eye . 
 
 [October, 
 
 {American Naturalist Extra , October , 1886.) 
 
 THE ARTHROPOD EYE. 
 
 BY J. S. KINGSLEY, SC.D. 
 
 T HE year 1886 has already seen several important studies 
 upon the eyes of Arthropods, some of which have mate- 
 rially altered our conception of the organ and of compound vision. 
 Of these studies by far the most important are those detailed 
 in Dr. William Patten’s “ Eyes of Mollusks and Arthropods ” 
 (Mitth. zool. Stat. Neapel, vi, pp. 542-756. pis. xxvm-xxxii, 
 1886). The paper is far too long for complete abstract here, 
 but some of the more important points relative to the compound 
 eye may be useful in supplementing the statements in the manuals 
 of comparative anatomy. Incidentally it may be remarked that 
 the matter pertaining to the eyes of mollusks is equally valuable. 
 
 As described in our hand-books, our knowledge of the eyes of 
 Crustacea, spiders and insects is based on Grenacher’s classic “ Se- 
 horgan der Arthropoden ” (1879), and no one (Graber excepted) 
 has ventured to criticise his results. Not so Dr. Patten. He has 
 shown that Grenacher is wrong in many fundamental points, and 
 that his conception of these organs is in some respects so erroneous 
 as to be all but worthless. The writer, in passing, may remark that 
 he has had occasion, in studies in a somewhat different direction, 
 to verify many of Patten’s statements, and so far as he has gone 
 he can confirm them. The figure of one of the elements of the 
 eye of the shrimp, illustrating this article, is drawn from his own 
 preparations, but in all essential features it agrees well with simi- 
 lar figures of the eyes of other Crustacea given by Patten. The 
 
The Arthropod Eye. 
 
 1886.] 
 
 863 
 
 ramifications of the distal end of the optic nerve were not seen 
 
 and have been inserted from Pat- 
 ten. 
 
 On the external surface of the 
 compound eye is the facetted 
 chitinous cornea, each facet of 
 which is regarded as a lens (/). 
 Immediately beneath this comes 
 a layer of epidermal (hypoder- 
 mal) cells, e , the existence of 
 which was utterly ignored by 
 Grenacher. These secrete the 
 cornea. Next in order are some 
 cells (retinophorae, r) which are 
 rather complex in their structure 
 and relations. There are four of 
 these to each facet, and they lie 
 exactly below the correspond- 
 ing epidermal cells. The nuclei 
 are placed in the outer ends, and 
 thence the protoplasm runs back- 
 wards to the basal limit of the ret- 
 inal portion of the eye marked by 
 the line at b in the figure. A short 
 distance from the surface they 
 contract to form a slender stalk 
 or style (.f), and these enlarge, in 
 a graceful manner, to form a ped- 
 icel (/). 
 
 Between the nucleated por- 
 tion of the retinophorae and the 
 style are some features existing 
 in the shrimp and some other 
 forms, but not in all. This part 
 of the cells in the form figured is 
 extremely thin, and the space be- 
 tween the four cells which make 
 up one optic element, or ‘ om- 
 matidium,’ is occupied by a trans- 
 parent body, the crystalline cone 
 {c). Below the cone the retin- 
 
864 
 
 The Arthropod Eye . 
 
 [October, 
 
 ophorae are slightly enlarged. It is, however, with regard to the 
 pedicel that one of Dr. Patten’s most important discoveries were 
 made. According to Grenacher, this part (his rhabdom) is 
 secreted by the surrounding pigment cells, but Dr. Patten claims 
 that it is in reality but the coalesced proximal ends of the four 
 retinophorae. 
 
 Surrounding the portions of the eye already described are a 
 varying number of pigment cells (about sixteen in Crangon); 
 around the nucleus these are large, but at either extremity they 
 thin out into fine threads or rods which, according to Dr. Patten, 
 extend, like the retinophorae, from the epidermis to the basal 
 membrane. I have not been able to trace these extensions except 
 in part, and hence have omitted them in the drawing. It seems 
 probable that a variation occurs in these rods with the Species, 
 though Dr. Patten (p. 637) is inclined to the contrary view. 
 
 At the base of the ommatidium a large nerve fiber ( a ) is seen 
 coming from the deeper portion to the base of the pedicel. Ac- 
 cording to Patten this divides just before reaching the basal mem- 
 brane, and gives off branches to each of the cells (pigment cells, 
 retinophorae) composing the optic element. And farther, branches 
 go to adjacent elements so that each ommatidium receives its 
 nerve supply from four different main bundles. The arrangement 
 of these is very complex, and need not be described. The fiber, 
 however, which penetrates the axis of the pedicel seems of more 
 importance to a conception of the phenomenon of vision, and 
 hence a word is necessary. It runs through the pedicel and 
 style and penetrates' the crystalline cone, where it gives off fine 
 fibrillae which radiate in every direction towards the outer wall. 
 These are points which seem to have escaped all previous ob- 
 servers. The writer has traced the axial fiber into the cone, but 
 has not seen the other details. 
 
 Of the various theories and conclusions advanced by Mr. Pat- 
 ten as results of his studies, we have room to mention but three : 
 First, the existence of the radiating fibers in the crystalline cone 
 (or in the retinophorae when the cone is absent) shows that at 
 that part of the eye the image is formed, and the suggestion at 
 once follows that by the depth of this layer of fibers there is 
 adequate compensation for lack of adjustment, for no matter 
 where the image produced by the lens may fall, it will fall upon 
 fibers of the nerves, So too these observations tend to throw 
 
1 886.] 
 
 The Arthropod Eye. 
 
 865 
 
 discredit on the “mosaic” theory of the vision of the compound 
 eye, a theory that already was too open to objection to be im- 
 plicitly accepted. Lastly, a point to be referred to again, Dr. 
 Patten comes to the conclusion that the compound eye can not 
 be regarded as evolved by a coalescence of ocelli. 
 
 A second important paper on the Arthropod eye has already 
 been mentioned in the pages of this magazine, but its connection 
 with the subject in hand will excuse its being brought up again. 
 Though several authors have mentioned facts in the development 
 of the arthropod eye — some, like Bobretzky, giving details of 
 importance — Mr. Locy was the first to indicate the most import- 
 ant feature in the process. In his paper entitled “ Observations 
 on the development of Agelena neevia ” (Bulletin Mus. Comp. 
 Zook, xii, pp. 63-103, 12 pis., 1886), he shows that in an early 
 stage the eyes appear as local thickenings of the epidermis fol- 
 lowed by an invagination of these thickened portions which thus 
 come to lie beneath the surface. The pouches thus formed, one 
 for each eye, are then cut off from the parent layer, and we have 
 now to deal with three layers. From the outer (epidermal) arises 
 the cuticle, cuticular lens and vitreous body of the adult ; from 
 the middle arises the retinal elements of the adult (exactly how 
 was not clearly determined) while the fate of the inner layer 
 was not traced. Locy points out that as a result of this mode of 
 development, one supposed difference between the eyes of artho- 
 pods and those of vertebrates disappears, and the rays of light 
 traverse the retinal elements of the one group in exactly the same 
 direction with regard to their origin as they do in the other. 
 
 Sedgwick (Quart. Jour. Micros. Sci., xxv, 1886) was the first 
 to point out that the eye in Peripatus was developed from an in- 
 vagination. The outer wall of this sac is described as forming 
 the epithelium outside the lens of the adult eye, while the inner 
 wall joins the cerebrum and gives rise to the retina. Hence, says 
 Sedgwick, the eye of Peripatus is a cerebral eye. 
 
 Kennel has also studied the development of the eye in the 
 same genus (Entwicklungsgeschichte von Peripatus, 11 Theil. 
 Arb. z. z. Inst. Wurzburg, vm, pp. 1-93, pis. 6, 1886), and gives 
 (pp. 31-33) further details. He too recognizes the invagination, 
 which becomes cut off from the parent layer, but says that its 
 inner wall has no close connection with the rudimentary brain, 
 but that the nervous connection with that organ is secondary. 
 
866 
 
 The Arthropod Eye. 
 
 [October, 
 
 This is the most important difference from Sedgwick’s very brief 
 account. Certainly Kennel’s figures do not support the view that 
 we have here to do with a ‘cerebral eye ;’ but it must be borne 
 in mind that he and Sedgwick are studying different species. 
 
 Next comes a preliminary communication on the development 
 of the ocelli of Hymenoptera, by Carriere (Zoologischer Anzei- 
 ger, ix, pp. 496-500, 1886). Here the optic epidermis becomes 
 at first two cells deep, and then these become obliquely invagi- 
 nated, both layers retaining their “ normal ” position. The outer 
 layer forms the lens-generating cells, the inner the retina-form- 
 ing ones. The cells which are not invaginated become elon- 
 gate and, together with the invaginated lens-building cells, form 
 the corneal lens. These cells never lose their connection with 
 the epidermis while those of the retina do. The cavity of in- 
 vagination does not close up but is occupied by the corneal lens. 
 Thus runs Carriere’s account ; but it is very difficult to under- 
 stand it, as it is not illustrated. 
 
 I have now a few observations of my own to record. In Cran- 
 gon the eyes arise from invaginated pits, and here as in the spi- 
 der, Agelena, we have three layers to deal with. These are the 
 unmodified epidermal layer and the two walls of the invaginated 
 pouch. The one of these latter which comes to be the more external 
 I have termed the retinogen because from it arises the retinal ele- 
 ments; the other, from analogous reasons, is the gangliogen. So 
 far the account is closely similar to that of Locy as outlined 
 above. I have, however, been able to trace the development of 
 all the parts of the adult eye, which in outline is as follows. The 
 cells of the gangliogen elongate and each divides, giving rise to a 
 row or series of cells which grow upwards toward the mathemati- 
 cal center of the eye and produce the chain of ganglia and nerve 
 fibers which lie in the stalk of the adult eye. In a somewhat 
 similar way the cells of the retinogen elongate and divide trans- 
 versely, each giving origin to five cells which also lie in a radius 
 of the eye. Of these the outer forms the retinophora of the 
 figure above while the others develop into the pigment cells. 
 All of the structures embraced in the bracket rg in the figure are 
 hence derived from the retinogen, while from g inward until con- 
 nection is made with the cerebral portions all is of gangliogenous 
 origin ; the space between the two indicates the position of the 
 cavity of the invaginated pouch. In the adult it is filled with 
 
1 886.] 
 
 The Arthropod Eye. 
 
 867 
 
 pigment, connective-tissue and nerve fibers. The latter grow out 
 from the ganglion, the others are of mesoblastic origin, and force 
 their way into the cavity at about the time when the cells of its 
 walls begin to elongate. The crystalline cone is plainly formed 
 by the walls of the retinophora and these same cells can also be 
 seen to elongate and unite to form the pedicel, thus clearly dem- 
 onstrating the truth of Patten’s position and the error in Gren- 
 adier's conception of the rhabdom. I hope soon to publish a 
 detailed account of my results with figures which will make clear 
 all the points indicated above. 
 
 This development of the compound eye from a single invagi- 
 nated pit shows conclusively that this organ could not have 
 arisen from a confluence of ocelli, but must have had its origin 
 from the division of a simple eye in some respects like that of a 
 spider. Farther, the close correspondence observable in the 
 development of the eye of a spider and that of a crustacean, as 
 outlined above, and the difference of both from that of Peripatus 
 and the ocelli of Hexapods, go far toward sustaining the position 
 I took last year (Inter-relationships of Arthropods, this journal, 
 xix, pp. 560-567 ; and Embryology of Limulus, Quart. Jour. 
 Micros. Sci., xxv, pp. 521-576) that the group Tracheata is not 
 a natural one, and that the spiders are far more closely related to 
 the Crustacea than they are to the Hexapods, with which they 
 are usually associated. 
 
 At the same time the structure of the eye in the adult Peri- 
 patus does not at least conflict with another point I suggested in 
 the paper in the American Naturalist just quoted, i. e., that 
 Peripatus, in spite of its tracheae, is not an Arthropod at all. To 
 be sure it arises by an invagination, but so does that of cephalo- 
 pods and, in a modified way, those of vertebrates. We know 
 almost nothing of the development of the eyes of other groups, 
 but the almost perfect similarity shown between the eye of the 
 adult Peripatus as figured by Balfour and that of the syllid worm 
 Autolytus as it is seen in my own preparations — a similarity ex- 
 tending to almost every detail — renders it a not very rash step to 
 predict that invagination will be found to play a part in the devel- 
 opment of the annelid eye as well. 
 
Bulletin of the Museum of Comparative Zoology, 
 
 AT HARVARD COLLEGE. 
 
 VOL. XIII. No. 8. 
 
 SIMPLE EYES IN ARTHROPODS. 
 
 By E. L. Mark. 
 
 With Five Plates. 
 
 CAMBRIDGE: 
 
 PRINTED FOR THE MUSEUxM. 
 February, 1887. 
 
No. 3. — Simple Eyes in Arthropods. By E. L. Mark* 
 
 That portion of Mr. Locy’s paper on the development of the spider + 
 which deals with the formation of the eye appears to possess importance 
 outside the objects of his special study. The discussion of the bearings 
 of his discoveries on the simple or monomeniscous eyes of Arthropods in 
 general, is the object of the present paper. 
 
 Two irreconcilable views have been held of late with regard to the 
 origin of the retina in the simple eyes of Arthropods. The writers upon 
 the subject have been pretty evenly divided in opinion. Grenadier, 
 Lankester and Bourne, and Carriere have all claimed, more or less dis- 
 tinctly, that the retina was derived from the “ hypodermis ; ” while Gra- 
 ber, Lowne, and Schimkewitsch have clearly inclined to the opinion that 
 it was an outgrowth from the cephalic ganglia. It has been impossible 
 for either party to establish its views beyond debate, on account of the 
 absence of the proper embryological information. 
 
 Grenadier (’79, p. 158) claims the ectodermic (hypodermal) origin of 
 the retina as duly established for certain cases, but admits that, although 
 highly probable, a similar origin has not been proved for eyes of the type 
 to which those of the spiders belong. He says : “ Her zweite Punkt auf 
 den es wesentlich ankommt, betrifft die Iierkunft des Retinaelementes. 
 Was diese anbelangt, so haben wir auch einige Beispiele kennen gelernt, 
 die uns in einer Weise, wie kaum etwas zu wunschen ubrig lasst, diese 
 Abstammung klarlegen ; nur schade, dass sich so wenige andere anftigen 
 lassen. Mit den ersteren meine ich die Augen der Schwimmkaferlarven 
 (Figg. 1-10, Taf. I), die uns so evident als moglich nicht nur die Ab- 
 hangigkeit des Retinaelementes, sondern auch aller ubrigen Augentheile 
 von dem Integument, der Hypodermis mit Cuticula, erkennen lassen. 
 Damit ist aber fur diese Thiere auch zugleich die Abstammung des Reti- 
 naelementes vom ersten, aus^eren embryonalen Keimblatt, dem Ectoderm, 
 gegeben. 
 
 “ Nicht so giinstig steht es mit den ubrigen Formen von Larvenaugen, 
 sowie den einfachen Augen der Spinnen und Insectenimagines. Wenn 
 
 * Contributions from the Zoological Laboratory of the Museum of Comparative 
 Zoology at Harvard College. Ho. XI. 
 
 + Wm. A. Locy, Observations on the Development of Agelena ncevia , Bull. Mus. 
 Comp. Zool., Vol. XII, No. 3, pp. 63-103, 12 pi., Jan., 1886. 
 
 VOL. XIII. — no. 3. 4 
 
50 
 
 BULLETIN OF THE 
 
 auch iiber die Herkunft einzelner Augentheile, liber die Abstammung der- 
 selben von der Hypodermis, namentlich bei den erstgenannten beiden 
 Catagorien, kein Zweifel obwalten kann, so ist doch hier die Retina in 
 den von mir untersnchten Zustanden ausser alter Continuitat mit ihr 
 und jenen Augentheilen, und der erforderliche strenge Nachweis dieses 
 jedenfalls hochst wahrscheinlichen ursprunglichen Zusammenhanges ist 
 erst noch zu fiihren.” 
 
 In his last paper on this subject Grenacher (’80, p. 430) reiterates his 
 inability to solve the problem, when he says : “ The genesis of the two- 
 layer 4 Stemma ’ out of the hypodermis, to which the conclusions from 
 analogy point, is still entirely obscure to me also, and is only to be made 
 out by direct observation.” 
 
 Lankester and Bourne (’83) have expressed their opinion on the 
 origin of the retina either in an incidental way or with a certain amount 
 of reserve. I have not hesitated to class them on this side of the ques- 
 tion, however, since they evidently incline in this direction. Of the lat- 
 eral eyes in scorpions they say (p. 182): “Both nerve-end cells and 
 indifferent cells of the lateral ommateum * apparently belong to the epi- 
 blastic layer, and are shut off together with the layer of hypodermis cells 
 from the subjacent connective tissue by a well-marked 1 basement mem- 
 brane,’ which in the region of the ommateum may be called the eye-cap- 
 sule, or, better, the 4 ommateal capsule.’ ” In this connection it should 
 be borne in mind that these lateral eyes are claimed by them to be 
 mwostichous.f They believe (p. 211), however, that “a few examples 
 clearly transitional between the monostichous and the diplostichous con- 
 dition have been described by Grenacher (among Myriapods).” There- 
 fore by inference their supposed diplostichous (in reality triplostichous) 
 condition must likewise have had both its layers derived from the hypo- 
 dermis. The difficulties in the way of this transition from monostichous 
 to so-called diplostichous eyes do not seem to have impressed themselves 
 so forcibly upon these observers as they did upon Grenacher, who, not- 
 withstanding his familiarity with the facts, confessed, as we have seen, 
 that the double-layer condition presented a still unsolved problem. 
 
 Finally, they have expressed J more precisely, although incidentally, 
 the conviction that the retina in the central (“ diplostichous ”) eyes of 
 the scorpions is of hypodermic origin ; but they have nowhere offered an 
 
 * “All the soft tissues of an arthropod eye, as distinguished from the cuticular 
 lens,” they call “ommateum.” 
 
 t “ An ommateum consisting of a single layer of cells.” 
 t See pp. 56, 57. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 51 
 
 explanation of the method of its formation other than that implied in the 
 allusion to Grenadier's researches on Myriapoda. 
 
 Carriere (’85, p. 178), basing his conclusions principally upon the ap- 
 pearances presented by the 4 stemma ’ in the pupa of an ant, believes that 
 it is derived from the hypodermis by, — first, a lenticular thickening of 
 the hypodermis produced by an elongation of the hypodermis cells ; and, 
 secondly, by the rearrangement of the latter into two layers, one above 
 the other, of which the outer remains in continuity with the permanent 
 hypodermis and constitutes the “ vitreous body,” while the inner is trans- 
 formed into the retina. The method by which this rearrangement is ac- 
 complished is to be learned a little farther on (p. 189), where he says : 
 “ According to my interpretation, therefore, the simple eye (Napfauge) 
 and the compound eye (Facherauge) of the Arthropoda are organs which 
 arise out of like components in a similar manner (through splitting of the 
 hypodermis into two layers), but in their further development diverge 
 from each other in two opposite directions.” 
 
 While the authors just quoted agree in believing the retina to be an 
 immediate derivative from the hypodermis, those cited below are at least 
 so far in agreement as to hold that the retina is not developed directly 
 from that layer. 
 
 Graber’s objection to the view that the retina is derived from the 
 •hypodermis was based principally upon its total separation from the 
 hypodermis and its derivatives (pigment-cells and “ vitreous body ”) by 
 means of the so-called pre-retinal septum or lamella * discovered by him. 
 Combating Grenacher’s conclusions, Graber (’79, p. 66) says : If really 
 the pigment-cells were directly continuous with the retinal cells, as 
 Grenacher’s Fig. 31 (Vespa) makes them, then there would be an unin- 
 terrupted transition from retina to hypodermis, and consequently the 
 typical two-layer “ stemma ” could be considered as only a modification 
 of the apparently one-layer eye of the Dytiscus larva. “Eine solche 
 directe Verbindung der Retina,” he adds, “ mit den das Auge umsaumen- 
 den Integumentzellen existirt aber nicht ; Hypodermis, Pigment- und 
 Krystallkorperzellen einerseits und Retina anderseits bilden vielmehr je 
 ein geschlossenes Ganzes fur sich, indem sich eben zwischen beiden 
 Straten unser praretinales Septum durch und durch zieht, und so vielleicht 
 auch fur die Zulassiglceit der Grenadier 1 schen Tlieorie bezilglich des hypo- 
 dermalen (wir sagen nicht ectodermatischen) Ursprungs der Arthropoden- 
 Retina eine schwer zu uberwindende Schranhe bildet In his resume 
 of the principal results of his paper Graber (p. 88) gives as the second 
 result : “Die Retina des Stemma ist in ihrer ganzen Ausdehnung durch 
 
 * I shall in the future refer to this structure as the “pre-retinal membrane .” 
 
52 
 
 BULLETIN OF THE 
 
 eine besondere cuticulare mit der Sclera zusammenhangende Zwischenla- 
 melle (praretinales Septum) vom integumentalen Epithel (Hypodermis, 
 “ Pigment-” und “ Glaskorper-” zellen) abgesondert. — Dies spricht (vom 
 rein topographischen Standpunct aus) fiir die Ausschliessung derselben 
 von der Hypodermis.” 
 
 Graber’s belief in the derivation of the retina from the nervous system 
 rather than from the hypodermis is still more emphatically expressed in 
 his subsequent paper on the eyes of Chsetopoda (Graber, 79 b , pp. 280 and 
 310), where he says of the retina of Alciope : “ Dieser [axis] Faden 
 spricht aber auch am meisten dafiir, dass die primaren Eetinazellen, resp. 
 deren mehrkernige Dilferenzirungsgebilde, oder die secundaren Eetina- 
 pallisaden, niclit direct von der ausseren (integumentalen) Zelllage, son- 
 dern aus der inneren (secundaren) Anlage des Nervensystems sich 
 ausbildet.” Concerning the eyes of Chaetopods in general, he adds 
 (p. 310) : “Der Augapfel als Ganzes besitzt keine eigne Umhiillung 
 (Sclerotica im Sinne der Wirbelthiere und Cephalopoden), wohl aber 
 kommt eine solche dem Eetinabecher zu, der als ein selbststandiger 
 Abschnitt vom allgemeinen oder integumentalen Theile abgelost werden 
 kann. Diese Eetina-Hulle ist eine diinne Cuticula und erweist sich topo- 
 graphisch als ein gestielter blasenartiger Anhang der Hirnkapsel.” He 
 says further (p. 312) : “ Die Eetina ist in ihrer gesammten Ausdehnung 
 nichts Anderes als die Endausbreitung des Sehnervs,” etc. 
 
 In the first published abstract of his paper on the structure and func- 
 tions of the eyes of Arthropoda, Lowne (’83, p. 142) claims that his 
 “Dioptron” of the compound eye apparently corresponds to the cornea, 
 the vitreous, and the fibrous membrane (Graber’s pre-retinal lamella) of 
 the simple ocellus. By implication the “Neuron” in the compound eye 
 and that in the simple eye are therefore also homologous. Concerning 
 the origin of these two parts he says (p. 142) : “ All the structures of the 
 Dioptron are developed from the cellular Hypoderm, whilst all the struc- 
 tures of the Neuron are formed from a solid papilla, or from a number 
 of papilloe which are outgrowths from the Cephalic Ganglia, so that in 
 this respect there is ground for a morphological comparison of the Dioptron 
 with the dioptric structures, and of the Neuron with the nervous structures 
 of the eye of a Vertebrate.” 
 
 In the paper as ultimately published by Lowne (’84), it is evident that 
 his conclusions relative to the origin of the “Neuron” from the cephalic 
 ganglia are based, so far as his own observations go, upon the develop- 
 ment of the compound eyes ; so that he only leaves it to be inferred that, 
 in his opinion, it has in the simple eyes the same origin. It is in that 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 53 
 
 sense, at least, that I understand the tenor of his criticism (p. 415) of 
 Grenacher’s belief : “ At present the origin of the retina of the simple eye 
 cannot be said to have been determined ; I have sought in vain for any 
 reliable indications as to its origin. Dr. Grenacher believes it to arise 
 by a modification of the cells of the hypoderm. His arguments in favor 
 of this origin are very unsatisfactory, and apparently indicate that the 
 vitreous, and not the retinal elements , arise from this layer.” 
 
 The conclusions reached by Schimkewitsch (’84) place him also with 
 those who regard the retina as an outgrowth of the cephalic ganglia. He 
 says (p. 10) : “According to my observations, the eye of Epeira and of 
 other spiders may be divided into two quite distinct parts : one part we 
 call epithelial, the other part retinal or neural. The first embraces a lens 
 and a vitreous body, and is separated from the second by a pre-retinal 
 membrane. The retinal part is formed by a collection of terminations of 
 the fibres of the optic nerve ; each termination is formed by an enlarge- 
 ment of the fibre, which supports, in the case of Epeira, a double bacillus 
 and nuclei. The two parts [epithelial and retinal] are enveloped by a 
 membrane — a prolongation of the neurilemma of the optic nerve — which 
 merges into ( se confond) the subcutaneous connective layer and the pre- 
 retinal membrane (lame)” At p. 14 of the same paper he adds : “ The 
 existence of a pre-retinal membrane is an argument — and such is also 
 the opinion of Graber — in favor of the development of the retina at the 
 expense of a neural rudiment, and not at the expense of an epithelial 
 redupiicature, as Grenacher supposes.* Besides that, we have the very 
 important observations of Bobretzky, who shows that the retina of the 
 compound eyes of the crayfish is certainly developed at the expense of the 
 neural rudiment.” His general conclusion on this matter is summarized 
 in the following words : “ Les couches epitheliales et mesodermiques 
 prennent aussi part a la formation des yeux, comme cela a lieu chez les 
 Yertebres.” 
 
 In his more recent paper on the embryology of spiders, Schimkewitsch 
 (’84 a ; does not deal with the origin of the eyes. 
 
 The answers to the questions concerning the source of the retina and 
 the method of its formation, now furnished by Locy, seem adequate to 
 
 * A part of the argument implied in the above quotations from Schimkewitsch 
 does not appear directly from the quotations themselves, hut rests upon his interpre- 
 tation of “the membrane which merges with the pre-retinal membrane and with the 
 so-called subcutaneous layer.” These three structures are, in his opinion, connective 
 tissue, and therefore of mesodermic origin. 
 
 Further along in the present paper this view will be discussed, and an explana- 
 tion will be offered of what seems to be the cause of the author’s apparent error. 
 
54 
 
 BULLETIN OF THE 
 
 settle these conflicting views, — so far, at least, as regards the eyes of the 
 spider-like type. While the formation of the retina from the epiblast, 
 independently of the cephalic ganglia, determines the controversy in favor 
 of those who have maintained its hypodermal origin, the method by 
 which it is formed shows that none of his predecessors have in the least 
 foreseen the true course of events. 
 
 He has discovered that in both types of retina exhibited by spiders 
 the retinal part of the eye is formed by an infolding. In the anterior 
 median eyes of Agelena * — and probably the same is true in all spiders’ 
 eyes which fall under the class called by Graber £>os£-bacillar, — this in- 
 folding gives rise to a pocket which is ultimately detached t from the 
 hypodermis. The two walls of the pocket soon come into contact, so 
 that this infolded, detached portion of the eye is composed of two layers. 
 The layers are of unequal thickness ; and -while one of them — the thinner 
 and deeper — remains normal, the other, by the process of infolding, be- 
 comes inverted. The cells of the thick, inverted layer are developed into 
 retinal cells. The bacilli are formed at what were originally the deep 
 ends of the ectoderm cells (Figs. 1, 8, 10, 20-22. Compare Locy, l. c. 
 PI. x.), and therefore in the inverted condition of the layer are in front 
 of the retinal nuclei.^ In the course of the involution the outer or thick 
 wall of the pocket becomes applied directly to the deep surface of that 
 portion of the ectoderm which lies immediately behind the infolding. 
 This region of the ectoderm is meantime being converted into a so-called 
 vitreous body. 
 
 The inversion of the retina proper is a fact of broader significance than 
 would at first appear, and it affords a satisfactory explanation of some 
 of the points in the anatomy and histology of simple eyes which have 
 been so earnestly discussed during the past few years. 
 
 After Grenacher (’79) it is especially Lankester and Bourne (’83) who 
 have emphasized the differences between what the latter authors have 
 named monostichous and diplostichous ommatea ; but how far they still 
 were from a full appreciation of the real differences is to be gathered both 
 from the name employed — c&pfostichous for an ommateum composed of 
 at least three originally distinct layers — and from the statement that 
 Grenacher had shown in Myriapoda stages intermediate between mono- 
 stichous and (their so-called) diplostichous conditions. From the latter 
 
 * The conditions in the remaining eyes of Agelena are described and discussed on 
 pp. 75 and 94. 
 
 t Compare footnote, p. 66. 
 
 J It seems to me more appropriate to refer the position of the bacilli to that of 
 the nuclei, rather than vice versa ; and I shall therefore speak of the two types of eye 9 
 as pre- and post -nuclear, instead of post- and pre-bacillar as Graber has done. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 55 
 
 we must infer, it seems to me, that their explanation is equivalent to say- 
 ing that the “ diplostichous ” condition has arisen by a gradual sinking 
 down of the retinal area, and a subsequent closing in of the adjacent 
 epiblast to constitute the outer layer of the ommateum. The funda- 
 mental difference between such a method and that shown by Locy to 
 exist in spiders is, that, according to the former assumption, there is no 
 inversion of the retinal area, whereas in spiders there is a complete inver- 
 sion of the more superficial of the two infolded layers. 
 
 It must be left to future observers to ascertain whether any of the 
 monomeniscous eyes of Arthropods are, as seems possible, actually formed 
 in the manner suggested by the condition in the Myriapods ; i. e., without 
 the inversion of the retinal area. Meanwhile one examines with fresh 
 interest the conditions hitherto described in order to ascertain, if may 
 be, the probable outcome of future studies. 
 
 Next in importance to the presence of two distinct cell-layers,* the 
 presence or absence of Graber’s pre-retinal membrane will be significant. 
 In all cases where there is an obvious pre-retinal membrane, and when 
 the “ vitreous ” is composed of a layer of cells which abut directly (per- 
 pendicularly) upon it, I believe there can be little doubt that the retina 
 has been formed by a process of inversion. Such I think is the case in 
 the eyes of all the Arachnoids hitherto carefully studied. 
 
 The cases among Arachnoids which will at first sight present the 
 greatest obstacle to the acceptance of this view are those of the scor- 
 pions ; it is therefore to these that most attention will be given. 
 
 Graber has given figures and descriptions of the median eyes in scor- 
 pions, which have been reviewed both by Grenadier (’80, pp. 421-425) 
 and by Lankester and Bourne (’83, pp. 191-193). Their criticisms deal 
 especially with the nuclear conditions of Graber’s “ Betinaschlauche.” 
 His “ parietal pigment- and matrix-zone of the retina ” was not reviewed 
 by Grenacher, but is considered at some length by his later critics, under 
 the head of “ Intrusive pigmentary connective tissue.” 
 
 * The presence of the third or posterior layer is unquestionably of the greatest im- 
 portance as a test of an invagination with inversion ; but I believe that it may be so 
 reduced in thickness in the adult that the negative evidence of its not having been 
 hitherto found in any particular case should not weigh too heavily in the interpre- 
 tation. I find, for example, in the case of some adults (Tegeneria, Theridium, 
 Thomisus) that the posterior layer is indicated only by the presence of very thin, 
 flattened nuclei, sometimes so densely enveloped in pigment-granules as to be 
 almost unrecognizable, but occurring at such regular intervals as to leave little doubt 
 about their real nature. 
 
56 
 
 BULLETIN OF THE 
 
 Graber ( ? 79, pp. 84, 85, Figg. 13, 14) found that in the median eyes 
 of Buthus there was left, after the action of caustic potash had made 
 the central portions of the sections paler, a rose-colored granular rim or 
 marginal zone, and that in this zone were to be seen a few, mostly in- 
 distinct, nuclei and markings perpendicular to the sclera, which together 
 might serve at first sight to suggest the presence of a tall cylindrical 
 epithelial sclera-matrix. This view Graber definitely puts aside, how- 
 ever, and concludes that the appearance is due to the oblique direction 
 of the section, the apparent epithelium being only the cut-off (anterior) 
 ends of “ Retinaschlauche.” But inasmuch as there are no other sub- 
 cuticular (subscleral)’ structures, these “retinal sacs” have assumed, in 
 his opinion, notwithstanding their other functions, the rdle of matrix- 
 cells. 
 
 Even without our present knowledge of the manner in which similar 
 eyes arise, this interpretation would be unsatisfactory, because the mar- 
 ginal zone is most sharply marked off from the retina in the 'posterior half 
 of the ball of the eye, and it would be difficult to imagine the course of 
 retinal cells which in this region could be so cut as to give rise to the 
 appearance figured. But I do not doubt the accuracy of the figure in 
 question (Graber’s Fig. 14), and believe that its interpretation becomes 
 easy when considered in connection with the probable origin of the re- 
 tina. If the median eye in Buthus was formed by an involution with 
 inversion of the retina, Graber’s “ Matrixzone ” would be the posterior 
 layer of that infolding, and its gradually merging into the retinal layer 
 in the anterior half of the ball of the eye would be entirely parallel to 
 what occurs in the formation of the “ pre-nuclear ” eyes in spiders. 
 
 Lankester and Bourne (’83) have also had under consideration this 
 pigment- and matrix-zone of Graber, and have arrived at conclusions 
 which are entirely new. It will be most satisfactory to quote their own 
 words upon what they call “ intrusive pigmentary connective tissue : ” 
 “ The structures which we consider as intrusive connective tissue in the 
 central eyes of the Scorpion may be compared to the interneural cells 
 of the lateral eyes. Like these, they are pigmentiferous, and serve 
 to fill up the spaces between the several nerve-end cells and between 
 these and the ommateal capsule. But whilst we regard the interneural 
 cells as ectodermal in origin, ... we find reasons for considering the 
 intracapsular pigmentary connective tissue of the central eyes of Scor- 
 pions as derived from mesoblast, and of the nature of connective tissue. 
 
 “ We have not embryological evidence for this conclusion, and depend 
 entirely upon the branching, inosculating character of the pigmentary 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 57 
 
 cells, and upon the analogy of the pigment-cells surrounding the reti- 
 nulse of the polymeniscous eyes of Insects and Crustacea, which are 
 very generally held to be of the nature of connective tissue, as also upon 
 that of the ‘ packing-tissue ’ to be described below in the central eye of 
 Limulus. 
 
 “We are by no means anxious to maintain that the more epithelium- 
 like cells amongst what we are about to describe as ‘ intrusive intracap- 
 sular connective tissue ’ may not be of distinct origin from other portions 
 of this pigmentiferous framework, and referable to interneural cells of 
 ectodermal nature ; but any such distinctions must be based upon em- 
 bryological facts which we do not possess. In the present state of 
 knowledge it seems most convenient and justifiable to hold that in the 
 central eyes of the Scorpions there are no interneural cells of ectodermal 
 origin, as there are in the lateral eyes, and that their place is taken by 
 intrusive connective tissue” (pp. 191, 192). 
 
 I believe the authors will agree with me that Locy has now furnished 
 the embryological facts which, by a fair use of reasoning from analogy, 
 will allow us to affirm with considerable certainty that at least their 
 “ epithelium-like cells ” (or, as they have in another place called them, 
 “ intracapsular pavement ” cells) are not intrusive , but are derived from 
 the ectoderm, — not, it is true, in so simple a manner as one might have 
 imagined by merely comparing them with the conditions (interneural cells) 
 which they have found in the lateral eyes. There is this fundamental 
 difference between their conceptions and those which are now presented 
 to us : in their view the “ intracapsular pavement ” cells, even if shown 
 by embryology to be derived from the ectoderm, would still be essen- 
 tially interneural cells ; i. e ., such as were originally interspersed among the 
 retinal cells (compare their explanation to Fig. 7). But in the present 
 aspect of the case that is not probable ; they are distinctly not comparable 
 with the interneural cells of the lateral eyes, — assuming that the latter 
 are “ monostichous,” * — but belong to an extra-retinal region of the 
 ectoderm. 
 
 What they are functionally, is to be inferred from their pigmented con- 
 dition. Their position indicates that they are, in addition, the matrix for 
 that portion of the basement membrane which has received the name 
 “ sclera.” 
 
 Whether the “ intracapsular epithelium ” represents the whole of the 
 posterior layer of the infolding, is a question which is intimately con- 
 
 * Whether Lankester and Bourne are right in claiming the lateral eyes of scorpions 
 to be “ monostichous ,” is quite another question, which will be discussed presently. 
 
58 
 
 BULLETIN OF THE 
 
 nected with the author’s theory of an intrusive (mesoblastic) connective 
 tissue. 
 
 At least three possibilities may be suggested to explain the inter-reti- 
 nular pigment-cells discovered by Lankester and Bourne in the central 
 eyes of scorpions : (1) They may be developed from indifferent hypo- 
 dermal cells practically in situ ; (2) they may be cells which have been 
 detached from the posterior layer of a retinal involution, and have grown 
 in between the retinuke from behind ; or (3) they may be, as claimed 
 by the authors, intrusive mesodermal cells. 
 
 If the lateral eyes are really “ monostichous,” that would seem to afford 
 an argument in favor of the first possibility, the interneural cells of the 
 lateral eyes being really pigment-cells developed in situ ; and in that 
 case the “ inter-retinular pigment-cells ” of the central eyes would cor- 
 respond to the interneural cells of the lateral eyes. 
 
 The above-quoted arguments (pp. 56, 57) in favor of the third possi- 
 bility do not seem to me to outweigh the fact that it is the liypodermis 
 and its derivatives which have in Arthropods the greatest tendency to 
 the pigmented condition. 
 
 Finally, the intimate connection between the other pigmented cells 
 and the “ intracapsular epithelium ” would be favorable to the second 
 view, — at least I cannot regard the intrusion between the retinal ele- 
 ments of pigment-cells from this source (posterior layer of the involution) 
 as any less probable than their migration through the “ ommateal cap- 
 sule ” and the intracapsular epithelium.* 
 
 No one, however, will think of arriving at a conclusive answer to 
 this question by other means than a careful histogenetic study of the 
 developing eyes of some of the scorpions. 
 
 So far, then, as regards the median (central) eyes of scorpions, they do 
 not present conditions sufficiently different from those of spiders to pre- 
 vent a similar interpretation of their parts. With the lateral eyes, how- 
 ever, the case is quite different. If the recent researches of Lankester 
 
 * There are other indications, besides that of a triplostichous condition, which 
 point to the probability of an involution of hypodermis as a source for all the post- 
 vitreous portions of the ommateum. In the scorpions, as well as in the spiders, the 
 emergence of the optic-nerve fibres is so eccentric (especially in Androctonus) that 
 one might almost venture to predict even the place and the direction of the invagina- 
 tion. (See theoretical considerations, below, pp. 91, 92.) 
 
 Perhaps MetschnikofF (71, p. 225, Taf. 16, Figg. 10, 11) was very near to discover- 
 ing the true relation of the eyes to the hypodermis when he explained that they 
 appeared as thickenings of the dermal fold which forms an overgrowth over the 
 cephalic ganglia. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 59 
 
 and Bourne are to be accepted, it would appear that the lateral eyes pre- 
 sent a much simpler type than the median eyes, — so far, at least, as 
 regards the relation of the retinal layer to the hypodermis, the point upon 
 which the interpretation essentially turns. 
 
 It is of importance in the consideration of this question that in neither of 
 their figures (Lankester and Bourne, Figs. 2, 3, 4) are the “ interneural ” 
 cells represented as reaching to the cuticular lens. They form a layer, 
 — uninterrupted except by the narrow nerve-fibre prolongations of the 
 retinal cells, — the individual elements of which are wedged in between 
 the posterior ends only of the cells composing the retina. Nothing in this 
 relation stands in the way of these interneural cells being directly com- 
 pared with the posterior layer of the retinal infolding in spiders’ eyes. 
 The only serious obstacle to a direct comparison with triplosticlious eyes 
 is the absence of a true “ vitreous.” 
 
 The authors affirm with great positiveness the entire absence of the 
 vitreous layer. There are two considerations which make it appear to me 
 possible that Graber in figuring that layer may not have been so grossly 
 in error as they claim. There are great differences in the thickness of the 
 “ vitreous ” in the adult eyes of different Arthropods. (Compare Gren- 
 adier, ’79, Figg. 28 and 31.) It is possible either that a very thin layer 
 of cells may have been overlooked by Lankester and Bourne, or that, 
 after secreting the substance of the cuticular lens, the “ vitreous ” cells 
 are in the adult crowded to the margin or completely obliterated. 
 
 If, then, it should happen from any cause whatever ( e . g. the extreme 
 thinness of the layer, or its prompt degeneration and disappearance after 
 secreting the lens) that the “ vitreous body ” had escaped the attention 
 of these authors, as suggested by Lowne (’84, p. 416), then one might 
 readily conceive that the lateral eyes of scorpions were formed on practi- 
 cally the same plan as the median eyes of the scorpion and the pre- 
 nuclear eyes of spiders. In that event the cells called by Lankester 
 and Bourne “interneural” would doubtless represent the posterior of 
 the infolded layers.* 
 
 Although Graber (’79, Fig. 4) has given a figure of the lateral eye 
 (Scorpio europaeus) which in some respects is much less satisfactory than 
 
 * If this were the case (compare Lankester and Bourne, op. cit., “Explanation of 
 the small italics in all figures ” and explanations of Figs. 7 and 8), the question 
 raised by the authors — whether the “ pigmentiferous cells” (pp) within the retinal 
 capsule of the central eye were equivalent to the “interneural epithelial cells” ( gg ) 
 of the lateral eyes, or were “ intracapsular (intrusive) connective tissue” — would 
 be answered in favor of the former of the two possibilities. 
 
60 
 
 BULLETIN OF THE 
 
 those of Lankester and Bourne, and although he has given no definite 
 description of a sclera-matrix in these eyes, yet one may fairly infer 
 (cf. 1. c., p. 77) his belief in such a matrix, and can find in his figure 
 (left side) indications of nuclear structures which easily admit of such an 
 interpretation. These (sclera-matrix 1) cells I consider to be, in any 
 event, the equivalent of what Lankester and Bourne have described as 
 “ interneural epithelial cells,” the nature of which, it will he observed 
 from their figures (Figs. 2, 3) and descriptions, differs considerably in 
 Euscorpius and Androctonus. 
 
 But in addition to the considerations presented by Lankester and 
 Bourne, there is another objection to the interpretation here proposed, 
 which at present I am not able to explain. The direct and apparently 
 primitive manner in which the retinal cells are continued into the nerve 
 fibres seems to point to a normal rather than an inverted condition of 
 the retina. 
 
 In either event, the nature of the lateral eyes in scorpions is deserving 
 of further study ; and it will not be surprising if it is found that they 
 arise by a process of infolding accompanied by inversion of the retina. 
 
 Grenacher (’78) has given a figure of an ocellus in one of the Phalan- 
 gidce which indicates the presence of a distinct layer of cells (“ vitreous ”) 
 in front of the retina ; and although he has not seen anything of a layer 
 behind the retina, these eyes present no more serious obstacle to an origin 
 by involution than do most of the hitherto published figures of the eyes 
 of spiders. 
 
 The conditions in the eyes of Myriapoda leave more room for doubt. 
 Graber, Grenacher, and Sograff are the only authors who have recently 
 given them any considerable attention. 
 
 The eyes in Myriapods — aside from Scutigera, in which they are of 
 a conspicuously different type — are apparently either monostichous 
 (Chilognatha) or so-called diplostichous (Chilopoda). The latter evi- 
 dently approach more nearly the conditions found in Arachnoidea, and 
 will be considered first. 
 
 Graber (’79, p. 59) claimed their substantial agreement with the ocelli 
 of the Arachnoids and Hexapods. While Grenacher’s subsequent work 
 has made much of Graber’s description appear illusory, there are still 
 some points in which it is probable that Graber has given reliable presen- 
 tations of the histological structure. There is, at least, one thing in 
 which I believe his observations deserving of more attention than they 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 61 
 
 have hitherto received. He has especially defended the cuticular inter- 
 pretation of -the “ sclera/’ and in connection therewith has urged the ex- 
 istence of a cuticular matrix. The nuclei of this matrix he has very 
 distinctly, and I am inclined to think very truthfully, figured (Fig. 18, k) 
 and described (pp. 64, 84) for Scolopendra. Even Grenacher (’80, p. 441) 
 has granted a conditional assent to their presence, although maintaining 
 that he did not feel entirely convinced.* 
 
 To anticipate a conclusion, the grounds of which will he presented 
 later, — in connection with a discussion of the nature of the pre-retinal 
 membrane, — I may say here that the existence of a distinct cell-layer 
 posterior to the retina, and inside the cuticular “ sclera,” appears to me a 
 strong argument in favor of the view that the retina in the Scolopendridse 
 has been formed by an involution with inversion. If Graber had realized 
 the probable identity of these posterior cell-layers in Myriapods and 
 scorpions, it is .possible he might have been saved the expression of his 
 sixth conclusion : “ The ends of the retinal sacs [cells] appear to form, 
 
 at least in part, the matrix of the sclera.” 
 
 There is a very palpable difference between the figures of the u vitre- 
 ous” by Graber, and the figures and descriptions by Grenacher (’80, 
 p. 434) ; nor is there any room to doubt that Grenadier’s work is, in 
 most particulars, incomparably the more satisfactory and reliable. But 
 Grenacher finds, if not a layer of uniformly fashioned cells, at least in 
 some individuals of one species (Branchiostoma) a vitreous composed 
 of an uninterrupted layer of cells, which differ from the vitreous cells 
 of spiders, for example, only in the more central position of their nuclei, 
 and the inclination of their axes towards (deep ends away from) the axis 
 of the eye. This exceptional condition of the vitreous — found only in 
 a few individuals — Grenacher brings into relation with the fact that the 
 lens in these cases was only partially developed, and deduces the con- 
 clusion that these animals had recently suffered a moulting, and that the 
 increased thickness of the hypodermis and vitreous is simply evidence of 
 increased functional activity. He recognizes the difficulty in the way of 
 
 * According to Grenacher (’80, Fig. 8) the pigment-cells which invest the eye 
 have the character of a continuous epithelium such as the posterior layer of the re- 
 tinal infolding in spiders does at an early stage ; but their relation to the thick strati- 
 fied cuticula (viz. outside the latter) forbids a comparison. If Grenadier's account is 
 correct, the Myriapods stand quite alone in having such a continuous mesodermic 
 investment of the eyes. 
 
 Compare also Sograff (’80), PI. Ill, fig. 17, where a nearly continuous layer of 
 cells is represented outside the thick cuticula of the eye, but inside only isolated 
 nuclei scattered among the nerve-fibres which occupy the space between the cuti- 
 cula and the basal ends of the retinal cells. 
 
62 
 
 BULLETIN OF THE 
 
 reconciling this condition of affairs with the typical one-layer condition 
 of the ommateum ; he seems to consider it, however, as only a phase in 
 the process of formation, which is insufficient to decide whether the eye 
 is to he regarded as a one-layer or a two-layer structure ; for he says : 
 “ Which of these two conditions, which are alternately realized in the 
 various phases of the life of the individual, shall we assume as the pri- 
 mary, in order to refer to it the other condition (a thing which presents 
 in itself no difficulty) ? Here, I believe, the observation of the first rudi- 
 ment of the development can alone give a reliable answer ; I at least feel 
 incapable of deciding solely upon the hitherto accumulated facts.” 
 
 It might have been unwise for Grenacher, and it may be even now 
 rash for one to hazard a conjecture as to which was the primary condi- 
 tion ; but in view of what is now known about spiders’ eyes, I think the 
 evidence favors the conclusion that the exceptional cases present the 
 more primitive condition. One or the other of two things is likely to 
 have taken place, — either the retina was formed by an involution which 
 allowed the “ vitreous ” to be from the first a continuous cell-area, or the 
 retina resulted from a depression of the hypodermis, followed by a ring- 
 like ingrowth of vitreous cells from the margins of the depression. The 
 obliquity of the axes of the vitreous cells, as seen in the finished eye, 
 might suggest the probability of simple ingrowth ; but in these excep- 
 tional growing eyes, the continuity of the layer, its nearly uniform thick- 
 ness, and the very slight oliquity of the central cells, while not absolutely 
 incompatible with such an origin, appear to me more favorable to the 
 supposition of a primitively uninterrupted vitreous layer. There is still 
 a wide difference between the one-layer condition figured by Grenacher 
 for Dytiscus larvae, and the completed eye of Scolopendra. If, as seems 
 probable, Grenacher is right in supposing the exceptional individuals of 
 Branchiostoma to have been engaged at the time of capture in the con- 
 struction of lenses, the lateral displacement of the vitreous cells had 
 probably only just begun ; but even when completed, the “ vitreous ” 
 and retina still continue to form two essentially distinct cell-layers. 
 
 Graber has claimed the existence of a pre-re tinal membrane in Myria- 
 pods ; but Grenacher asserts that he assigned to it an impossible position. 
 It is true Graber has not carefully described, nor very precisely repre- 
 sented it ; but I fail to understand how it was possible for Grenacher to 
 speak of it as located in an impossible place. However inaccurately 
 Graber may have described the cell-layers which constitute “ vitreous ” 
 and retina, they certainly are in contact, even according to Grenadier’s 
 own description ; and it is along this region of contact that I understand 
 Graber to have located the pre-retinal membrane. Even Grenacher’s 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 63 
 
 own figures (l. c., Taf. XX, Figg. 2-4) seem to me favorable to the presence 
 of a cuticular partition between the two cell-layers under consideration. 
 
 If there are some features of the eye in Chilopoda which seem to favor 
 a method of formation similar to that traced in spiders, there are almost 
 none in the case of the Chilognatha, provided the figures by Graber are 
 to be superseded by the account given by Grenacher. Neither Graber 
 nor Grenacher has figured anything that could be compared to the pos- 
 terior layer of a retinal involution ; and Grenacher denies, in addition, 
 the existence of a “vitreous.” In brief, according to the latter author, 
 the whole eye is composed of a single continuous layer of cells formed 
 into a cup-like depression ; all, except the cells at the margin of the cup, 
 are bacilli-producing elements. Whether all the cells of the depressed 
 region, or only the marginal ones, are engaged in the production of the 
 lens, the author does not suggest. Apparently, the only chance of there 
 having been a distinct “ vitreous ” in this case, would rest upon the pos- 
 sibility that these marginal cells at first meet in front of the retina, and 
 afterwards suffer a complete centrifugal displacement ; but of this there 
 is as yet no direct evidence. 
 
 The apparent improbability of an involution with inversion in the case 
 of the Chilognatha is not without weight in considering the nature of the 
 eyes in Chilopoda, since the arrangement of the retinal cells is so strik- 
 ingly similar in the two groups as to render a fundamental difference 
 between them highly improbable. Further, the almost strictly sym- 
 metrical (radial) arrangement of the parts in all Myriapoda stands in 
 contrast to a very common obliquity in the eyes of spiders. So, not- 
 withstanding the several arguments which I have presented in the case 
 of the Scolopendridae favorable to an involution with inversion, I am not 
 entirely certain that such has really taken place. While the evidence 
 strongly inclines me to a belief in a process of inversion for Chilopoda, I 
 agree with Grenacher that nothing short of a study of the development 
 of the eyes is likely to afford an absolutely satisfactory answer. 
 
 I am not able to read Sograff’s paper (’80), published in Russian ; but 
 in his preliminary paper (’79) he does not seem to have recognized any 
 difference between the structure of the eyes of coleopterous larvae and of 
 spiders.* 
 
 In the case of Hexapoda the simple eyes of the larvae and the ocelli 
 of the adult are sufficiently different to require separate consider- 
 
 * “ The eyes of the Lithobidae and Scolopendridse are exactly like the eyes of the 
 larvae of Acilius and other Coleoptera, as well as those of the spiders ” (Sograff, ’79, 
 
 P- 17). 
 
64 
 
 BULLETIN OF THE 
 
 ation.* There are no satisfactory observations on the course of events 
 during development in either of these cases. 
 
 The simple eyes of the larvae of Dytiscus and Acilius have figured as 
 types of the one-layer condition since the time of Grenacher’s masterly 
 work ; and indeed there seems at first sight little or no opportunity for 
 any other interpretation, even though Graber (’80) at first suggested, and 
 then (in a footnote, l. c., p. 67) definitely claimed the existence of a 
 pre-retinal membrane in the case of Dytiscus. But the direct and evi- 
 dent continuity of the “ vitreous ” cells with the retinal cells, especially 
 the uniformity in the positions of the nuclei in the two regions, makes 
 an inversion of the retinal layer extremely improbable. Even in the 
 larger dorsal eyes of Acilius, where there is a perceptible difference in 
 the size of the nuclei in the “ vitreous ” and the retina, the continuity 
 appears from Grenacher’s figure ( l . c., Fig. 4) absolutely uninterrupted. 
 There is a striking similarity between this eye and the anterior median 
 eye of Salticus ; but the presence of (even a few) nuclei just in front of 
 the anterior face of the retina in the latter case (compare Grenacher, ’79, 
 Fig. 28) is sufficient evidence of an interruption in the continuity be- 
 tween “ vitreous ” and retina in Salticus, and makes a substantial differ- 
 ence between the two at least possible. However improbable a like 
 interruption in the continuity of these cell-layers may be in Acilius, it 
 is not to be overlooked that a complete separation of retina from 
 “vitreous” even here could easily have been followed by conditions 
 like those figured by Grenacher ; for to accomplish this it would only 
 have required a subsequent displacement of the basal ends of the “ vit- 
 reous ” cells containing pre-retinal nuclei to the margin of the pigmented 
 cylinder. That such a displacement — accompanied, perhaps, with 
 partial obliteration — has really taken place in the case of Salticus, 
 seems probable from the paucity of the pre-retinal nuclei figured, t and 
 their entire absence from the funnel-shaped depression in the middle 
 of the retina. 
 
 Finally, in the ventral eye of Acilius figured by Grenacher (79, Fig. 10), 
 the appearance of the vitreous is certainly not more favorable to a mono- 
 stichous than to a so-called diplostichous condition. While in the dorsal 
 eyes the basal (nucleated) ends of the vitreous cells abut upon the peri- 
 phery of the cylindrical ocular mass, in the ventral eyes they appear to 
 end directly in front of the retina, to the surface of which they are 
 almost perpendicular. They consequently appear in the figure to form 
 
 * The “compound ocelli” are not so directly comparable with the types of eyes 
 with which the present paper is concerned. 
 
 t I can confirm the fact from my .own observations. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 65 
 
 a continuous cell-layer in front of and concentric with the retina. The 
 critical region — where the pigmented hypodermis passes into the layers 
 behind the lens — is not satisfactorily portrayed in the figure. On one 
 side (the right) the hypodermis seems to be directly continuous with the 
 retinal layer ; upon the other side it is continuous with the layer form- 
 ing the vitreous body , the retina being on this side more detached from 
 it. Not finding nuclei in the vitreous layer, Grenacher admits that they 
 may have entirely disappeared ; but he is more inclined to the opinion 
 that they are grouped with nuclei of the ring-shaped pigmented zone at 
 the anterior border of the retina, — where the nuclei are too numerous 
 to be supposed to belong exclusively to the pigment zone, — and that 
 the finely attenuated posterior ends of the cells, bent outward towards 
 the nuclei, escaped direct observation. 
 
 If the nuclei of the “ vitreous ” have completely disappeared, it is diffi- 
 cult to see how this could he regarded as a monostichous eye. There is 
 nothing, it is true, in the second assumption which precludes the idea 
 that the ommateum consists of a single layer of cells ; but it is equally 
 clear that it does not preclude the possibility that the nuclei of the 
 “ vitreous ” have been displaced towards the margin of the lens ; and this 
 would be compatible with a true involution of the retinal cells. I think 
 that such a displacement of the nuclei from the central portion of the 
 “ vitreous ” — in a manner analogous to that which Grenacher believes to 
 have taken place with the retinal nuclei in the case of Salticus (Grenacher, 
 ’79, Fig. 25 K) — is more probable than either their total disappearance 
 or their having primitively held a marginal position. 
 
 In all these cases there is the opposing argument that no third layer of 
 cells was discovered. 
 
 The ocelli of the imagines also seem from previous descriptions to be 
 destitute of a third layer, — at least no one, so far as I am aware, has 
 claimed it. From one of Grenacher’s figures (that of Crabro, l. c. } Fig. 34) 
 I infer that a third layer may nevertheless exist as a thin sheet of cells, 
 forming, as in spiders, the matrix of the so-called sclera.* 
 
 The only observations on the development of the simple ocelli of the 
 imago are those of Carriere already alluded to. They are too incomplete 
 to serve as a safe guide. I am, moreover, persuaded, from the examina- 
 
 * Grenacher (’79, p. 60) speaks of the nuclei as belonging to this fine cuticula, and 
 in the copy of his paper which I have, the (blue) nuclei lie on the inner side of the 
 cuticula. Since the “registering” appears to be very accurate for the “vitreous” 
 cells, I have no doubt that the nuclei of the sclera are printed as Grenacher intended, 
 although no mention of their position in relation to the cuticular membrane 
 (“sclera”) occurs in the text. 
 
 VOL. XIII. — NO. 3. 
 
 6 
 
66 
 
 BULLETIN OF THE 
 
 tion of some of the early stages in the formation of the ocelli of Vespa, 
 which Mr. F. A. Houghton is investigating, that a process of involution 
 takes place ; and I believe that here also it will be shown that there is an 
 inversion of the retinal area.* 
 
 If the presence of a distinct and continuous layer of “ vitreous ” cells in 
 front of the retina possesses any weight in favor of an involution after the 
 type of spiders’ eyes, then the simple ocelli of adult Hexapods are likely 
 to have followed the same plan of development as the eyes of Arachnoids. 
 That the cells of the vitreous layer are usually so fiat and thin that they 
 have sometimes been overlooked, does not in the least diminish their im- 
 portance as an index to the manner in which the retina was produced. 
 Indeed Carriere (’85, p. 178) has shown conclusively that the cells com- 
 posing the thin layer which represents the “ vitreous ” in the completed eye 
 of Vespa, are much reduced in size as compared with their condition dur- 
 ing the formation of the lens. The figure which he has given (Fig. 142) 
 of the eye of the wasp during this stage is very instructive, for it shows 
 that, however obvious the continuity of hypodermis and retina may ap- 
 pear in the finished state of the eye (compare Grenacher, ’79, Fig. 31), 
 they are separated during this earlier condition by a wide interval, and 
 that consequently the supposed continuity can have no such importance 
 as might otherwise be attributed to it. Although Grenacher has not fig- 
 ured anything which may be fairly taken to represent Graber’s pre-retinal 
 membrane, it is evident from Carriere’s figure of the earlier condition that 
 retina and ‘(vitreous” are sharply separated by a line which seems to be a 
 continuation of the inner cuticula of the hypodermis, much as in the eyes 
 of spiders ; and Grenacher himself, criticising Leydig’s views, has insisted 
 upon the sharp separation of the two cell-layers. 
 
 * Since the above was written, Carriere (’86) has published an article in the 
 Zoolog. Anzeiger (Jahrg. 9, no. 217, pp. 141-147), in which he has reverted to the 
 histological conditions of the ocelli in the Diptera and Orthoptera ; but he has not 
 given any further evidence concerning their development. 
 
 Postscript. — Under date of June 1, Prof. Carriere writes me that he has arrived 
 (independently) at the conclusion that the ocelli in Hymenoptera and Diptera are 
 formed by a process of involution, but that the infolded region does not become 
 detached from the hypodermis. 
 
 It is possible that this difference of opinion is more formal than real, since there 
 is probably no period in the formation of the ocellus, after the earliest stages of 
 involution, during which the involuted portion is not in contact with the hypodermis 
 in the region of the “vitreous but the ultimate intervention of the pre-retinal 
 membrane is to me sufficient evidence of an interruption in the original continuity 
 of the cell-layers. That is all I should wish to claim by saying the infolded portion 
 of the hypodermis became “ detached ” from the permanent hypodermis. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 67 
 
 I have referred especially to the ocelli of Hymenoptera because of the 
 evidence of a third layer, and the certainty of there being a “ vitreous ” 
 which undergoes a great reduction during the development of the eye. 
 Even if a “ vitreous ” should in some instances appear to be wanting in 
 the adult, the condition could be fairly explained as a result of ultimate 
 atrophy. The evidence for the existence of a third layer is in most cases 
 still wanting. When Grenacher (’79, p. 57) claimed a substantial agree- 
 ment in the morphology of the ocelli of insects and the eyes of spiders, 
 he based his conclusion on the presence of two distinct cell-layers, — a 
 vitreous and retina. With the present knowledge of the development in 
 the case of spiders, it again becomes an open question whether the mor- 
 phological change in insects follows the same fundamental plan. It is not 
 impossible that there are among insects two methods of development for 
 the ocelli, — one with, the other without, retinal inversion. A conspicu- 
 ously reduced “ vitreous,” and the probable existence of a distinct post- 
 retinal layer of cells in Hymenoptera, inclines me to the opinion that in 
 some cases, at least, there is an inversion. 
 
 One of the questions which is most intimately connected with that of 
 the origin of the retina concerns the nature and significance of the pre- 
 retinal membrane. In connection with this I shall consider the inner 
 cuticula or basement-membrane of the hypodermis and the “ sclera” 
 
 Graber (’79, pp.64-67) was the first to call attention to the existence of 
 a homogeneous cuticula-like membrane (“ prseretinale Zwischenlamelle ”) 
 between the “ vitreous body ” and the retina, and, as we have seen, to lay 
 stress upon its existence as an argument against Grenacher’s supposition 
 that the retina was derived from the hypodermis. The question in his 
 mind turned upon the direct continuity of the hypodermis (pigment) cells 
 with the cell-layer forming the retina. Such a continuity being precluded 
 by the presence of his pre-retinal membrane, the inference of a hypoder- 
 mal origin for the retina became for him untenable. 
 
 Grenacher subsequently (’80, pp. 429, 430) conceded the existence of 
 such a structure in the case of scorpions and spiders, but was unwilling 
 to follow Graber in his generalization that all “ Stemmata ” possess this 
 membrane. Unable to disprove Graber’s claims in the case of Dytiscus by 
 a re-examination of the subject, he was still unwilling to give them any 
 weight, because Graber “ claimed with equal certainty the existence of 
 such a cuticular membrane for Myriapods, but assigned to it an entirely 
 impossible location.” But the problem of reconciling a pre-retinal mem- 
 brane with the supposed hypodermal origin of the retina, was not attempted 
 by Grenacher. 
 
68 
 
 BULLETIN OF THE 
 
 While the existence of a pre-retinal membrane, as claimed by Graber, 
 is corroborated for eyes of the “ pre-nu clear ” type, and its presence made 
 readily comprehensible by the observations of Locy, the conclusions drawn 
 by Graber from this anatomical fact have received the reverse of con- 
 firmation. Whether eyes of the post-nuclear type exhibit this membrane, 
 is not so easily determined'; but the question will be considered in a 
 subsequent part of the present paper. 
 
 Lankester and Bourne (’83, p. 182) apply the name “ ommateal cap- 
 sule ” to that portion of the “ basement-membrane ” (inner cuticula) which 
 lies in the region of the ommateum * of the lateral eyes of scorpions, and 
 then extend the use of the term •)• to “ diplostichous ” eyes, so as to cover 
 what has been called by the earlier writers “sclera.” Denying the existence 
 of the separate “ vitreous ” claimed by Graber for the lateral eyes, they 
 of course find in these eyes nothing equivalent to Graber’s pre-retinal 
 membrane. In the central eyes, however, it exists as " a strong lami- 
 nated membrane,” forming a septum which divides the vitreous body 
 from the rest of the ommateum. The ommateal capsule, of which the sep- 
 tum, they say, forms a part, is “ finely laminated and devoid of nuclei.” 
 
 The “ ommateal capsule ” in the lateral eyes of Limulus ( l . c., p. 203), 
 “ whilst well marked in every other region, is deficient immediately below 
 the retinula, where the group of optic-nerve filaments passes out of or into 
 the capsule.” The authors regard this deficiency of the capsule as related 
 to the intrusion of connective tissue into the eye ; for it is around the 
 optic nerve that the intrusion appears to take place. 
 
 In the central eyes of Limulus they “ could not define an ommateal 
 capsule,” the intrusive connective tissue being much more abundant than 
 in the lateral eyes ; but a vitreous body composed of short cells is sepa- 
 rated from the retinal body behind it by “ firm membrane,” not very 
 clearly indicated in their figures, but apparently continuous with the 
 basement-membrane of the liypodermis. 
 
 It seems to me possible that the great difficulties attending the investi- 
 gation of these eyes account for the fact that the authors have not dis- 
 covered a post-retinal capsule. 
 
 * Compare the quotation in the footnote, p. 50. 
 
 t However appropriate this terminology may be for monostichous eyes, it evi- 
 dently is not sufficiently distinctive in the case of “ diplostichous ” eyes. It would 
 doubtless be better to adopt a terminology which should express the topographical 
 relation of the basement-membrane to the retina. The whole capsule might then be 
 called the “ retinal capsule." In diplostichous eyes the “sclera” could then be 
 called the peri-retinal (or better, perhaps, the post-retinal ) membrane , in contradis- 
 tinction to the remaining portion, already appropriately named by Graber “ pre- 
 retinal membrane.” 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 69 
 
 The views held by Schimkewitsch (’84, pp. 8, 9, 12) are widely at 
 variance with those of all the other writers. He is without doubt right 
 in bringing the “ inner cuticula,” the so-called “sclera,” and the pre-retinal 
 membrane into a single category ; but misled, as I think, by appearances 
 of the sclera that can readily be explained in another manner, he has 
 concluded that all these structures are cellular * 
 
 Schimkewitsch finds that at the point of insertion of the dorso-ventral 
 muscles of the abdomen this “ inner cuticula ” is continuous with the 
 sarcolemma of the muscular bundles. Reasoning from Froriep’s (78) 
 conclusion that the sarcolemma of the striate muscles in vertebrates is to 
 be regarded as connective tissue, he maintains that this internal cuticula 
 in Arthropods must also be regarded as a connective [-tissue] formation. 
 He reaffirms the fact stated by Graber ; viz., that this same cuticula is 
 prolonged in the form of a pre-retinal layer, and that it merges with the 
 envelope of the eye (“sclera”), — “although it tends to prove the chiti- 
 nous nature of this envelope ; but,” he adds, “ nuclei are readily visible in 
 its thickness .” f Finally, in Lycosa saccata during development there 
 lies beneath the integument, directly under the chitinogenous layer and 
 outside the future subcutaneous muscular layer, a series of very flat 
 cells ; and they represent, so he claims, the future “ internal cuticula ” of 
 Graber. 
 
 Neither the nuclei in the thickness of the internal cuticula, nor the 
 conditions observed in the development of Lycosa, are figured, so that it 
 would be very difficult to judge of the value of Schimkewitsch’s conclu- 
 sions, were it not that he has figured the same conditions, which recur 
 in the envelope of the eye (sclera). “ I have already shown,” he says 
 
 * MacLeod (’80, pp. 31-34), it is true, has urged a similar proposition respecting 
 the so-called membrana externa , or m. propria of the tracheal tubes, as well as the 
 basement-membrane of the integument ; but his conclusion is based upon theoretical 
 considerations rather than upon satisfactory direct evidence. Until the demonstra- 
 tion in this membrane of nuclei distinct from those of the epithelial cells (chiti- 
 nogenous matrix) is possible, the question cannot be considered as settled in favor of 
 the connective-tissue nature of the membrana propria of the integument. 
 
 Grenacher (’80, p. 26) has also spoken incidentally of the fact that the thin, inner 
 cuticula of the hypodermal cells in the larvae of Dytiscus are “ stellenweise kerntra- 
 gende ; ” but I do not understand that he directly commits himself to the opinion 
 that these nuclei belong to cells which have served as the matrix of what he calls 
 “ Cuticula,” much less to the opinion that this membrane is a cellular structure. 
 
 t “ La meme cuticule interne se prolonge en forme de lame preretinenne dans les 
 yeux et se confond avec l’enveloppe de l'ceil, comme l’a demontre Graber, et je puis 
 affirmer le fait, bien qu’il tende a prouver la nature chitineuse de cette enveloppe ; 
 mais des noyaux dans son epaisseur sont bien visibles” (p. 9). 
 
70 
 
 BULLETIN OF THE 
 
 (p. 12), “that Graber’s cuticula ought to be considered as a connective 
 [-tissue] layer, and in the envelope of the eyes I have been able to estab- 
 lish oval nuclei (PI. II, Fig. 4, nrk) ; and I claim that the pre-retinal layer, 
 which is merged with these membranes, is also of connective nature.” 
 
 The conditions of the eye-envelope are somewhat differently repre- 
 sented in each of Schimkewitsch’s three figures illustrating its cellular 
 composition. In one figure (Fig. 4, PI. II) the nuclei appear to lie on 
 the outer surface of the homogeneous membrane ; in another (PI. Ill, 
 Fig. 4) they are distinctly on the inner surface ; while in the third (PI. 
 Ill, Fig. 11) they are less definitely related to the membrane, a portion 
 of the nuclei appearing simply as thickenings in it. Figure 4, PI. Ill, 
 is evidently drawn to the largest scale, and also, I believe, represents 
 more truthfully than the others the relations of the nuclei to the mem- 
 brane ; they are simply tangent to the inner surface of the double-contoured 
 membrane. I believe they are without the least doubt the nuclei of the cells 
 which constitute the posterior of the two layers resulting from the involution 
 of the hypodermis to form the retina. 
 
 With this explanation of the nuclei supposed to lie in the “ sclera,” 
 the theory of the connective-tissue nature of the “ internal cuticula ” is 
 deprived of an apparently valuable support, and now seems to rest on 
 quite as unsatisfactory evidence as ever before. 
 
 Lowne (’84, p. 415) believes that “the columnar cells immediately be- 
 neath the cornea (Grenacher’s vitreous) represent the dioptron.” “ They 
 are separated from the retina by a fibrous membrane which apparently 
 corresponds to the membrana hasilaris of the compound eye.” This 
 basilar membrane the author has previously defined as a cuticular struct- 
 ure. But it is evident from the context that the author rests his conclu- 
 sions on the peculiar fibrous pre-bacillar layer which is found in Salticus, 
 and which Grenacher (’78, p. 51) considered to be composed of fibres 
 from the anterior ends of the retinal cells. Lowne, it is true, denies the 
 direct connection (claimed by Grenacher) of these fibres with the marginal 
 ring of nuclei ; and adds : “ In some of these sections the fibrous mem- 
 brane has completely separated from the bacilla, just as the membrana 
 basilaris separates from the retina in the compound eye.” It should be 
 remembered, however, that Grenacher also found nuclei in this fibrous 
 layer, and that Lowne’s statement in no way affects the validity of that 
 observation,* nor does he (Lowne) attempt any explanation of the fact. 
 
 * From the examination of sections of the eyes of an adult Salticus made by Mr. 
 Locy, and of those of Theridium tepiilariorum, C. Koch, by Mr. G. H. Parker, I am 
 able to confirm Grenacher’s observation. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 71 
 
 I think it is sufficiently evident that this “ fibrous membrane ” in Salticus 
 cannot be considered the morphological equivalent of the pre-retinal mem- 
 brane originally described by Graber ; for if it were, it would be the only 
 known case in which the pre-retinal membrane was composed of inter- 
 lacing fibres (compare Grenadier, J 78, Figs. 25, 27), to say nothing of the 
 occasional presence of nuclei within it. Hence, while I agree with the 
 implied conclusion of Lowne that the vitreous layer and the retina are 
 separated by a cuticular structure, I regard his reasons as altogether 
 uncritical, and such as would lead, if logically pursued, to an entirely 
 different conclusion. 
 
 Carriere (’85, p. 187), considering it probable that the two layers of the 
 monostichous eyes have originated by a process of delamination, as in the 
 compound eyes, finds it in no way remarkable — even though the sepa- 
 ration is much more distinct than in the latter case — that the outer 
 layer of the monostichous eye (a genuine epithelium) develops a “ Basal- 
 membran ” after the manner of the ordinary epithelium of Arthropods. 
 “ But this membrane not only separates, it also joins the upper with the 
 lower layer ; at least I have never met a case in which the two layers 
 had become separated from each other.” Although not precisely stated, 
 there can be no doubt that Carriere regards his “ Basalmembran ” as the 
 equivalent of Graber’s pre-retinal membrane, and as a cuticular structure. 
 
 Locy’s observations and the conclusions which directly result from 
 them not only place the retina in a more satisfactory relation to the hypo- 
 dermis, but also afford at the same time a fair explanation of the condi- 
 tion and mutual relations of sclera, pre-retinal membrane, and the internal 
 cuticula of the hypodermis. It now becomes probable — unless in spe- 
 cial cases the reverse is proved by direct observation — in all those 
 instances where a pre-retinal membrane is demonstrable in the adult 
 “ stemma,” first, that the retina has been produced by an involution of 
 the ectoderm (hypodermis), which has inverted the more superficial of the 
 two infolded cell-layers ; and consequently, secondly, that the eye is not 
 simply two-layered, as supposed by Grenacher as well as all subsequent 
 observers, but is really three-layered (triplostichous). 
 
 In the light of this process of involution the deep cuticular layer 
 (“ Binnen- Cuticula , ” Graber) appears in readily appreciable relations. 
 Whether as an “ inner cuticula ” to the permanent hypodermis and the 
 pigment-cells, as the so-called sclera which invests the retinal bulb, or as 
 a pre-retinal membrane, it is really one and the same thing. These three 
 structures have a like origin, — they are the continuous product of the 
 basal ends of ectoderm cells ; and the pre-retinal membrane alone requires 
 
72 
 
 BULLETIN OF THE 
 
 the further modifying statement that it may he double, whereas the oth- 
 ers are the result of the activity of only a single layer of cells. Grabers’ 
 conception of the “ Zwischenlamelle ” — as a direct prolongation of the 
 integumental “ Binnen-Cuticula,” from which the sclera proper branches 
 off on the inner or deep side towards the nervus opticus — is to be so far 
 amended as to make both sclera and cuticula to branch from the “ Zwi- 
 schenlamelle,” rather than the sclera and “ Zwischenlamelle ” to branch 
 from the cuticula. Either conception is to that extent faulty that there 
 is no such thing as a branching off or a splitting, but quite the contrary, 
 — a fusion. It is possible that some of the lines seen by Graber within 
 the “ Zwischenlamelle ” (and explained by him as the result of the ordi- 
 nary stratification of cuticular membranes) are indications of the plane 
 along which the fusion between the component layers of this pre-retinal 
 membrane took place. 
 
 The brilliancy of the eyes of many spiders, to which Duges (’36, 
 p. 177) was the first to call attention, was investigated by Leydig, but it 
 has received little or no attention from recent writers. 
 
 Leydig (’55, p. 439 ; ’57, p. 254 ; ’64, p. 48) describes the structure 
 • which is the cause of this brilliancy as a tapetum , which is either con- 
 tinuous, lining completely the fundus of the eye, or, in some species 
 (Clubonia claustraria, Hahn, and Theridium sp. T), interrupted by a band 
 of black pigment which traverses its middle in wavy lines. * 
 
 In Phalangium the tapetum is not continuous, but consists of isolated 
 scales (“ Flitterchen”). In still other cases (Lycosa saccata, and several 
 species of Epeira) it forms a narrow band at the anterior rim of the eye- 
 pigment, but becomes visible (as radial streaks lodged in the dark pigment) 
 only after the eyes are dissected out. The tapetum usually consists of 
 scales of the same kind as those which are met with in the tapetum of the 
 fish’s eye. They are minute, iridescent plates, which lie close together, 
 and are separable only when subjected to strong pressure. In other 
 cases (Phalangium, Micryphantes) the tapetum is composed of spherules 
 larger than the pigment-granules. 
 
 Graber’s (’79) account of the tapetum in Tegenaria domestica 
 (“ Scheitelauge ”) is limited to the description of his Eigs. 27 and 30. 
 In the former, the “blaulich grim schimmerndes Tapetum” is repre- 
 sented as composed of numerous minute plates, forming a stratum on 
 both sides of the pre-retinal membrane (!), the long axes of most of the 
 
 * In Clubonia claustraria this black wavy line corresponds, according to Leydig, 
 with the major axis of the oval eye. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 73 
 
 plates being perpendicular (!) to the membrane. In Fig. 30 the “ kry- 
 stalloide Plattchen ” of the tapetum appear as irregular, angular, more or 
 less lozenge-shaped bodies, composed of a granular central mass and a 
 broad rim of uniform thickness, in the substance of which is located the 
 pigment which gives the “ Plattchen ” their peculiar color. 
 
 Graber has apparently fallen into an error both as regards the location 
 and the direction of the elements which compose the tapetal layer. It is 
 not likely that the tapetum in Tegenaria differs so fundamentally from 
 that of Agelena. It is probable that Graber has mistaken the posterior 
 ends of the retinal cells for the corresponding ends of the so-called 
 vitreous cells.* 
 
 Grenacher (’79, p. 55) omitted a consideration of the tapetum for two 
 reasons, — because (1) it presents nothing of importance for the compre- 
 hension of the simple eyes and their relation to the compound eyes ; and 
 (2) the method of examination would of necessity have been different, 
 since the employment of nitric acid to depigment the eye destroys the 
 tapetum in a very short time, without leaving a trace of it. 
 
 Without entering into a discussion of the nature of the tapetum, or its 
 prevalence in the eyes of spiders, I wish to call attention to a few facts 
 which appear to me of deep interest, and possibly of fundamental impor- 
 tance, in any attempt to appreciate the morphological bearings and the 
 functional capabilities of such eyes. 
 
 No one, I believe, has hitherto called attention to the distribution of 
 tapeti further than to indicate, as Leydig has done, that certain spiders 
 do, and others do not, possess this structure. My examinations have not 
 been sufficiently numerous to allow a very trustworthy conclusion to be 
 drawn from them ; but I have been impressed by the fact that, in the 
 few cases examined, the tapetum, when present, was limited to the lat- 
 eral anterior and to the posterior eyes ; that the anterior median pair 
 does not possess such a layer. When it is remembered that a division 
 of the eyes into two groups is necessitated by the different types of 
 bacillar development, t and that, so far as' at present observed, the groups 
 
 * Postscript. — An examination of sections of the posterior median eyes ( Scheitelau- 
 gen) of Tegenaria domestica, which Mr. Parker made at my suggestion soon after his 
 return to Cambridge in August, has confirmed my opinion that this species does not 
 differ essentially from Agelena in the position of the tapetum. It is certain that 
 it lies beneath the retinal layer, and is in no sense adjacent to the pre-retinal 
 membrane. 
 
 + For convenience of reference I shall call the group embracing only the anterior 
 median pair in Agelena the group with pre-nuclear bacilli, or, briefly, pre-nuclear 
 group (Graber’s post-bacillar); the remaining six eyes in Agelena will then consti- 
 
74 
 
 BULLETIN OF THE 
 
 founded on the position of the bacilli, and those based on the presence 
 or absence of a tapetum, correspond,* one can hardly avoid the convic- 
 tion that these two features are in some way connected, and that the 
 dimorphism first pointed out by Grenadier is emphasized in other matters 
 than those to which his attention was directed. 
 
 The origin of the tapetum and the exact method of its formation are 
 not yet sufficiently clear to me ; but I hope to be able before long to 
 acquire more information upon the obscure points. In connection with 
 the development of the eyes of the “post-nuclear” group, Locy (’86, 
 p. 89) has mentioned a structure which separates the two layers of the 
 retinal infolding, and he has described it as a “ much-folded chitinous 
 layer, probably homologous with the cuticular covering of the body, with 
 which, in the earlier stages, it appears to be continuous.” 
 
 After renewed examinations of his preparations, and others of a simi- 
 lar nature from other spiders, I have arrived at the conclusion that this 
 layer is without any doubt the tapetum, and that there is no certainty of 
 its having been at first continuous with the external cuticula of the body. 
 As understood by Locy, it was a natural inference, with regard to its 
 formation, that it resulted, like the cuticula, from the secretive activity 
 of the ends of the cells composing one or both the layers of the retinal 
 infolding. This view seems at first to receive confirmation from the 
 early appearance of the tapetum, its apparent continuity (in many cases) 
 with the external cuticula, its greenish-yellow color, and the peculiar 
 shape of the separate elements which ultimately make up this layer. I 
 find also that in Theridium + it is composed of tolerably regular, elon- 
 gated, hexagonal plates (PI. Ill, Fig. 17), neatly fitting edge to edge (as 
 though secreted by a pavement-epithelium) ; and in one instance I have 
 noticed distinct perpendicular markings in some of the scale-like plates 
 when seen edgewise. If the plates were really comparable with the 
 cuticula, these markings might be the equivalents of “ pore canals.” I 
 should add, however, that they were so strong as to suggest rather the 
 composition of the plates out of numerous perpendicular rods of uniform 
 size. 
 
 But notwithstanding all this, the tapetum may be the result of a cell- 
 
 tute the post-nuclear (Graber’s pre-bacillar) group. If the relation suggested above 
 should be realized, “ pre-nuclear ” eyes might with equal propriety be designated as 
 non-tapetal , and “post-nuclear” as tapetal. 
 
 * In Thomisus vulgaris, Hentz, I have not been able to find any evidence of the 
 existence of a tapetum either upon sagittal or transverse sections. However, the 
 only sections at my disposal are such as have not been depiginented. 
 
 + Theridium tepidariorum, C. Koch. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 75 
 
 metamorphosis rather than a simple secretion. Of one thing, at least, I 
 am convinced, — the tapetum owes its origin to a limited number of cells, 
 the nuclei of which become very much elongated during the process of 
 involution. How this takes place can best he shown in connection with 
 a general account of the changes accompanying the formation of the eyes 
 which possess a tapetum. 
 
 The hypodermal infolding in the eyes of the “ post-nuclear ” group was 
 not studied in detail by Locy ; it appears to be considerably more com- 
 plicated than in the case of the anterior median pair. This I have been 
 able to make out from the specimens which Mr. Locy has kindly placed 
 at my disposal.* Most of the figures on the accompanying plates are 
 intended to illustrate these conditions. The first seven figures (PI. I) 
 present the median faces of successive sagittal sections from the left half 
 of the head of an individual about four days after hatching. The first 
 section is the one nearest the median plane. The directions of the in- 
 foldings are such that sagittal sections are more favorable for the study 
 of the posterior median and anterior lateral eyes than for the posterior 
 lateral. The nature of the infolding-process is most readily understood 
 by the aid of sagittal sections of the posterior median eye ; and hence I 
 begin the description with that eye. 
 
 Of all the sections studied, those which are represented in Figures 
 8 and 9 (PI. I) are in some respects the most satisfactory, but in other 
 respects they are possibly misleading. There is a considerable thickening 
 of the hypodermis in two regions, and these two thickened tracts appear 
 to be connected by a continuous row of nuclei (tap.) so arranged as to 
 suggest that an S-shaped-folding of the hypodermis has taken place. The 
 principal difference between this condition and that described by Locy 
 appears at first sight to be due to the relative thickness of the three com- 
 ponents of the “ S.” In the anterior median eye the middle part is from 
 the beginning the thickest ; in the present case it is the thinnest. In one 
 
 * This paper was begun in the belief that there was no important difference 
 in the method by which the pre-bacillar and the post-bacillar types of ocelli are de- 
 veloped. After a large portion of the paper was already written, the author received 
 (March, 1886) from Mr. Locy for re-examination the preparations which had served 
 as the basis of his paper. The results of the re-investigation of his material, al- 
 though not sufficiently complete to form an entirely satisfactory presentation of the 
 subject, are incorporated here because they are deemed of importance, and because to 
 have waited until the questions to which they give rise could have been more ex- 
 haustively studied would have necessitated both an extension of the paper beyond 
 the original plan and an undesirable delay in its publication. 
 
76 
 
 BULLETIN OF THE 
 
 of these figures, however (Fig. 9), there is some evidence that the row of 
 nuclei (tap.) is not single, but double, and that it is the result of an out- 
 folding of cells {tap.) lying between the regions pr. and r. This conclu- 
 sion is strengthened by the condition of the anterior lateral eye as shown 
 in Fig. 4, tap. It is almost certain, from the shape and direction of the 
 nuclei, that the equivalent region in this case is a fold, open below. If 
 this middle region really represents a double rather than a single layer 
 of hypodermal cells, then the S-shaped appearance is deceptive ; and one 
 must suppose that half of the fold has become merged in one of the 
 thickenings (or otherwise obscured), while the other half remains as the 
 only apparent means of connection between the two thickenings. It is 
 further evident that this owtffolded middle region must be in the nature 
 of a re-entrant fold from the apex * of an original involution, of which 
 the two thickenings constitute the walls. 
 
 The condition and connections of this middle region are of great im- 
 portance in deciding upon the morphological relations of the retina, and 
 it is therefore to be regretted that the evidence as to its real nature is not 
 more conclusive. 
 
 In the tract nearest the anterior median eye (Fig. 8 ,p r.) the thickening 
 results simply from a displacement and a slight elongation of the cells 
 and their nuclei, the latter overlapping each other like so many tiles. But 
 the posterior thickening is more complicated ; it consists of two parts. 
 The anterior part is composed of cells, the nuclei of which have their 
 long axes nearly parallel with the surface of the head ; they collectively 
 form a broad band (r.) nearly perpendicular to the surface of the head ; 
 the nuclei are wedged between each other so as to form two or three 
 irregular rows. Behind this, and more or less in continuity with it, is a 
 region {pr r.) which gradually diminishes from a thickness nearly equal- 
 ling the length of the “ perpendicular band,” to the thickness of the or- 
 dinary hypodermis. The nuclei in this triangular region are, in the main, 
 perpendicular to the surface of the head, although showing a tendency to 
 radiate from a point near the deep end of the “ band.” There are, then, 
 four more or less distinct tracts already recognizable. These may be 
 named from behind forward, pre-retinal {pr r.), retinal (r.), tapetal (tap.), 
 and post-retinal {p r.), respectively. The same regions may also readily 
 
 * It is possible that the re-entrant fold was not confined to the bottom (apex) 
 of the eye-pocket, but extended along its margins , and that the “ fissure ” in the 
 tapetum, subsequently referred to, is to be explained as resulting from the failure of 
 these two lateral ingrowths into the pocket to unite along the axis of the latter. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 77 
 
 be traced in Fig. 2, although in this section the nuclei of the “ band ” 
 (r.) are more regularly polygonal. 
 
 The further changes and the ultimate fate of each of these four tracts 
 seem fairly evident from a simple comparison of this figure (Fig. 2) with 
 Fig. 12, which shows a corresponding view of the same eyes (but from 
 the right side of the head) of an individual killed eight days after hatch- 
 ing. (Consult also Figs. 11, 16, 20-24, and the explanations of the 
 figures.) The relative positions of the parts have become slightly changed 
 in the later stage, owing to a continuation of the process of folding and 
 the closer approximation to each other of the three anterior regions. 
 
 Numbering from behind forwards, it will be seen that the fourth or 
 last tract ( p r. Fig. 12) has grown backward until it now lies underneath 
 nearly the whole of the other three regions, and that the first tract 
 {pr r.) has grown forward in a corresponding manner, and thus intervenes 
 between the cuticula and the greater portion of the rest of the ocellus. 
 In the place of the third tract {tap.) the “ tapetum ” now appears, with 
 here and there a greatly elongated nucleus, and in the second tract (r.) 
 the ends of the cells, which were previously directed forwards, and are 
 now directed downwards, — i. e., toward the tapetum, — have developed 
 the bacilli ( hac .) characteristic of retinal cells. 
 
 From this stage onward, the significance of each of the four layers is 
 evident, and the determination of the homologies with the three layers 
 of the other type is to a certain extent possible. 
 
 The first or posterior tract ( pr r.) becomes the most superficial layer 
 and secretes the lens (Figs. 12, 22) ; it is the equivalent of the so-called 
 " vitreous body.” * The cell-boundaries in this, as in the other layers, are 
 not made readily distinguishable by the process of preparation employed ; 
 but the shape and direction of the well-stained nuclei show that they are 
 quite oblique to the surface of the lens, and that some of them are 
 
 * This layer of cells, which I have hitherto called “ vitreous body ” or “ vitreous,” 
 in conformity to the prevalent nomenclature, deserves a designation more in keeping 
 with its primitive function, — the secretion of a cuticular lens. Any designation 
 intended to replace so simple a word as “vitreous” must be equally brief in order to 
 be acceptable. I propose the name lentigen as a substitute for “ vitreous body.” 
 I believe this substitution is the more desirable since, according to the best present 
 information, there are probably some cases (e. g. Dytiscus) in which “ lentigen ” and 
 “ vitreous body ” would not be strictly identical. According to Grenacher’s descrip- 
 tion, certain of the pre-retinal cells in Dytiscus do not abut upon the lens, and their 
 participation in its production may therefore be questioned. They do intervene 
 between the lens and the sensitive surface, however, and may appropriately retain 
 the title “ vitreous ” cells. 
 
78 
 
 BULLETIN OF THE 
 
 slightly S-shaped. The line of demarcation between this and the second 
 tract, or next deeper layer, is not always sufficiently distinct to allow one 
 to claim with certainty the presence of an internal cuticula (basement- 
 membrane) equivalent to the pre-retinal membrane of Graber. In some 
 cases (Fig. 24) I have seen a sharp limiting membrane between the pre- 
 retinal and retinal layers ; but in other cases (Figs. 20-22) it has been 
 impossible to find the least indication of such a membrane. The form 
 and relation of these two tracts indicate a gradual slipping of the first 
 upon the second, rather than a typical folding ; but this is probably to 
 be considered as simply a modification of what originally was a true fold- 
 ing at the retino-lentigen margin of the retinal pocket. The overgrowth 
 of the lentigenous cells finally results in the same relation between the 
 two tracts as was originally produced by the ingrowth (infolding) of the 
 retinal layer. In the original method the retinal layer formed one wall 
 of a free pocket (compare Locy, ’86, PI. X, Fig. 64); in the modified 
 process it is from the beginning in contact with the lentigen. The pos- 
 terior region of the latter is finally extended (Figs. 11, 12) so as partially 
 to envelop the posterior margin of the retina. 
 
 The relations of the second tract (r.) are not equally clear upon all the 
 sections. If Figs. 8 and 9 were taken to represent the original unmodi- 
 fied condition of the hypodermal foldings, the conclusion might be that 
 there had been an outfolding Having the second tract for its wall on one 
 side, and the third tract on the other. If this were the typical method, 
 there could be no doubt but that that face of the second layer which at 
 this stage is directed forwards, and in which are developed the bacilli, 
 would correspond to the originally deep surface of the hypodermis. The 
 bacilli would therefore be developed here, as in the anterior median eyes, 
 at that end of the cells which in the original position of the hypodermis 
 must have been turned away from the light. But of the justice of this 
 conclusion I am not convinced ; for in other cases (Fig. 4) the out- 
 folding, as stated above, appears to involve only the second tract, and in 
 still others there is not sufficient evidence of a true folding of any kind. 
 In Figs. 2 and 16, for example, the conditions are such as might have 
 been produced by a detachment (delamination*) of the cells of the third 
 tract (tap.) from those of one of the adjacent layers, without the forma- 
 tion of any owtffolding. If either of the latter suppositions represents 
 the true state of the case, then the anterior face of the retinal tract (r.) 
 
 * The delamination might possibly have resulted from an abbreviation in the 
 process of forming the tapetum, which originally took place exclusively by means 
 of an outfolding of the tapetal cells. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 79 
 
 corresponds to the superficial ends of the component hypodermal cells, 
 and the bacilli accordingly occupy the ends of the cells which were 
 originally directed towards the light. 
 
 Upon either supposition there is a difficulty in instituting a comparison 
 with the eyes of the “ pre-nuclear ” group. Upon the first assumption, 
 while the bacilli would occupy the originally deep ends of the cells, as in 
 the other type, the retinal layer as a whole would have been only par- 
 tially and temporarily inverted, — not permanently, as in that type, — and 
 therefore a strict homology could not be claimed. But upon the second 
 assumption, while the infolding would result in an inversion of the 
 retinal layer, as in the simpler type, the bacilli would occupy the 
 originally superficial ends of the cells, and this would also present a 
 serious obstacle to a satisfactory comparison. 
 
 I have not, perhaps, a sufficient number of successive stages to place 
 the matter beyond question, but believe that the evidence from the ma- 
 terial which I have, and also certain theoretical considerations, point 
 towards the second assumption — that the retinal layer is inverted — as 
 the more probable. 
 
 In the later stages (Figs. 11, 12, 21-24) it is not always easy to dis- 
 tinguish at once between the nuclei of the first and second layers ; but 
 careful attention to the shape and inclination of the nuclei, as well as to 
 the intensity of their staining, allows one to determine fairly well the 
 extent, if not the exact boundaries, of each layer. In Figure 22, for 
 example, the nuclei of the “ lentigen ” were excessively flattened and 
 apparently degenerating ; those of the retinal layer were much paler, less 
 broken, and less granular. 
 
 The origin of the third tract (tap.) is involved in the question just 
 considered ; but whatever this origin, — whether it arise by delamination, 
 or by an outfolding which affects only its own cells, or wffiether it result 
 from an outfolding one wall of which is the retinal layer, — the ultimate 
 condition of this tract can scarcely be called in question ; it produces the 
 tapetum. Its nuclei (compare also Figs. 18-22) often undergo a remark- 
 able elongation, and conform in shape to the curved direction of the layer. 
 
 In all the eyes of the “ post-nuclear ” group in Agelena the tapetum 
 has the form of a short canoe, the cavity of which is directed towards 
 the retina. Its greatest length corresponds with the direction of the ecto- 
 dermic infolding. The end corresponding with the bottom of the pocket 
 of involution is narrower than the opposite end, and does not approach so 
 near to the surface of the head as the latter. The variations in the curva- 
 ture from end to end are often considerable, amounting in some cases to 
 
80 
 
 BULLETIN OF THE 
 
 a sharp bend in the middle (Fig. 24), and the inclination of the sides to 
 each other may also vary several degrees. The tapetum does not carpet 
 the whole fundus of the eye, being, even in its broadest part, much nar- 
 rower than the latter (Figs. 13, 14, PL III) ; but it appears to he as exten- 
 sive as the layer of bacilli developed in front of it. Corresponding in 
 position to the keel of the canoe, is a narrow interruption, or fissure, which 
 extends through the whole length of this layer. It is sometimes slightly 
 curved, S-shaped, and its edges are not always clear cut. It is probable 
 that the appearance which Leydig described as “ a band of black pigment 
 traversing the middle of the tapetum ” was due to the presence of a 
 similar fissure. In some instances the broad outer end of the tapetum 
 appears to abut directly upon the inner surface of the external cuticula ; 
 but even in such cases I have not found in its vicinity any modifications 
 of the cuticula, neither an infolding, nor any marked interference with 
 its regular course. In no case, have I been able to trace a direct con- 
 tinuity of cuticular and tapetal substances. Often the tapetum cannot 
 be followed up to the external cuticula ; but where the conditions of 
 the sections were favorable for its study, I have never failed to find that 
 the narrow, deep end of the tapetum reaches to, and is apparently con- 
 tinuous with, the internal cuticula, or basement-membrane. This con- 
 dition seems to afford confirmation of the opinion that the tapetum 
 results from an owtfolding of cells which previously occupied a position 
 at the bottom of an early hypodermal infolding, involving the “ retinal ” 
 and “ post-retinal ” tracts. For if the tapetal cells originally grew into 
 the cavity of the hypodermal pocket from its deepest end, they would 
 naturally retain a direct connection with that portion of the basement- 
 membrane where they were at first situated. The region of this ingrowth 
 into the cavity of the original pocket may have extended along the two 
 margins of the pocket for a greater or less distance, and the interrup- 
 tion in the tapetum (“fissure”) may possibly have resulted from the 
 failure of these two regions of ingrowth to meet along the axis of the 
 original pocket. The absence of a direct connection with the external 
 cuticula is in itself a strong argument against considering the tapetum 
 homologous with that layer ; this is further strengthened by a consid- 
 eration of the chemical differences between the two, referred to by 
 Grenacher. 
 
 The tapetum in Agelena consists of small, thin, slightly curved, scale- 
 like, iridescent structures which are superposed and closely packed. 
 The whole layer has a considerable thickness, and when viewed in lon- 
 gitudinal section, a peculiar wavy, fibrous appearance. If these scales 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 81 
 
 were the product of a cuticular secretion on the part of the cells of the 
 tapetal layer, one would rightly expect the nuclei of the cells to retain 
 some constant relation to the scales. They should all be located on one 
 or both surfaces of the scale-like layer, or they should all lie in the 
 middle between two sheets of such structures. But I have been unable 
 to find any such constancy of relation, the few nuclei being distributed 
 through the layer apparently without any regard to their distance from 
 either surface of the tapetum (Figs. 15, 18-22, tap.). For these reasons 
 I believe it must be admitted that the tapetal scales are formed by a 
 metamorphosis of the cell-substance of the cells forming what I have 
 called the third, or tapetal tract, and not by a process of cuticular secre- 
 tion. I have not traced the development of the separate scales within 
 the body of a cell, but from the small number of nuclei present it is 
 evident that each cell must give rise to a large number of the scale-like 
 elements. 
 
 The fourth or deepest layer apparently corresponds with the deepest or 
 third layer in the eyes which present the simpler structure, — the pre- 
 nuclear ocelli. There is no doubt that it owes its formation here, as well 
 as there, to a process of hypodermal infolding (Figs. 2, 5-9, 16, 18, 19), 
 and it retains, even after the formation of the tapetum, an evident con- 
 tinuity with the indifferent hypodermis immediately in front of it. Like 
 the deep layer in the eyes of the “ pre-nuclear ” group, it also becomes 
 the seat of an early and intense pigmentation. That it subserves the 
 ordinary functions of a pigment-layer to the retina can scarcely be 
 doubted ; but instead of progressively diminishing in thickness and indi- 
 viduality, as in the pre-nuclear eyes, it here seems to increase in thick- 
 ness, and may perhaps fulfil important functional relations not shared 
 by the corresponding layer in the simpler ocelli. In the more advanced 
 stages (Figs. 20-24) this layer is considerably augmented in bulk as com- 
 pared with earlier stages and in comparison with the mass of nuclear 
 material. Its anterior border overlaps the anterior margin of the other 
 layers (Fig. 22), much as the superficial layer (pr r.) at an earlier stage 
 (Fig. 12) envelops the posterior margin of the layers underlying it. 
 From its connection with the optic nerve it has acquired a somewhat 
 conical shape (Figs. 23, 24). A portion of the nuclei still forms a more 
 or less continuous layer near the surface (Fig. 20) ; others (Fig. 22) lie 
 near its axis. Throughout the whole of its substance very fine striations 
 are now distinguishable. The direction of the striations makes it evi- 
 dent that they are due to the radiation of the fibres of the optic nerve, 
 towards which they all tend. But I am not yet entirely certain about 
 VOL. XIII. — NO. 3. 6 
 
82 
 
 BULLETIN OF THE 
 
 the method of the distribution of the nerve-fibres to the retinal layer. 
 It will require a more careful study of maceration-preparations in con- 
 nection with sections in different planes to settle this important question. 
 It seems to me improbable that the nerve-fibres pass directly through the 
 tapetum. From what I have seen, I think that most of them pass 
 around the margins of that layer to join the anterior ends of the retinal 
 cells, though I have reason to think that some of them reach the retina 
 through the fissure in the tapetum. 
 
 The position of the other eyes is not quite so favorable for study by 
 means of sagittal sections ; and yet an examination of Figs. 4-6 is suffi- 
 cient to show that the infolding does not take place in the same direc- 
 tion in both of the lateral eyes. In the anterior laterals the retinal mass 
 lies in front of the infolding, whereas in the posterior laterals the retinal 
 mass lies, as in the posterior median eyes, behind the infolding. 
 
 In the anterior lateral eye (Figs. 4, 5) the four tracts are readily dis- 
 tinguishable ; and it is necessary only to compare Figs. 4 and 5 with the 
 later stage in Fig. 15, and the still older one of Figs. 18 and 19, in order 
 to learn that the fate of each is the same as in the eye already described ; 
 a further description is therefore unnecessary. 
 
 That all the layers — especially that producing the tapetum — are not 
 seen with the same distinctness in the posterior lateral eye, is, without 
 question, due to the direction of the axis of that eye. The sections are 
 cut in a plane which makes a considerable angle with the main axis of 
 the eye and of the infolding, and the figures therefore give a more ob- 
 lique view of the cells of the tapetal layer, which consequently are not 
 so readily distinguishable from those of the retina. The earlier sections 
 (Figs. 4, 5) pass through the fundus, — the last (Fig. 7) through the 
 margin of the infolding, where the first and the fourth layers begin to 
 merge into one another. (Compare Explanation of Figures.) 
 
 It can be seen from the figures of a later stage (Figs. 13-15) that the 
 axis of this ocellus continues to be nearly perpendicular to the sagittal 
 plane. Of the three sections, that which is nearest the surface of the 
 head (Fig. 13) shows the greater portion of the tapetum,* with its 
 median fissure, nearly en face; there are also shown, between the ob- 
 server and the tapetum, faintly expressed markings nearly perpendicular 
 to the fissure. I could not discover that they were continuous across 
 the region of the fissure. They are undoubtedly due to the differentia- 
 
 * A part of the posterior end of this structure was cut away with the preceding 
 section. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 83 
 
 tion of bacilli, — the intervals between the markings corresponding very 
 well with the intervals shown when the plane of sectioning is nearly 
 parallel with the direction of the fissure (compare Fig. 19), — but I am 
 uncertain whether it is to be concluded from this that the bacilli have 
 the shape of broad plates , or whether these plate-like structures are really 
 composed of rows of rods , which the method of preparation and mount- 
 ing (Canada balsam) has made incapable of optical resolution. There 
 is a suggestive resemblance between these plate-like markings and the 
 sinuous figure formed by the peculiar arrangement of the bacilli in the 
 posterior eye of Lycosa as given by Grenacher (’78, Taf. Ill, Fig. 24) ; 
 but I was not able to satisfy myself that these plates presented the 
 folded-back-and-fortli arrangement shown in Grenadier’s figure. . From 
 what is known of the form of the bacilli in other simple eyes, it seems 
 most reasonable to suppose, however, that the plates are composed of 
 rows of bacilli. 
 
 The second section (Fig. 14) shows the remaining portion of the tape- 
 turn, belonging principally to the anterior end of that structure ; if there 
 were portions of the bacilli present upon this section, they were too faint 
 to be discerned. 
 
 Finally, the third (deepest) section (Fig. 1 5) passes entirely below the 
 tapetum, cutting through the post-retinal layer. 
 
 The presence or absence of a pre-retinal membrane in the eyes of the 
 present type is of some interest, and yet it may not be of radical impor- 
 tance. Whether the change in the relative positions of the retinal and 
 pre-retinal tracts during development is due to a true folding, or to a 
 slipping of one layer over the other, may depend simply upon how faith- 
 fully the original method of transposition (folding) is adhered to. With 
 the gradual substitution of a slipping for a folding, the opportunity 
 for the formation of a pre-retinal membrane may have gradually disap- 
 peared ; nevertheless, I am of opinion that evidence of such a membrane 
 will usually be found during some stage in the formation of the ocellus. 
 
 In some spiders (Tegenaria, Theridium) the development of the re- 
 tinal infolding and the secretion of the lens are accompanied by a grad- 
 ual displacement of the deep ends of the “ lentigenous ” cells towards 
 the margin of the eye, so that in the adult the pre-retinal membrane is 
 almost in contact with the posterior surface of the lens, especially near 
 the margin opposite that towards which the nuclei of the “lentigen” are 
 displaced. This of course increases the difficulty of discerning the 
 membrane. 
 
84 
 
 BULLETIN OF THE 
 
 The method of connection between retinal cells and optic-nerve fibres 
 is a fact upon which Grenacher has placed great importance, since upon 
 it depends largely, in his opinion, the interpretation given to the func- 
 tional value of the individual elements of the retina. According to 
 Grenacher’s investigations (79 and ’80) the posterior (deep) end of each 
 cell of the retina (in the “ Stemma ”) is prolonged into a single nerve- 
 fibre, the optic nerve being composed of a bundle of such fibres, pre- 
 sumably as numerous as the retinal elements. This condition — espe- 
 cially well marked in Dytiscus, in the posterior dorsal eyes of Epeira 
 (Grenacher, 79, Figs. 1, 18, 20), in Lithobius, lulus, and Glomeris 
 (Grenacher, ’80, Figs. 9, 11, 13) — has also been confirmed by Lan- 
 kester and Bourne (’83, Figs. 2, 4, 7, 11) for Scorpionidse and other 
 Arthropods. 
 
 Without being prepared to question the accuracy of the observations 
 of these authors in the cases cited, I am of opinion that there are suffi- 
 cient reasons for not accepting as universal this mode of union between 
 retinal cells and optic-nerve filaments. I do not wish to be understood 
 as opposing the idea of the independent communication of the elements 
 of the retina with the nerve-centre, but only as claiming that generali- 
 zations as to the manner of union between retinal elements and optic- 
 nerve fibres cannot be as quickly and safely drawn as might be inferred 
 from previous writers. 
 
 The nature of the optic-nerve connection in the anterior eye of Epeira as 
 described and figured by Grenacher (79, p. 44, Fig. 18, A) is in itself suffi- 
 cient to raise doubts concerning the universality of the method claimed 
 by him ; viz., a direct prolongation of the (ultimately) posterior ends of 
 the retinal cells. Grenacher says that the peripheral fibres of the optic 
 nerve are continued without sharp limitation directly into the neighbor- 
 ing (“ herantretenden ”) retina-cells ; but the inner [axial] fibres enter 
 into the interior of the retina, where they divide into two bundles, — a 
 smaller dorsal, and a larger ventral, — which then spread out in single 
 fibres, which in turn join the ends of the corresponding [retinal] cells. 
 That which seems to me unwarranted in his conclusions is, that the axial 
 fibres are joined to the ends of the retinal cells. It is not quite clear 
 from the figure cited how this union could be easily effected. The same 
 feature, but in a more marked degree, is also shown in Mr. Locy’s sec- 
 tions of the anterior median eyes of Agelena a few days after hatching 
 (PI. II, Figs. 10, 11, and PI. V, Figs. 23, 24), and in the adult eyes (an- 
 terior median) of Theridium tepidariorum, C.K., which I have examined. 
 
 Grenacher himself called attention to a want of symmetry in the eyes 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 85 
 
 in question (Epeira), the entrance of the optic nerve being slightly dor- 
 sal ; but the significance of this fact was not perceived by him. 
 
 The same peculiarity is also noticeable in the figure of Epeira diadema 
 given by Schimkewitsch (’84, PI. II, Fig. 4), where, besides, the radiating 
 fibres of the two bundles described by Grenacher are also figured. They 
 are, however, erroneously assumed by the author to be muscle fibres.* 
 
 In these cases (and doubtless similar conditions prevail in many others) 
 the optic nerve leaves the bulb of the eye not directly opposite the lens, 
 and not always at the point which corresponds to the shortest distance 
 between the eye and the brain. It is noticeable that the place of emer- 
 gence is in some instances (Figs. 10, 11, 20, 23, 24, n. opt.) very near to 
 the superficial border of the retina. If the opinion held by Grenacher 
 were to be substantiated in these cases, we should expect to find the major 
 part of the optic-nerve ramifications bending abruptly backward as soon 
 as they had entered the cuticula of the bulb, and forming behind the 
 bulb a kind of nerve-fibre sheath, which would gradually become thinner 
 
 * Schimkewitsch (p. 14) finds in these nerve-fibres the sphincter described by 
 Leydig. .“But,” he adds, “I have never seen that this sphincter takes its origin 
 from the integument, as claimed by Grenacher. . . . The action of the muscle as a 
 constrictor has been observed by Leydig ; but I am not able to understand how the 
 muscle would be able to change the visual axis, [even] if it were attached to the in- 
 tegument, as Grenacher supposes, since the cornea-lens is quite immovable.” 
 
 Leaving aside the question as to the accuracy of Grenacher’s conclusions about a 
 change in the direction of the visual axis, it must be sufficiently evident upon com- 
 paring the figures given by the two authors (Grenacher, 79, Fig. 18, M, M') that the 
 structures in question have nothing in common. Whatever may be the effect of its 
 contraction, the muscle figured by Grenacher encircles the eye, lying, as he expressly 
 states (l. c., p. 46), outside the cuticula which invests the eye ; whereas that to which 
 Schimkewitsch attributes the. function of a sphincter lies wholly within the cuticular 
 envelope. 
 
 Leydig (’58, p. 441) observed powerful, jerking contractions of the pigmented 
 layer in the eyes of several living spiders. It is a long step that Schimkewitsch 
 has to take when he says Leydig has observed the action of his supposed sphincter 
 muscle. It is the more surprising that he should have adopted such an interpreta- 
 tion of the fibres, when a much more natural one had already been given, as above 
 quoted, by Grenacher. He adduces no argument to prove the contractile nature of 
 the fibres, and, it would seem, must have arrived at his conclusion rather hastily, 
 and without the remembrance of Grenacher’s description of the optic nerve. 
 
 If it were necessary to strengthen with special arguments the natural interpreta- 
 tion given by Grenacher, one might insist — in addition to the observed direct 
 continuation of the fibres with the optic nerve — upon the absence of transverse 
 striations, and a susceptibility to staining reagents like that of nerve-fibres rather 
 than that of more deeply staining inuscle-fibres. 
 
86 
 
 BULLETIN OF THE 
 
 towards the side opposite the place of entrance, as the fibres one after 
 another effected a union with the basal ends of the retinal cells. But 
 nothing of the kind seems to exist in either of the cases cited or in those 
 which have come under my own observation. The fibres, instead of 
 following the surface of the bulb beneath the post-retinal membrane 
 (“ sclera ”), traverse directly the retinal layer in several groups.* * * § Their 
 connection with the retinal cells, however, is not — as one would fairly 
 infer from Grenacher’s account — at the posterior (originally free) ends, but 
 rather with the anterior parts of the cells, t — at least it may be designated 
 as certainly pre-nuclear. J The evidence of this rests partly upon the 
 position and general direction of the nerve-strands in a region behind 
 the forming bacilli and in front of the nuclei, and partly on the modifi- 
 cation of form which many of the retinal cells and their nuclei exhibit 
 in consequence of this relation. The elongation of the anterior ends of 
 the nuclei § is so evidently a result of the peculiar position and connec- 
 tions of the nerve-filaments (PI. V, Figs. 23, 24) that I cannot for a 
 moment think it attributable to any other cause. 
 
 There is also reason to believe that a similar condition exists in the 
 eyes of the “ post-nuclear ” type, and that the nerve-fibres which appear 
 to emerge from the deep surface of the retinal layer really pass around 
 the margins of the tapetum (somewhat as in Pecten), to join the now super- 
 ficial ends of the retinal cells. This in turn increases the probability of 
 the inversion of the retina in “ post-nuclear” eyes. (Compare Explanation 
 of Figures.) 
 
 I shall return to a consideration of the manner in which this interest- 
 ing connection is brought about in the pre-nuclear eyes, and of the prob- 
 
 * Since the groups do not necessarily lie in the plane of the section, they are not 
 all seen in one section ; but I am satisfied, from the examination of several cases, that 
 such a division of the fibres usually takes place. 
 
 + That such a method of nerve-connection with sensory cells is not wholly with- 
 out parallel, will he evident upon comparing the conditions here described with the 
 account of the termination of the radial nerve of the cochlea in mammals as given 
 by Lavdowsky (76, pp. 529, 530, Taf. 35, Figg. 10 A, 10 C). 
 
 + The connection here (after inversion) called “pre-nuclear” is of course equiva- 
 lent to a post-nuclear connection before inversion. The nerve-fibre, which I believe 
 reaches the nucleus itself, therefore retains as nearly as possible its original method 
 of connection with the retinal cells ; i. e., it approaches the nucleus from what w r as 
 originally the deep end of the hypodermal cell. 
 
 § The nuclei present no such modification of form in the earlier stages of the 
 formation of the eye, before the appearance of the optic nerve, but are similarly 
 rounded at both ends. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 87 
 
 able cause of its existence, in the following portion of the paper, devoted 
 to theoretical considerations. 
 
 What have been the causes, and what is the real significance, of the 
 hypodermal infolding accompanying the formation of ocelli 1 
 
 The following speculations are an attempt at the solution of these prob- 
 lems. It is not supposed that they offer a complete explanation of the 
 phenomena, but it is hoped that they may stimulate criticism on the part 
 of future observers, which will ultimately lead to a satisfactory elucidation 
 of the conditions. 
 
 The case of ocelli with pre-nuclear bacilli , in which there has been an 
 involution with inversion of the retinal layer, will be considered first. 
 One meets here a problem similar to that which is encountered in 
 endeavoring to explain the origin of the retina in vertebrates. If the 
 retina in the ancestors of vertebrates was a patch of ectoderm in its 
 normal position, then there are two questions to be settled in explain- 
 ing the present condition. One is, What could have been the advantage 
 in the assumption of the inverted position of the retinal cells in rela- 
 tion to the direction of the waves of the light-stimulus 1 The other, 
 How could the retina have remained functional during the whole of 
 the involution-process which accompanied the formation of the neural 
 tube? 
 
 Here, in the “ pre-nuclear eyes,” the same questions arise : If the retina, 
 which is formed by a process of inversion, was once a normally located 
 portion of the “ hypodermis,” how could it have remained functional 
 during the process of inversion, and what could have been the motive 
 which led to the inversion ? 
 
 The question of the immediate cause may perhaps be more readily 
 answered in the case of vertebrates than here ; for in vertebrates the ulti- 
 mate inversion of the retinal cells is only a necessary consequence of a 
 much more fundamental change, — the involution of the central nervous 
 system, — which may find its adequate explanation in something ( e . g. the 
 protection of the nervous system) very remotely, if at all, connected with 
 the functions of the eye. But in the case of spiders’ eyes it is different. 
 The retina is formed comparatively late in embryonic life, and, so far 
 as is yet known, independently of any such neural infolding. Unless, 
 then, the retinal inversion can be connected with the formation of the 
 cephalic portion of the central nervous system, the cause of this remark- 
 able complication must be sought in some advantage secured to the eye 
 itself. It is not necessary that the motive be one that is constantly oper- 
 
88 
 
 BULLETIN OF THE 
 
 ating to produce the original result ; it is only necessary to show how this 
 influence once operated to bring about the end achieved. 
 
 Protection to the retina may have been one of the objects gained ; but 
 it is not easy to see how that is better accomplished by an inversion than 
 by a simple depression of the retinal area. 
 
 The influence of the light itself, especially the direction of the rays 
 which gain access to the retinal cells, may have been more important. 
 Either a gradual shifting in the position of the original lenticular thicken- 
 ing of the cuticula, or the development of a new lenticular region, may 
 have been the means by which this new and transforming influence was 
 brought to bear on an already existing retina; for unless the involu- 
 tion can be connected with the formation of the central nervous system, 
 this complicated ocellus must be imagined to have been developed from 
 a more simple functional eye. 
 
 It is assumable that this primitive eye was composed of a single layer 
 of modified hypodermal cells occupying the normal position (perpendicu- 
 lar) in relation to the surface of the head,* that the proximal (deep) 
 ends of the sensory cells were in connection with the nervous centre by 
 means of nerve-fibres, and that it was in the distal (free) ends of the 
 cells that the bacilli were formed.t 
 
 * Either these cells at first all shared in the secretion of the corneal lens, or else 
 this function was confined to a portion of the cells/ evenly distributed over the sensi- 
 tive area, only isolated cells being modified into sensory elements. The latter con- 
 dition is at present realized in the eyes of many of the invertebrates, and one might 
 at first be inclined to regard it as the result of a differentiation accomplished in the 
 cells of the sensitive area during its development as an organ of special sense. If 
 that were the most reasonable assumption, it would become very doubtful whether 
 the ocelli of Arthropods have ever passed through any such stage of differentiation, 
 unless the lateral eyes of scorpions prove to be truly monostichous, as claimed by 
 Lankester and Bourne. But the results of modern inquiries into the origin of sen- 
 sory organs have made it more and more probable that this differentiation of epithe- 
 lium into sensory cells and indifferent cells (“ Stutzzellen ”) is to be carried back to a 
 period which antedates the formation of all special-sense organs. In the light of this 
 important generalization a sensitive area, composed exclusively of sensory cells, must 
 be looked upon as a highly modified condition resulting from the atrophy or dis- 
 placement of the indifferent cells, or, possibly, their gradual conversion into sensory 
 elements. 
 
 t There is nothing to favor the supposition that these ocelli were developed from 
 retinal cells which contained bacilli at their deep ends before the process of inversion 
 began, for there is not a single case among the invertebrates in which such a condi- 
 tion exists, where other complications do not make it probable that there has been an 
 inversion. The principal cases of “ post-nuclear ” bacilli are found in the dorsal 
 eyes of Onchvdium, and the eyes at the margin of the mantle in certain Lamellibranchs 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 89 
 
 I know of no example among Arthropods in which this condition is 
 strictly realized, provided the still problematic development of the lateral 
 eyes of scorpions is not taken into consideration. Even the simplest are 
 considerably modified. 
 
 It is not certain along what line of modifications the eye with inverted 
 retina has been developed. Not all triplostichous eyes are necessarily 
 like the pre-nuclear type in spiders. A triplostichous condition might 
 be produced by a simple depression of the retinal area and a subsequent 
 closing together of the surrounding hypodermis, ultimately giving rise to 
 an inner and an outer corneal layer, as in many of the mollusks. The 
 condition of the eye in Peripatus suggests such a method of formation in 
 this primitive Tracheate. 
 
 It is not unreasonable to suppose, however, that all the triplostichous 
 eyes have passed through a condition of simple sac-like depression, in 
 which originally the retinal cells are not inverted, and that from this 
 simple condition two others have originated, — (1) By a closing together 
 and fusion of the lips of the original depression a more or less volu- 
 minous cavity (filled with a so-called lens) is formed in front of the still 
 uninverted retina and behind a double layer of hypodermis, — a triplos- 
 tichous condition such as is realized in Peripatus (Carriere, ’85, p. 124 ; 
 Kennel, ’86, p. 32, Taf. Ill, Fig. 34). (2) By an approximation of the 
 
 walls of the depression its cavity is reduced to an axial fissure ; the cells 
 corresponding to the “ outer cornea ” in the first case become the “ lenti- 
 gen ; ” those corresponding to the “ inner cornea ” become a “ vitreous ; ” 
 the retina still remains uninverted, — a monostichous (potentially tri- 
 
 (Pecten, Spondylus, etc.). It seems to me there is little doubt but that in both 
 these cases there has been at some time an inversion of the retinal area. The peculiar 
 course of the optic-nerve fibres and their method of joining the sensory cells (at their 
 anti-bacillar ends), as well as the position of the bacilli, point to this conclusion. 
 They are not, it will be observed, in any sense monostichous eyes. 
 
 The eyes of Planarians, also, may possibly be interpreted as having bacilli of the 
 “post-nuclear” type; but here, too, the course of the nerve-fibres points to an in- 
 version of the retina, and, in addition, it is doubtful if the eye is monostichous. 
 
 Postscript. — Although Dr. Patten informs me that there is no inversion of the 
 retina in the case of Pecten, I believe that an inversion at some time during the 
 phylogeny of the eyes of Pecten has been the cause of their present condition. But 
 whether there is an inversion during the ontogeny of Pecten or not, the question im- 
 mediately before us is little affected by it ; for eyes like those of Pecten are already 
 too complicated to have served as the primitive condition of the triplostichous ocelli 
 of Arthropods. It may therefore still be safely assumed that the cells of the primitive 
 ocelli had pre-nuclear bacilli. 
 
90 
 
 BULLETIN OF THE 
 
 plostichous) condition such as is realized in Dytiscus as described by 
 Grenadier . 
 
 The triplostichous eye with inverted retina may have begun, like that 
 with the normal retina, in the sac-like depression ; but it has probably 
 passed through a stage in which there was an early obliteration of the 
 original cavity, as in the second case above. Perhaps the eye in Dytis- 
 cus or in some of the Myriapods is the nearest approach — in the hith- 
 erto described ocelli of Arthropods — to this earlier condition. Here, at 
 any rate, none of the cells in the retinal area retain the function of se- 
 creting cuticula, and the area is therefore relieved from the necessity of a 
 fixed topographical relation to the lens, — an important consideration in 
 the development of the theoretical views which follow. 
 
 Of the two possible ways suggested, in which a change due to the ac- 
 tion of the light may have been brought about, I will first consider that 
 which assumes, — (1) that light gained access to some portion of the 
 periphery of the eye-bulb through other parts of the cuticula than that 
 which originally served for the transmission of light ; and (2) that this 
 light from a new direction operated to develop a practically new eye out 
 of a 'portion of the already existing retinal cells. 
 
 To make this hypothesis more intelligible, one may begin with the con- 
 crete case of the anterior median eye in spiders. (Compare Figs. 25, 26, 
 30—32.) It may be assumed that the eye from which this “ pre-nuclear ” 
 type was produced had the form and position * indicated in Fig. 30 ; 
 that the light which hitherto affected the retina entered through the cuti- 
 cular lens (Ins.), in the direction indicated by the arrow, A ; but that, after 
 the development of the eye up to a certain stage, light also gained access 
 in the direction of the arrow P through another region of the cuticula. The 
 same influences which originally tended to the production of an eye un- 
 derneath the cuticular region (Ins.) may now have operated on that por- 
 tion of the cells of the already formed retina which were directed towards 
 the new lens; and in time these retinal cells may have developed the 
 characteristic bacillar structures at the ends of the cells nearest to this 
 new lens (Ins'. Fig. 31). 
 
 * This primitive eye has been assumed to have occupied the angle of the forehead, 
 as at present (Fig. 11), and to have had its axis inclined to the horizon at an angle 
 of 45°. It might have been parallel with the horizon, or even more nearly perpen- 
 dicular to it, without having materially affected the problem. If, however, it had 
 been perpendicular, the newly admitted light would have been in front, and the new 
 lens in front of, instead of behind, the original lens, and as a consequence the 
 involution would have been directed forward instead of backward. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 91 
 
 The advantages of vision in the new direction may have been due to 
 the more favorable relation of the cells to the direction of the newly ad- 
 mitted light as compared with that which came along the original course, 
 inasmuch as the latter was nearly perpendicular to the axes of the retinal 
 cells (and therefore not favorable, upon Grenacher’s theory, to the per- 
 ception of distinct images), whereas the former would be parallel to the 
 axes of some of the retinal cells, and therefore competent to furnish (upon 
 the development of the lens) a more distinct image. 
 
 Any advantage of this nature would gradually lead to an extension of 
 the favorably located portion of the retina, and even to any modification 
 of the form of the layer as a whole whereby it should be brought into 
 still more favorable optical relations to the newly admitted light. This 
 might be accompanied by a gradual regressive modification of parts of the 
 retina not so situated as to be capable of profiting by light entering from 
 the new direction. In this way the originally symmetrical condition 
 would be replaced by conditions more and more unsymmetrical.* 
 
 Thus in time a new lens might be formed and the old one atrophy ; 
 one region of the original retina might become converted into a new retina 
 with new bacilli at the deep ends of the cells, and the cells of the remaining 
 regions sink from their function of percipient elements to that of simple 
 pigment-cells. The disappearance of the original bacilli in the persist- 
 ently functional area of the original retina might be complete, or only 
 partial. 
 
 A strong indication that the anterior median eye in Agelena previously 
 existed in the condition of a functional monostichous eye, the deep ends 
 of whose retinal cells were directly continuous with the optic-nerve fibres, 
 is found in the relation of the optic nerve to the present eye, and espe- 
 cially in its relation at different stages of its growth. Without some such 
 assumption the peculiar connection of the optic nerve with the retina 
 would remain apparently inexplicable ; but upon this assumption the 
 conditions appear as a natural consequence of the changes accompanying 
 involution. In the earliest stage in which the connection of the optic 
 nerve with the retina has been figured, before the appearance of the 
 bacilli (Figs. 1, 2), the nerve-fibres emerge from the outer and posterior 
 
 * Grenacher has shown that there is an unsymmetrical condition of the retinal 
 cells and their bacilli in the anterior median eyes of Lycosa. (See Grenacher, 78, 
 Taf. III. Fig. 22 A, and text, p. 48.) This must doubtless be regarded as a secondary 
 differentiation, — i. e. as evolved after the infolding and from a more symmetrical 
 triplostichous condition ; but it is instructive as indicating the possibility of 
 regressive changes due to the altered functional requirements imposed on the retina. 
 
92 
 
 BULLETIN OF THE 
 
 border of the retinal infolding immediately underneath the “ lentigen .” 
 Upon the development of the bacilli the fibres emerge farther and farther 
 back from the surface of the head, until finally a considerable interval 
 separates the nerve from the lentigenous cells (Figs. 10, 23, 24, 20). 
 This is exactly what might have been expected if the eye had been de- 
 veloped phylogenetically by the inversion of a layer of cells which were 
 already in functional activity before the process of inversion began , and the 
 deep ends of which were connected with the optic nerve.* It is also consist- 
 ent with the formation at the deep ends of the retinal cells of secondary 
 bacilli , which may be regarded as the physical cause of a recession (onto- 
 genetic) of the place where the optic nerve emerges. 
 
 If the fibres of the optic nerve were originally joined to the proximal 
 ends of the sensory cells, it is natural that they should have retained 
 this connection for a longer or shorter period after the beginning of the 
 involution which finally inverted the retina. The nerve-fibres are ulti- 
 mately connected to post-bacillar parts of the retinal cells. There can 
 be no doubt that the formation of the bacilli is a progressive process ; 
 they are not begun throughout their whole, extent at the same time, but, 
 beginning at the originally deep ends of the retinal cells, they increase 
 in length by successive additions to the ends of the rods which are di- 
 rected towards the nuclei. It is equally evident that there is a gradual 
 shifting in the region to which the nerve-fibres are distributed, so that 
 this region is always post-bacillar. Nothing seems more reasonable, in 
 view of these facts, than that the secondary condition of the nerve-fibre 
 distribution results from the gradual development of bacilli in the region 
 of the original distribution, whereby the nerve-fibres are excluded from 
 their primitive mode of connection with the sensory cells. If this is the 
 true explanation of the cause of the shifting of the nerve-fibres, it offers 
 a valid argument in favor of the secondary (i. e. recent) origin of the pre- 
 nuclear bacilli. 
 
 But if these bacilli are not the original rods, what has become of the 
 latter h Were it not for this marked influence of the developing bacilli 
 on the course of the optic-nerve fibres, one might have assumed that the 
 new bacilli were not absolutely new structures, but only the original 
 bacilli migrated from one end of the retinal cells to the other, pari passu 
 with the process of retinal inversion, being therefore new only in the 
 sense that they occupy new positions. Such a view seems, for the rea- 
 
 * This explanation of the peculiar position of the optic nerve as it emerges from 
 the eye was first suggested to me by Dr. Whitman. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 93 
 
 son assigned, untenable. It is more likely that the primitive bacilli 
 have, with loss of function, atrophied, and that consequently the pre- 
 nuclear bacilli of inverted retinae are not homologous with the pre-nuclear 
 bacilli of uninverted retinae. 
 
 It is possible that the primitive bacilli do not in all cases completely 
 atrophy. There are at least certain problematic bodies in the retina of 
 scorpions which may find an explanation in connection with this hy- 
 pothesis. I have in mind the structures which Graber described for 
 Androctonus as “ posterior nuclei,” — subsequently claimed by Grenacher 
 (’80, pp. 423, 424) to be only peculiar, highly refractive bodies, — and 
 the structures which Lankester and Bourne (’83, pp. 185, 193) have 
 seen in the central eyes of Euscorpius Italicus, and have described under 
 the name of “ phaospheres” 
 
 It may be an obstacle that the “ phaospheres ” are also sometimes 
 found in front of the nuclei, and further, that the rhabdomeres are not 
 formed within, but at the surface of the retinal cells. The variability in 
 the relation of the phaospheres to the nuclei may be regarded as an ab- 
 erration rendered possible by the loss of function, rudimentary structures 
 being more liable to vary than such as are at the height of their func- 
 tional activity. (Compare Darwin, Origin of Species, chap, v.) The 
 second obstacle is probably not of great importance, since it still remains 
 to be shown that intra-cellular and extra-cellular rod-like structures are 
 essentially different. Besides, it is conceivable that the 'primary bacilli 
 may have been intra-cellular, while the secondary bacilli are extra- 
 cellular. 
 
 A more serious obstacle arises from the fact that similar structures 
 (phaospheres) also exist in the lateral eyes of Euscorpius Italicus (Lankes- 
 ter and Bourne, ’83, p. 185), in the case of which, evidences of an in- 
 folding and inversion are not so satisfactory as with the median eyes. If 
 the lateral eyes do not result from an infolding and inversion of the re- 
 tinal layer, this explanation of the “ phaospheres ” would go for little or 
 nothing, since their presence in the lateral eyes could not be explained 
 on the same hypothesis. I have endeavored, however, to show (p. 59) 
 the great probability of an inversion of the retina in these lateral eyes, 
 and must await a satisfactory disproval of that opinion before allowing 
 this possibility to outweigh the considerations in favor of the explanation 
 of phaospheres which is here attempted. 
 
 In the above hypothesis regarding the origin of “ pre-nuclear ” ocelli, 
 the two points demanding explanation have been kept in view, — the 
 continuance of functional activity, maintained by means of the simul- 
 
94 
 
 BULLETIN OF THE 
 
 taneous operation of light from two directions, and the advantage to 
 vision secured through the more favorable relation of the retina to the 
 direction of the newly admitted light ; and there are, in addition, some 
 hitherto unexplained anatomical features which gain by this hypothesis a 
 reasonable explanation. 
 
 Changes similar to those imagined above might possibly have accom- 
 panied a gradual shifting in the position of the original lens (compare 
 Figs. 25-29), rather than the substitution of a new lens. Such a shift- 
 ing, from whatever cause, might have concentrated the light upon one 
 portion of the retina at the expense of remaining parts. The less-favored 
 parts might have been degraded in functional importance, and might 
 have atrophied. So far not much difficulty would be encountered in 
 appreciating the assumed conditions ; but how the light, acting through 
 the original, though shifted, lens, could have afforded any advantage 
 which would have been competent to initiate an inversion, or to carry 
 forward such a process when once begun, is not so easy to comprehend. 
 
 In considering the development of “ post-nuclear ” eyes, however, it 
 will be possible to show how such a migration on the part of the lens 
 may have been an important factor in the process of inversion. 
 
 The structure of ocelli with “ post-nuclear ” bacilli , both in the adult 
 condition and in such stages of development as are at present known, is 
 only conditionally referable to what has been assumed above as the 
 primitive state of the eye, and the development is not so easily explained 
 as that of eyes with pre-nuclear bacilli. 
 
 The difficulty depends partly upon the uncertainty as to the exact 
 changes through which the eye passes in its ontogeny. Further study 
 will unquestionably soon determine this in a more satisfactory way. 
 But even when it has been definitely established that the retinal layer 
 either does or does not become inverted , it will not even then follow that 
 the relations of the two types to each other, and to a primitive antece- 
 dent condition, will at once become evident. One naturally looks for a 
 development of both types from a common origin, and, for a time at 
 least, along a common line. 
 
 If the retina is inverted, a general comparison with the retina of “pre- 
 nuclear” eyes becomes possible ; but the bacilli cannot be strictly homol- 
 ogous, since they do not occupy equivalent ends of the retinal cells. 
 
 If the existence of an inversion were established, a common line of 
 development could be fairly maintained ; the “ post-nuclear ” type must 
 then be considered less modified, as far as regards the retina, than the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 95 
 
 “ pre-nuclear ” type: the “ post- nuclear ” eye without tapetum (if such 
 exist) would, to a certain extent, represent a common antecedent of both 
 types, one of which might have been produced by the substitution of 
 new (pre-nuclear) for old (now become post-nuclear) bacilli, and the other 
 by the addition of a tapetum without change in the bacilli.* 
 
 On theoretical grounds this seems to be the more probable phylo- 
 genetic course ; but upon this assumption — that there is an inversion of 
 the retina — the explanation of the motive to the infolding offered above 
 for “ pre-nuclear” eyes could not be simply extended to eyes of the post- 
 nuclear type, since the cause of the development of new bacilli in one 
 case, and their non-development in the other, would then be left 
 unexplained. 
 
 There are grounds for supposing that the retention of the original bacilli 
 in “ post-nuclear ” eyes is due to the development of a tapetum , — a subject 
 to which I shall return directly. 
 
 If the retina is not inverted, even a general comparison with the retina 
 of “ pre-nuclear ” eyes becomes difficult ; for the involution in that event 
 affects only the tapetal and post-retinal layers, not the retina itself. In 
 that case, too, the primitive condition of the eye must be assumed to 
 have been unlike any primitive conditions at present known ; viz., with 
 bacilli at the deep ends of the hypodermal (sensory) cells. f 
 
 If there has been no inversion of the retina, the obstacles to an expla- 
 nation of the development are considerable. What can have been the 
 cause of an infolding which involves only the tapetal and post-retinal 
 layers, or of the peculiar outfolding between retinal and tapetal layers ? 
 I have been unable to form any idea of how this condition could have 
 been produced from a primitively monostichous retina with joo^-nuclear 
 bacilli, consistently with the retention of the functional activity of the 
 eye during all the changes. Neither has it been possible to comprehend, 
 upon the same assumption, how the optic nerve came to emerge from the 
 post-retinal layer. 
 
 * But if the retention of the original bacilli in the inverted retina was at first 
 directly dependent on the existence of a tapetum, this “ common antecedent ” con- 
 dition (without tapetum) would not have been realized, except as the result of a re- 
 gressive modification of the “post-nuclear” eyes, involving the disappearance of the 
 tapetum. 
 
 t It is not entirely impossible that eyes may have arisen which in the primitive, 
 uninverted condition possessed post-nuclear bacilli ; but it is very improbable that 
 such was the case, because we have not at present, in any animal, a single instance 
 of monostichous eyes in which that condition obtains. (Compare the footnote to 
 pp. 88, 89.) 
 
96 
 
 BULLETIN OF THE 
 
 On the other hand, if it be assumed that there has been an inversion, 
 some of the steps in the process appear more easily explainable. Figures 
 25-29 have been drawn to indicate a possible line of development by 
 inversion, having two stages (Figs. 25, 26) common to this and the “pre- 
 nuclear ” type. The direct cause of the beginning of the inversion has 
 been assumed in this instance to be a gradual shifting in the position 
 of the original lens, rather than the appearance of a second lens bring- 
 ing light from a different direction. The shifting — so one may reason 
 — is accompanied by a gradual atrophy of one side of the retina, the 
 simultaneous development of a tapetum, and a peculiar modification in 
 the course of the fibres of the optic nerve which arise from the persistent 
 portion of the retina. 
 
 A lens changing in its relation to the retina, as indicated in the 
 figures, might easily allow a part of the eye to remain functional during 
 the process of inversion ; but alone it would afford no explanation of the 
 cause of the inversion, since it would not begin to have an influence 
 (similar to that ascribed to the new lens in “ pre-nuclear ” eyes) until 
 the change in the direction of the axis of the retinal depression (the 
 thing to be explained) had become sufficient to make some of the retinal 
 cells parallel to the axis of the lens. It must be admitted, then, that, 
 alone, this shifting of the lens is not an adequate explanation. It may 
 be, however, that the formation of a tapetum is the cause, in con- 
 nection with the shifting of the lens, both for the atrophy of one side 
 of the retina, and the inversion of the other side. 
 
 If the formation of a reflecting structure (tapetum) were accompanied 
 by a slight shifting on the part of the lens, the tapetum would practi- 
 cally cut off the light from one face of the retina and reflect it to the 
 sensitive elements of the opposite face. That would result in an atrophy 
 of the part robbed of light, and an increased development of that on 
 which additional (reflected) light fell. 
 
 The direction of the reflected rays may, in addition, have influenced 
 the shape of the retina : if the tapetum were at first a straight band 
 parallel with the original axis of the optic depression (compare Fig. 26), 
 the light falling upon it would be reflected at nearly equal but very 
 oblique angles, no matter upon what portion of the band it fell. If, how- 
 ever, the deep portion of the band became slightly curved (concave towards 
 the persistent portion of the retina), — as would be altogether natural 
 with an increase in the thickness of the retinal laj^er on one (functional) 
 side, and a corresponding decrease in thickness on the other (atrophied) 
 side, — the rays reflected from the curved portion of the tapetum would 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 97 
 
 fall upon the sensitive surface more nearly perpendicular to it than they 
 would have done without such a curvature. The advantage of this, even 
 if an increase in the intensity of the light were the only end achieved, 
 is evident ; but, in addition to the increased illumination afforded by this 
 part of the tapetum, it is probable that the rays of reflected light would 
 take directions more nearly parallel with the axes of the corresponding 
 retinal cells (Fig. 27), and that thus conditions favorable for more distinct 
 vision — perhaps even for the perception of images — would be realized.* 
 Such an advantage once secured at the deep end of the tapetum, it is 
 easy to appreciate how an increase in the extent of the curved portion of 
 the band would enlarge the more successfully reflecting area, thus en- 
 hancing the total effect of the light, and possibly affording a more exten- 
 sive (reflected) image. Once begun, this process would not cease until 
 it had involved the entire eye. 
 
 This, it seems to me, would be sufficient to explain the curvature 
 actually found in the adult eyes, where the retinal cells are all perpen- 
 dicular to the tapetum, and would besides afford an explanation of the 
 retention of the original bacilli at the (primitively) free ends of the cells. 
 
 It is no longer probable that the iridescent scales of the tapetum are 
 referable to the cuticular secretions of the hypodermis. It is more likely 
 that the tapetum is formed from cells which grow from the apex of the 
 original retinal involution into the cavity formed by that involution, and 
 that they take the form of an outfolding. Whether the tapetal cells, 
 phylogenetically considered, originally constituted a distinct portion of the 
 hypodermis embracing the area corresponding to the apex of the subse- 
 quent involution, it is at present impossible to decide ; but it seems less 
 probable than that they should have been gradually differentiated from 
 a portion of the retina after the involution (but not the inversion) had 
 begun. It may even be imagined that the tapetal scales in some way 
 represent the metamorphosed bacillar elements of the cells from which 
 they are developed, although I know of no direct evidence of it. Unless 
 they are formed from cells which have previously possessed the function 
 of retinal elements, their source and the cause of their appearance will be 
 still more problematical. 
 
 There is reason to suppose that the course of the optic-nerve fibres 
 through the post-tapetal layer is a secondary condition. If — as is prob- 
 
 * That this curvature finally became so great that the light was reflected outward 
 through the lens, and thus served to help in the illumination of outside objects, 
 does not necessarily interfere with this assumed primitive function of the tapetum. 
 
 VOL. XIII. — NO. 3 . 7 
 
98 
 
 BULLETIN OF THE 
 
 able from previously presented arguments — these post-nuclear eyes were 
 developed from functional monostichous eyes, the deep ends of whose reti- 
 nal cells were directly connected to the nerve-fibres, the fibres should 
 retain their connection with the deep ends of the cells, and should ex- 
 hibit, even in advanced stages, a course similar to that pursued by the 
 nerve in “ pre-nuclear ” eyes at an early stage (Fig. 1). Instead of that 
 they traverse the post-retinal layer, which may have acquired the func- 
 tions of an optic ganglion in addition to its duties as a pigment-layer. 
 The narrowness of the tapetal band makes it probable that most of the 
 nerve-fibres pass around its margins in making their way from the retina 
 to the post-retinal layer. Although this is a modification of, it is not 
 fundamentally different from, the condition in pre-nuclear eyes. In the 
 latter the fibres are collected into a single bundle at the deep end of the 
 pocket, and therefore emerge at the posterior border of the eye only ; in 
 the post-nuclear type the fibres pass over the lateral margins of the pocket 
 (and the outer edges of the tapetum) as well as its deep end (compare 
 Figs. 28, 29) before they are joined into a single trunk. The only real 
 difference between the two is in the share which the “ post-retinal ” layer 
 appears to take in the formation of eyes of the “ post-nuclear ” type. It 
 is conceivable that this condition may have been brought about gradually 
 during the stages of inversion, — that the nerve-fibres of the aborted half 
 of the eye, instead of undergoing complete atrophy, acquired relations 
 with the persistently functional parts of the retina and their nerve-fibres, 
 and thus influenced the course of the latter. 
 
 Cambridge, June 22, 1886. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 99 
 
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 Carridre, J. 
 
 ’ 85 . Die Sehorgane der Thiere vergleichend-anatomisch dargestellt. Miinchen 
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 Dug&s, A. 
 
 ’ 36 . Observations snr les Araneides. Ann. des Sci. nat., 2 e ser., Zool., Tom. 
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 Froriep, A. 
 
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 Kennel, J. 
 
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 Lankester, E. R., and A. G. Bourne. 
 
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MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 101 
 
 EXPLANATION OF FIGURES. 
 
 LETTERS. 
 
 The following letters are used to designate respectively : — 
 
 A. 
 
 = Anterior. 
 
 mu. = Muscle. 
 
 bac. 
 
 = Bacillus. 
 
 mu'. = Muscle, cut cross-wise. 
 
 en c. 
 
 = Brain. 
 
 n. op. = Optic nerve. 
 
 fis. tap. 
 
 = Tapetal fissure. 
 
 P. = Posterior. 
 
 gi. 
 
 = Poison gland. 
 
 p r. = Post-retinal cell-layer of the eye. 
 
 Ing. 
 
 = “ Lentigen, ”= “ Vitreous ” 
 
 pr r. = Pre-retinal cell-layer. 
 
 
 (auct. ). 
 
 r. = Retina. 
 
 Ins. 
 
 = Cuticular lens. 
 
 tap. = Tapetum. 
 
 Figures 1-24 were all drawn, with the aid of the Oberhauser camera, to the same 
 scale (X 515 diam.) from balsani-mounted sections cut from objects stained in alco- 
 holic borax carmine (Grenacher’s) and imbedded in paraffine. Figures 1-16 and 
 18-24 relate to Agelena ncevia ; fig. 17 to Theridium tepidariorum , C. K. Figures 
 1-16, 18, 19, 23, 24 are from preparations by Mr. W. A. Locy; figs. 17, 20-22 from 
 preparations by Mr. G. H. Parker. 
 
 PLATE I. 
 
 Figs. 1-7. Median faces of successive sagittal sections from the left half of the 
 head of a young Agelena ncevia , about four days after hatching. The position of the 
 portion of the brain nearest to the eyes is indicated at en c. 
 
 Fig. 1. The plane of the nearer surface of the section passes through the middle of 
 the anterior median eye, cutting its optic nerve obliquely. The latter emerges from 
 the retina immediately beneath the “lentigen.” The distal ends of the elongated 
 nuclei in the “lentigen ” are scarcely discernible, not being sharply marked off from 
 the surrounding substance, nor so deeply stained as at their proximal ends. Behind 
 the anterior eye, and beyond its optic nerve, are the muscles which separate the pos- 
 terior median eyes, and then pass obliquely forward and downward, in part beneath 
 the anterior median eye, in part between it and the anterior lateral eye (compare 
 Figs. 2 and 3). Beyond these muscles, and partly obscured by them, is the layer of 
 cells composing the median wall of the posterior median eye. The muscle-cells are 
 traceable through the “ hypodermis ” to the cuticula at the surface of the head. 
 
 Fig. 2. This section embraces a large portion of the lateral wall of the anterior 
 median eye, and the middle region of the posterior median eye. In the latter there 
 are four well-marked regions, — post-retinal, tapetal, retinal, and pre-retinal. 
 
 Fig. 3. The lateral wall of the posterior median eye is embraced in this section, so 
 
102 
 
 BULLETIN OF THE 
 
 that the four regions are not as distinctly shown as in Fig. 2. In the post-retinal 
 region (anterior margin of the eye) there is a single cell which differs from the ordi- 
 nary liypodermal cells and resembles the cells with spherical nuclei found through - 
 out the body-cavity. I am unable to say whether it is a hypodermal cell preparatory 
 to division, or an intrusive element of different origin. The region in front of this eye 
 embraces three successive layers, — nearest the median plane a portion of the lateral 
 wall of the anterior median eye; beyond this, a portion of the muscles above de- 
 scribed, distinguishable by the direction of their very large (seen flat-wise?) nuclei; 
 and finally beyond the latter the median wall of the anterior lateral eye. The nuclei 
 of the latter are reproduced in 
 
 Fig. 3 a , to show more accurately the arrangement of the cells. The four smaller 
 nuclei near the middle of the group correspond in position with the faintly stained 
 nuclei of the tapetum in the following figure, and undoubtedly belong to the tapetal 
 layer. 
 
 Fig. 4. This section embraces the middle portion of the anterior lateral eye, the 
 muscular bands which pass between the post, median and post, lateral eyes, and a por- 
 tion of the median wall of the latter (post-retinal tract). The nuclei of the tapetal 
 region are arranged as though resulting from an outfolding between retinal and post- 
 retinal layers. Most of the nuclei in the anterior layer of this fold are less deeply 
 stained than those of the posterior layer. In this and the three following figures the 
 position of the poison-glands is shown at gl. 
 
 Fig. 5. The region of tangency between the lateral eyes and their mutual flatten- 
 ing is shown. The post-retinal tracts of both eyes are in contact. The tapetal cells 
 and the post-retinal tract of the anterior eye are separated by the space of the original 
 infolding. The distinction between the different tracts of the posterior eye is not 
 readily to be made out, since the section embraces a part of its median wall; but some 
 of the nuclei near the middle probably are tapetal. In the next section, 
 
 Fig. 6, the posterior lateral eye is cut nearly through the middle. The axis of the 
 eye being nearly perpendicular to the plane of the section, the latter embraces in the 
 centre only retinal cells flanked by a few tapetal cells, the latter being separated by a 
 narrow interval from the post-tapetal tract. The lateral region of the anterior eye, 
 which appears in this section, is composed principally of pre-retinal cells. 
 
 Fig. 7 shows the extreme lateral margin of the anterior (lateral) eye, and a section 
 of the posterior eye near its lateral margin. In the latter are to be seen in the centre 
 the nuclei of the retinal cells; to the left and beyond them, those of the pre-retinal 
 cells ; and to the right the post-retinal cells, separated from the retinal elements 
 by a clear space. 
 
 Fig. 8. Lateral face of a sagittal section through the anterior and posterior median 
 eyes of the left side. The tapetal tract appears to be represented by a single row of 
 nuclei. Consult the text, pp. 75-83. 
 
 Fig. 9. Median face of a sagittal section through the posterior median eye of the 
 right side. A single faintly-stained nucleus in front of the retinal nuclei apparently 
 belongs to a tapetal cell, and thus suggests the existence of a fold in the tapetal 
 layer. This opinion is strengthened by the prolongation of the other tapetal cells 
 towards the region of the supposed outfolding. The tapering ends of the tapetal 
 nuclei point to the same region, hut the lines representing the cell- boundaries have 
 not been printed with sufficient distinctness. Consult the text, pp. 75-83. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 103 
 
 • PLATE II. 
 
 Fig. 10. Median face of a sagittal section through the anterior median eye of the 
 left side, several days after hatching. The bacilli have begun to appear, and the 
 fibres of the optic nerve are seen to be distributed to the retinal cells near their 
 
 nuclei, between them and the forming bacilli. The flattened nuclei of the post- 
 
 retinal tract still indicate the presence of a distinct layer of cells behind the retina. 
 The distance between the place where the optic nerve emerges and the “ lentigen ” is 
 greater than at first. (Compare Fig. 1.) 
 
 Fi<rs. 11, 12. Two successive sagittal sections of the anterior and post, median eyes 
 of the left side (median face) and of the same age as the preceding. The tapetum 
 of the posterior eyes is already formed. 
 
 Fig. 11. The optic nerve of the post, eye communicates with the middle of the 
 post-retinal layer. A large portion of the median half of the tapetum removed with 
 this section. The optic-nerve fibres of the anterior eye (not drawn) were distributed, 
 as in Fig. 10. 
 
 Fig. 12. The general direction of the original infolding is still evident in the pos- 
 terior eye. The post-retinal layer is continuous with the hypodennis in front of the 
 eye, and the pre-retinal behind. The bacilli at the anterior edge of the eye are partly 
 obscured by an overlying portion of the tapetum, in which are to be seen the elon- 
 gated nuclei. Beneath the outline of the anterior eye the optic nerve of the anterior 
 lateral eye of the same side is cut obliquely. 
 
 PLATE III. 
 
 Figs. 13-15. Lateral faces of three successive sagittal sections, through the lateral 
 eyes of the right side. From the same spider as Figs. 11 and 12. 
 
 Fig. 13. Beyond the plate-like bacilli (consult the text, pp. 82, 83) is the tapetum 
 with its longitudinal, uneven fissure, flanked on either side by the nuclei of the reti- 
 nal cells, which are scanty immediately in front of the tapetum. 
 
 Fig. 14. Section through the bottom of the canoe-shaped tapetum of the post, eye, 
 showing the fissure and some of the nuclei of the flanking retinal cells, as well as 
 some of the narrower marginal nuclei of the post-retinal layer. In the anterior eye 
 are to be seen the post-retinal layer and some of the cells of the retina. 
 
 Fig. 15. Only the nuclei of the post-retinal layer indicate the posterior eye. The 
 tapetum of the anterior eye is cut lengthwise near its middle. All four layers are 
 distinguishable, the bacilli being already developed. 
 
 Fig. 16. Lateral face of a sagittal section through the posterior and anterior median 
 eyes (the latter in outline) from the left side of a specimen one day after hatching, 
 showing the four tracts of the posterior eye before the appearance of tapetum or 
 bacilli. 
 
 Fig. 17. Theridium tepidariorum, C. K., adult. Portion of a sagittal section — 
 lateral face — passing though the tapetum of the posterior lateral eye of the right side. 
 The outer border of the tapetum is obscured by pigment-granules of the post-tapetal 
 layer,' extending outward to the oval outline. The tapetal plates are large and quite 
 regularly arranged; the tapetal fissure is broad and irregular in outline. 
 
104 
 
 BULLETIN OF THE 
 
 Figs. 18, 19. Two successive sagittal sections of the anterior lateral eye of the left 
 side. Agelena ncevia, eight days after hatching. The tapetum does not reach the 
 external cuticula. 
 
 Fig. 18. The distant surface of the section nearly coincides with the plane of the 
 tapetal fissure, only a small portion of its left half being shown, near the muscle-fibres, 
 mu’. The greater portion of the tapetum is seen from behind, and nearly perpendicu- 
 lar to its surface. It obscures the bacilli in front of it. There are five greatly elon- 
 gated tapetal nuclei still visible. 
 
 Fig. 19. The nuclei of the “post-retinal” layer are large and closely packed. 
 Three tapetal nuclei are visible at the edges of the tapetum, which is viewed more 
 nearly edgewise than in Fig. 18. The bacilli are perpendicular to the corresponding 
 parts of the tapetum, and therefore do not all point to a common imaginary centre. 
 
 PLATE IY. 
 
 Figs. 20-22. Median faces of three successive sagittal sections through the right an- 
 terior and posterior median eyes, after the formation of pigment-granules has begun. 
 
 Fig. 20. The position of the optic nerve of the anterior eye is sketched in from 
 the two preceding sections. It will be seen that its place of emergence is still farther 
 from the “lentigen ” than in Fig. 10. The near edge of the tapetum in the posterior 
 eye shows two elongated tapetal nuclei. The pre-retinal (lentigenous) nuclei are dis- 
 tinguishable from the retinal nuclei by their deeper color, greater flatness, and more 
 granular appearance. 
 
 Fig. 21. In the posterior eye five tapetal nuclei are visible, and in the “post- 
 retinal ” layer the radiating branches of the optic nerve. The near edge of the bacil- 
 lar layer is cut; the nuclei of retina and “ lentigen ” appear as in the previous section. 
 
 ^ Fig. 22. In the anterior eye the post-retinal layer has apparently joined the retinal 
 layer. (Compare Fig. 20.) Many of the retinal nuclei present a peculiar vacuolated 
 appearance at their outward ends. The bacilli have attained a greater length than 
 in Fig. 10, their deep ends being covered by pigment-granules. The anterior eye is 
 joined by a muscular (?) band — inserted near the emergence of the optic nerve — to 
 the anterior margin of the posterior eye. In the latter the tapetum reaches the in- 
 ternal cuticula, but not the external cuticula; it contains four tapetal nuclei. Many 
 of the retinal nuclei are elongated, and taper at one end (compare the retinal nuclei 
 of the anteri&r eye in Figs. 23, 24), probably indicating the region of their connec- 
 tion with a fibre of the optic nerve. The bacilli are evidently more numerous than 
 the retinal nuclei. 
 
 PLATE Y. 
 
 Fig. 23. Median face of a sagittal section ; anterior and posterior median eyes. 
 The course of the optic nerve to the posterior eye sketched in from the preceding 
 sections. In the anterior eye the fibres of the optic nerve can be traced to the elon- 
 gated anterior ends of the retinal nuclei, which exhibit shapes dependent on this 1 
 union. 
 
 Fig. 24. Lateral face of a sagittal section through the anterior and posterior 
 median eyes of the right side of the same specimen as the last figure. The distribu- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 105 
 
 tion of the fibres of the optic nerve in the anterior eye as in Fig. 23. The post-retinal 
 nuclei still distinguishable, although considerably flattened. In the posterior eye the 
 retinal cells are separated from the “lentigen” by a distinct pre-re tinal membrane. 
 Bacilli already well developed in the anterior half of the eye. The tapetum reaches 
 to the external cuticula as well as to the internal cuticula, and contains several tape- 
 tal nuclei. The optic nerve and its radiating fibres are shown in the “ post-retinal ” 
 layer. 
 
 Figs. 25-32. Diagrammatic figures to show a possible course of development of 
 the several layers of the simple eyes from the hypodermis. Consult the text, pp. 
 87-98. 
 
 Figs. 25, 26, 30-32. To illustrate the development of eyes of the “pre-nu clear ” 
 type. The primary bacilli, at the distal ends of the hypodermal cells, are colored yel- 
 low ; the secondary bacilli, at the (originally) deep ends of the cells, red. Optic- 
 nerve fibres are yellow ; they should have been continued up to the nuclei. Arrows 
 in Figs. 30, 31, indicate the directions of the light from two different sources. (The 
 letters A. and P. were accidentally omitted.) 
 
 Figs. 25-29. To illustrate the development of eyes of the “post-nuclear” type. 
 The primary bacilli (yellow) retained on one side of the eye ; replaced on the other 
 side by the tapetum (here assumed to have been developed from the remaining half 
 of the original retina). Optic-nerve fibres, red; they should have been continued 
 up to the nuclei on the side away from the bacilli. 
 
 CONTENTS. 
 
 I. Source and Relations of Retina. page 
 
 1. Historical Summary 49 
 
 2. Discussion of the Conditions in — 
 
 a. Arachnoids 55 
 
 b. Myriapods 60 
 
 c. Insects 63 
 
 II. Pre-retinal Membrane and “Sclera” 67 
 
 III. Tapetum 72 
 
 IV. Development of “Post-Nuclear” Eyes in Agelena. 
 
 1. Posterior Median Eye . . •• 75 
 
 2. Lateral Eyes 82 
 
 V. Optic-Nerve Termination 84 
 
 VI. Theoretical: Causes of Inversion in Eyes with — 
 
 1. Pre-Nuclear Bacilli 87 
 
 2. Post-Nuclear Bacilli 94 
 
 VII. Bibliography 99 
 
 VIII. Explanation of Figures 101 
 
Mark -Simple Eyes. 
 
 Pl. II 
 
 E.L.Mark, del. 
 
 B.Meisel, lith. 
 

 
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Mark - Simple Eye s 
 
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 f is. tap. 
 
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 B.Meisel, lith. 
 

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 Mark -Simple Eyes. Pl. IV. 
 
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THE LIBRARY 
 OF THE 
 
 UN!V C OF ILLINOIS 
 
E.L.Mark, del. 
 
 B.Meisel, lith.. 
 

 OF THE 
 
i88o.] 
 
 The Structure of the Eye of Trilobites. 
 
 503 
 
 (From the American Naturalist, July, 1880.) 
 
 THE STRUCTURE OF THE EYE OF TRILOBITES. 
 
 BY A. S. PACKARD, JR. 
 
 B EYOND the fact that the entire eye of certain Trilobites, and 
 enlarged views of the outer surface of the cornea of the eye, 
 have been described and figured in Burmeister’s work on the 
 organization of Trilobites and in various palaeontological treatises 
 in Europe and North America, especially by Barrande in his 
 great work on Trilobites, I am not aware that any one has given 
 a description of the internal structure of the hard parts of the 
 eye of Trilobites. 
 
504 The Structure of the Eye of Trilobites. [July, 
 
 The full bibliography of treatises relating to these animals in 
 Bronn’s Die Classen und Ordnungen des Thierreichs, carried up 
 to 1879 by Gerstacker, contains references to no special paper on 
 this subject, and the resume by Gerstacker of what is known of 
 the structure of the eye, only refers to the external anatomy of 
 the cornea, the form of the facets and their number in different 
 forms of Trilobites. He shows that observers divide them into 
 simple and compound ; the former (ocelli) are found in the genus 
 Harpes. These “ocelli’’ are said to be situated near one another, 
 and are so large that the group formed by them can be seen with 
 the unaided eye; the surface of the single “ocellus” appears, under 
 the glass, smooth and shining. From the description and the 
 figure of the eye enlarged, from Barrande, it would seem as if 
 each eye was composed of three large simple ones; so that these 
 eyes are really aggregate, and not comparable with the simple 
 eye or ocellus of Limulus and the fossil Merostomata. 1 More- 
 over the situation of these so-called ocelli is the same as that of 
 the compound eyes of other Trilobites. 
 
 The Trilobites with compound eyes are divided into two 
 numerically very dissimilar groups ; the first comprising Phacops 
 and Dalmanites alone, and the second embracing all the remain- 
 ing Trilobites, excepting of course the eyeless genera, Agnostus, 
 Dindymene, Ampyx and Dionide. The eyes of Phacops and 
 Dalmanites are said by Quendstedt and Barrande not to be com- 
 pound eyes in the truest sense, but aggregated eyes ( Oculi congre - 
 gati). But judging by Barrande’s figures of the eyes of Phacops 
 fecundus and P. modestus (Barrande, Vol. 1, Suppl. PI. 13, Figs. 12 
 and 22), and bur observations on the exterior of the eye of an 
 undetermined species of Phacops , kindly sent us by Mr. J. F. 
 Whiteaves, Palaeontologist of the Canadian Geological Survey, 
 we do not see any essential difference between the form and 
 arrangement of the corneal lenses of Phacops and Asaphus, and 
 are disposed to believe that the distinctions pointed out by the 
 above named authors are artificial. 
 
 For my material I am mainly indebted to Mr. C. D. Walcott, 
 who has so satisfactorily demonstrated the presence in Trilobites 
 of jointed cephalo-thoracic appendages. On applying to him for 
 specimens, and informing him that I wished to have sections 
 
 1 The eyes of the fossil Merostomata (Eurypterus and Pterygotus) are evidently 
 in external form and position, judging by Mr. Woodward’s figure, exactly homolo- 
 gous with the ocelli and compound eyes of Limulus. 
 
i88o.] The Structure of the Eye of Trilobites. 505 
 
 made of the eyes of Trilobites to compare with those of Limu- 
 lus, he very generously sent me his own collection of sections of 
 the eyes of Asaphus gigas and Batliyurus longistrinosus, which he 
 had prepared for his own study, also other eyes, and especially 
 the shell or carapace of a large Asaphus, from Trenton Falls, 
 showing the eye and the projecting points of the corneal lenses. 
 Prof. Samuel Calvin kindly sent me the eyes of an unknown Tri- 
 lobite from the Trenton limestone, one specimen showing the pits 
 made in the mud by the projecting ends of the corneal lenses, 
 while to Mr. Whiteaves I am indebted for a well preserved eye 
 of Phacops. To Dr. C. A. White, Palaeontologist of the U. S. Geo- 
 logical Survey, I am also indebted for eyes of Calymene. 
 
 First turning our attention to the casts and natural sections ; 
 that of the interior of the carapace, including the molted cornea 
 of Asaphus gigas , is noteworthy. When the concave or interior 
 
 a 
 
 Fig. 1. — Section of hard parts of eye of Limulus ; c, cornea; ini, integument : pc, 
 
 pore canals; cl, corneal lens. X 30 diams. Fig. la, optical section of facets. 
 
 surface of this specimen is placed under a magnifying power of 
 fifty diameters, the entire surface is seen to be rough with the 
 ends of the minute solid conical corneal lenses which project 
 into the body-cavity. This is exactly comparable with the cast 
 shell of Limulus and its solid corneal lenses projecting into the 
 body cavity (Fig. 1). Those of Asaphus only differ in being 
 much smaller and more numerous, and perhaps rather more 
 blunt. Without much doubt the ends of the corneal lenses of 
 Asaphus, as in Limulus, were enveloped in the retina, the animal 
 molting its carapace, the hypodermis with the retina being 
 retained by the trilobite, while the corneal lenses were cast with 
 the shell. 
 
 In the specimen of the unknown trilobite from Iowa, received 
 from Prof. Calvin, the corneal lenses, seen externally, are quite far 
 apart, arranged in quincunx order ; the lenses are round and decid- 
 edly convex on the external surface. In a natural section, where the 
 
 VOL. XIV. — NO. VII. 
 
 33 
 
506 The Structure of the Eye of Trilobites. [July, 
 
 eye has been broken into two, the conical lenses are seen to 
 extend through the cornea as cup-shaped or conical bodies, and 
 are quite distinct from the cornea itself. In another broken eye 
 of the same species, the cornea is partly preserved, and two of the 
 corneal lenses are seen to extend down into and partially fill two 
 hollows or pits ; these pits are evidently the impressions made in 
 the fine sediment which filled the interior of the molted eye or 
 cornea ! 
 
 Thus in the Asaphus gigas noticed above, we have the entire 
 inside of the cornea with the cone-like lenses projecting from the 
 concave interior ; while in the last example we have the impres- 
 sions made by the cones in the Silurian mud which silted into the 
 cornea after the trilobite had cast its shell. 
 
 Farther evidence that the trilobite’s eye was constructed on the 
 same pattern as that of the living horse-shoe crab is seen in the 
 sections made by Mr. Walcott. We will first describe, briefly, 
 
 the eye of Limulus. Fig. I 
 represents a section through 
 the cornea of Limulus ; c , the 
 cornea, which is seen to be a 
 thinned portion of the integ- 
 ument; pc , indicates one of 
 the nutrient or pore canals, 
 which are filled with connec- 
 tive tissue extending into the 
 integument from the body 
 cavity; cl , is one of the series 
 of solid conical corneal lenses. 
 These are buried partly in the 
 black retina, and the long 
 slender optic nerve just before 
 
 Fig. 2 .— Section through the eye of a tril- reaching the eye subdivides, 
 obite; lettering as in Fig. i. x 5° diams. sending a branch to each facet 
 
 or cornea, impinging on the lens. Fig. I a represents a vertical 
 view of the corneal lenses or facets, magnified fifty diameters, as 
 seen through the transparent cornea. It will be seen that they 
 are slightly hexagonal and arranged in quincunx order ; their 
 external surface is flat, though that of the ocelli is slightly 
 convex. 
 
 Now if we compare with the horse-shoe crab’s eye that of the 
 
i88o.] The Structure of the Eye of Trilobites. 507 
 
 trilobite Y Asaphus gigas, Fig. 2), we see that the eye is raised 
 upon a tubercle-like elevation of the carapace ; the integument 
 (int) is about as thick as that of Limulus, and it contains similar 
 pore-canals (pc); the eye itself, or cornea, occupies a rather 
 small area; its exterior surface, instead of being smooth as in 
 Limulus, is tuberculated, or divided up into minute convex areas; 
 these convexities are the external surfaces of the corneal lenses, 
 which extend through the cornea, so that its surface is rough 
 instead of smooth as in Limulus ; cl indicates one of the corneal 
 lenses which are arranged side by side ; they are of slightly dif- 
 ferent lengths and thicknesses, and the rather blunt free ends pro- 
 ject into the cavity of the eye, which in the fossil is filled .with a 
 translucent calcite. 
 
 It is quite apparent that we have here the closest possible 
 homology between the hard parts of the eye of Limulus and the 
 Asaphus. Another point of very considerable interest is a toler- 
 ably distinct dark line (rt) which seems to run across from one 
 lens to another, and which may possibly represent the external 
 limits of the retina or pigment mass in which the ends of the 
 lenses were probably immersed; should this be found to be the 
 indications of the outer edge of 
 the retina, it would be a most 
 interesting fact in favor of our 
 view of the identity between the 
 eyes of the two types of Palceo- 
 carida under consideration. 
 
 Another section sent us by Mr. 
 
 Walcott is represented by Fig. 3 ; 
 it is from Asaphus gigas , but rep- 
 resents a less elevated and broader Fig. 3. — Cornea of Asaphus. 
 part of the eye than that seen in Fig. 3 ; the section does not so 
 well exhibit the free ends of the corneal lenses. Fig. 3 a repre- 
 sents a transverse view of the eye of Asaphus gigas } 
 showing the hexagonal form of the facets, and their 
 quincunx arrangement. 
 
 This hexagonal appearance of the corneal lenses is still Fig. 3a. 
 retained in natural vertical sections of eyes of the same genus ; 
 where with a good Tolies lens the sides of the cones are seen to 
 be angular. Fig. 4 represents a few such cones. I do not under- 
 stand to what this hexagonal appearance is due ; for both in 
 
508 
 
 The Structure of the Eye of Trilobites. 
 
 [July, 
 
 coup 
 
 Fjg. 4. — 
 Lenses of 
 Asaphus. 
 
 Limulus and the Trilobites the corneal lenses appear usually to 
 be round, and yet in making a camera drawing (as are 
 all those here represented) of the cornea of Limulus 
 from above, they present the same hexagonal appear- 
 ance as in the Trilobites. The cause of this I leave 
 to others to explain. 
 
 In a section (transverse) of the cornea of Bathyurus longistrino- 
 sus, received from Mr. Walcott, the lenses are seen to be very 
 irregular, five or six-sided, and very irregularly grouped, not 
 arranged in distinct rows. 
 
 From the facts here presented it would seem evident that the 
 hard parts of the eye of the Trilobites and of Limulus are, 
 throughout, identical. The nature of the soft parts will, as a 
 matter of course, always remain problematical ; unless the dark 
 line indicated in Fig. 3 ( cl) really represents the outer edge of 
 the pigment of the retina; but however this may be, judging by 
 the identity in structure of the solid parts, we have, reasoning 
 by analogy, good evidence that most probably the eye of the 
 Trilobites had a retinal mass like that of Limulus, and that the 
 numerous small branches of the long slender optic nerve (for such 
 it must have been) impinged on the ends of the corneal lenses. 
 It has been shown by Grenacher and myself that the eye of Lim- 
 ulus is constructed on a totally different plan from that of other 
 Arthropods; I now feel authorized in claiming that the trilobite’s 
 eye was organized on the same plan as that of Limulus; and thus 
 when we add the close resemblance in the larval forms, in the gen- 
 eral anatomy of the body-segments, and the fact demonstrated by 
 Mr. Walcott that the Trilobites had jointed round limbs (and 
 probably membranous ones), we are led to believe that the two 
 groups of Merostomata and Trilobites are subdivisions or orders 
 of one and the same sub-class of Crustacea, for which we have 
 previously proposed the term Palceocarida. 
 
Bulletin of the Museum of Comparative Zoology, 
 AT HARVARD COLLEGE. 
 
 Vol. XIII. No. 6. 
 
 THE EYES IN SCORPIONS. 
 
 By G. H. Parker. 
 
 FOUR 
 
 WITH PLATES. 
 
 CAMBRIDGE: 
 
 PRINTED FOR THE MUSEUM. 
 December, 1887. 
 
No. 6. The Eyes in Scorpions. By G. H. Parker.* 
 
 The subject discussed in the following pages has already attracted the 
 attention of able investigators, and were it not that the authors of the 
 later papers have pointed out questions only partially answered, a recon- 
 sideration of the subject might appear presumptuous. It is hoped that, 
 in discussing these questions from an embryological as well as histological 
 standpoint, the additional evidence obtained may throw some light on 
 their solution. 
 
 The study of the eyes in Arthropods requires so much technical skill 
 that until very recently satisfactory work in this field has been almost 
 impossible. Aside from papers mainly of historical interest, the most 
 comprehensive publication for the student to-day is Grenacher’s “ Unter- 
 suchungen fiber das Sehorgan der Arthropoden.” This appeared in 1879, 
 and contained an admirable study of the eyes in spiders ; it did not, 
 however, touch upon the organs of sight in scorpions. The same year, 
 Graber, in his paper entitled “ Ueber das unicorneale Tracheaten- 
 Auge,” severely criticised Grenacher’s conclusions. Grenadier, in order 
 to answer his critic, turned his attention to the eyes in scorpions, and, in 
 his paper on the eyes in Myriapods, published in 1880, he included a 
 reply to Graber. Three years later a comparison of the eyes in the scor- 
 pion and king-crab was published by Lankester and Bourne. The sub- 
 stance of these four papers, when viewed in the light of newly discovered 
 embryological facts, has recently been fully discussed by Mark. Previous 
 to the appearance of Mark’s paper, Patten, after having made a compara- 
 tive study of the eyes in certain mollusks and arthropods, included in his 
 general description an account of the histology of the eyes in scorpions. 
 The five papers quoted, namely, those of Graber (’79), Grenadier (’80), 
 Lankester and Bourne (’83), Patten (’86), and Mark (’87), are the only 
 ones in which the histology of the eyes in scorpions is considered. 
 
 The publications on the development of the eyes are even less exten- 
 sive than those on the histology. Metschnikoff (’71, p. 225), in his 
 
 * Contributions from the Embryological Laboratory of the Museum of Compara- 
 tive Zoology at Harvard College, under the Direction of E. L. Mark. — No. XII. 
 A Thesis presented for the Degree of S. B. 
 
 VOL. XIII. — no. 6. 11 
 
174 
 
 BULLETIN OF THE 
 
 paper on the development of the scorpion, barely alludes to the eyes. 
 Their probable method of development is described by Patten (’86, p. 
 672). His conclusions as far as they touch upon the eyes in scorpions 
 are based upon inferences drawn from the method of development in 
 other forms, not from actual observations. In a preliminary communica- 
 tion by Kowalevsky and Schulgin (’86, pp. 530-532) the method of 
 development for the median eyes is described at some length. On ac- 
 count of incompleteness in their studies, these authors were forced to 
 omit a description of the lateral eyes. Later in this paper, the substance 
 of their communication will be considered. 
 
 The species of scorpions previously studied have been numerous. 
 Graber (’79, p. 71) examined the eyes in Scorpio europceus , Schr., and 
 Buthus afer, L. Grenadier’s investigations (’80, p. 42) were made upon 
 Buthus afer , Ischnurus caudicula , and Lychas americanus. Androctonus 
 funestus , var. citrinus, Ehr., Euscorpius italicus, Poess. and E. carpathi- 
 cus , were the species studied by Lankester and Bourne (’83, p. 180). 
 The embryological researches of Kowalevsky and Schulgin were made 
 upon Androctonus ornatus. 
 
 The species which I have studied belongs to the genus Centrums.* 
 In July and August, 1886, through Mr. C. W. Johnson, gravid females 
 were obtained from Florida. At intervals during the following winter 
 Mr. Johnson and Mr. F. S. Schaupp of Texas supplied me with fresh 
 material. I am also indebted to Dr. H. A. Hagen for some alcoholic 
 specimens from Arkansas. 
 
 In preparing the eyes for study by means of sections, the two chief 
 difficulties encountered were the presence of chitinous lenses and dense 
 pigment. It is difficult to cut the lens, and often this structure is in 
 part torn away, thus destroying the surrounding tissue. In the median 
 eyes, by careful dissection, the soft parts may be separated from the lens 
 and cuticula, and cut without the interference of these hard structures. 
 The separation is best accomplished after the tissues have been hardened. 
 The method of dissection cannot be applied to the lateral eyes, for they 
 are almost completely surrounded by chitine. In these eyes the best 
 results were obtained by trimming off the chitine around the eyes, and 
 cutting the retina and the lens after the removal of as much chitine as 
 possible. 
 
 The pigment is so abundant and so dense that even the thinnest sec- 
 
 * I am unable to state what species this is. I have not succeeded in finding 
 it described anywhere. Specimens in the collection of the Museum marked by 
 Simon as “ Centrurus sp. incog.” are of the same species as those here described. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 175 
 
 tions cannot be studied to advantage until they have been depigmented. 
 For this purpose I know of only two classes of successful reagents, acids 
 and strong alkalis. Grenacher has generally employed the first, Graber 
 the second. 
 
 Of the acid reagents strong solutions are required. Lankester and 
 Bourne (’83, p. 180) employed 5 or 10% solutions of nitric acid. In 
 the eyes which I have studied, this mixture did not remove the pigment, 
 even after the lapse of a week ; and I was forced to use stronger and 
 stronger grades, till 50% was reached. This mixture gives fair results, 
 but must be made and used with much caution. A given volume of acid 
 should be poured slowly into an equal measure of alcohol, never the re- 
 verse, and the mixture should be kept cool, otherwise the acid may attack 
 the alcohol. In such an event the solution is rendered worthless, and, 
 should the specimens be in it at the time, the heat generated by the 
 reaction gives the acid such additional dissolving power that the sections 
 are at once destroyed. A more efficient acid reagent is a mixture of 
 equal parts hydrochloric and nitric acids. A 35% solution of this mix- 
 ture in strong alcohol gives better results than the pure nitric acid at 
 50%, and does not so readily attack the alcohol. 
 
 Of the alkalis, weak ammonia, sodic hydrate, and potassic hydrate 
 are most serviceable. The solids are to be preferred to the ammonia, 
 since from them solutions of a definite strength can more easily be made. 
 An aqueous solution of J or J% potassic hydrate has given the most 
 satisfactory results. 
 
 The method of using the depigmenting fluid is as follows. Unstained 
 material is cut in paraffine ; the ribbons are mounted on a slide with 
 Schallibaum’s fixative ; when the sections are fixed, the paraffine is re- 
 moved with turpentine ; the slide with the sections is then successively 
 washed with alcohol of 98%, 90%, 70%, and so on, till a grade homo- 
 geneous with the depigmenting fluid is reached. Into a shallow white 
 dish filled with the depigmenting fluid the slide is now gently lowered. 
 In a few seconds the pigment, dissolving, will be seen as a reddish cloud. 
 The process is usually completed in less than a minute, and the slide is 
 promptly transferred to a dish of clean water or alcohol and there gently 
 rinsed. The sections are next stained by exposure to the dye in a shal- 
 low dish. After being sufficiently stained, they may be washed and 
 mounted in glycerine, or, after the proper steps in dehydrating and clari- 
 fying, mounted in benzol-balsam or other mounting medium. 
 
 The dyes which have been found the most serviceable are some of the 
 carmines and hematoxylin. The aniline dyes have almost invariably 
 
176 
 
 BULLETIN OF THE 
 
 given poor results. For general purposes Grenadier’s alcoholic borax- 
 carmine is excellent. In both embryonic and adult material Czoker’s 
 alum-cochineal gave fine nuclear outlines. In the adult eyes, the rhab- 
 domes and the cell boundaries were most distinctly shown by Kleinen- 
 berg’s hsematoxylin. A very faint coloration with this dye gave the best 
 results for nerve-fibres. 
 
 For the isolation of the retinal elements two maceration fluids were 
 used. A weak solution of chromic acid, as employed by Patten (’86, pp. 
 736, 737), gave good results ; but since the mycelium of a fungus is often 
 developed in very dilute solutions of this reagent, it can be used only 
 when it is carefully watched and its results are controlled by another 
 method. It was employed in the following manner. The retina, after 
 the removal of the lens and surrounding tissue, was placed for five or ten 
 minutes in a \°J 0 solution. After this treatment, which slightly hardened 
 the tissues, the first solution was replaced by a second of 5*0 %. In this 
 the retina remained for three or four days, at the end of which time the 
 retinal cells were easily separable. The most satisfactory method of iso- 
 lating the cells is to place on a slide in dilute glycerine a small portion of 
 the macerated retina, and, having protected it with a cover-glass raised 
 on wax feet, to gently tap the cover-glass till the cells are separated. 
 One part of 0.2% solution of acetic acid in sea-water mixed with an 
 equal volume of 0.04% osmic acid in sea- water, although only partially 
 successful as a maceration fluid for the retina in scorpions, is a reliable 
 check for the results obtained from chromic acid. 
 
 After the cells have been isolated, the abundance of pigment which 
 they contain so obscures their contents that scarcely more than their out- 
 lines can be studied. The removal of the pigment is on the whole more 
 successfully accomplished before than after isolation. For this process, 
 as for simple isolation, the retina should be subjected to the action of 
 \oj 0 chromic acid for five or ten minutes, and then transferred to a solu- 
 tion of \°J 0 potassic hydrate. In this the pigment dissolves, forming a 
 reddish cloud. After about a minute the retina should be removed to 
 distilled water, rinsed, and transferred to Grenadier’s alcoholic borax- 
 carmine. This reagent performs both the office of a maceration fluid and 
 a dye. In from twelve to twenty-four hours the retinal cells can be iso- 
 lated, and present in different regions of the retina three principal condi- 
 tions. First, those from the exterior of the retina are seriously altered 
 by the continued action of the potash ; second, those from the centre of 
 the retina remain almost unchanged, still retaining most of their pig- 
 ment; third, those from an intermediate position, without being other- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 177 
 
 wise much altered, lose most of their pigment. It is from these last 
 that the best results were obtained. 
 
 The eyes in scorpions are situated on the prosomatic shield. According 
 to their position they may he classed into two natural groups, the median 
 and the lateral eyes. As their name implies, the median eyes are situated 
 close to the sagittal plane. They are a little in advance of the centre of 
 the shield, two in number, and always symmetrically placed. The lateral 
 eyes form two isolated groups, one on either side, at the edge of the shield 
 where its anterior border meets its lateral margin. In different genera, 
 the number of eyes in each group varies from two to seven. Two kinds 
 of lateral eyes have been distinguished ; the larger or “ principal,” and 
 the smaller or i( accessory ” eyes. As will be shown later, no essential 
 difference exists between these two groups ; the smaller and larger eyes 
 are constructed on the same plan. 
 
 On account of the marked dissimilarity in the structure of the median 
 and lateral eyes, they will be described separately. 
 
 The Median Eyes. 
 
 Grenacher (’79, p. 40) first pointed out that the vitreous and retinal 
 layers in the eyes of spiders were separate. Graber in the same year con- 
 firmed this discovery, and showed that the median eyes of scorpions had 
 a similar structure. These two-layered eyes were designated by Lan- 
 kester and Bourne (’83, p. 195) as diplostichous, and among them were 
 included the median eyes in scorpions. Up to this time all authors 
 agreed that the median eyes of scorpions were two-layered. 
 
 One of the results of Locy’s work (’86, p. 85), as Mark has indicated 
 (’87, p. 71), is that in spiders the so-called diplostichous eyes are in 
 reality three-layered, or triplostichous. The embryological facts on which 
 this statement is based will be referred to later. For the present it is 
 sufficient to note that an interesting question presents itself, namely, if 
 the so-called diplostichous eyes in spiders have been shown to be triplo- 
 stichous, may we not look for a similar condition in the median eyes 
 of scorpions ? Some of the reasons for believing this have already been 
 stated by Mark (’87, pp. 55-58), but the final settlement of the question 
 can only be reached through embryological means. It was my principal 
 object in beginning these studies to reach a satisfactory conclusion in 
 this matter. 
 
 Patten (’86, p. 672) had already claimed that the median eyes in 
 
 VOL. XIII. — no. 6 . 12 
 
178 
 
 BULLETIN OF THE 
 
 scorpions were three-layered, and that they were probably formed from 
 a cup-like involution of ectoderm. The closure of the cup produced an 
 optic vesicle, the deeper half of which became retina, while the more 
 superficial half was probably represented by a structure to be described 
 hereafter, the preretinal membrane. The details of this method of de- 
 velopment, as will be seen later, are not confirmed by my observations ; 
 but nevertheless it remains to Patten’s credit that he was the first to 
 insist that the eyes in scorpions were three-layered, and not two-layered 
 as had been previously held. 
 
 Metschnikoff (’71, p. 225), in his paper on the development of the scor- 
 pion, did not discuss the formation of the eye further than to claim for 
 it a hypodermal origin. His evidence on this point can scarcely be consid- 
 ered as conclusive, for his studies were made from superficial views only. 
 
 The youngest material at my disposal was already somewhat advanced ; 
 but the eyes were still sufficiently undifferentiated to give adequate evi- 
 dence as to their origin, and thus to afford a trustworthy basis for the 
 interpretation of structures in the adult. 
 
 In the earliest stage examined, the eyes appear on surface view as a 
 pair of oval, slightly pigmented areas. They are situated at the anterior 
 end of the head, one on either side of the median line, and somewhat 
 above the mouth. In a slightly older stage a sagittal section a little at 
 one side of the median plane shows the region of the pigmented areas to 
 be already composed of three layers of hypodermis (PL III. fig. 12, prr ., 
 r., andpr.). The hypodermis of the prosomatic shield (prr.) extends 
 downward toward the mouth, and preserves its indifferent condition ; 
 before reaching that opening, it is folded upon itself, the deeper arm (r.) 
 of the fold passing dorsally in contact with the deep face of the external 
 portion. The ventral third of the infolded layer is as thin as the ex- 
 ternal layer of hypodermis, but the remaining two thirds are considerably 
 thickened and contain much pigment. This thickened layer, becoming 
 rapidly thinner at its dorsal end, is also folded upon itself to form a 
 third layer (p r.), which passes ventrally next the deep face of the thick- 
 ened portion, and at the point of first folding becomes continuous with 
 the external hypodermis as it proceeds in the direction of the mouth. 
 
 This condition is practically an involution of the hypodermis. The 
 infolded layers take the form of a flattened sac, or pocket, the open end 
 of which is situated in the median plane between the mouth and the 
 previously described pigmented areas. From its opening the pocket 
 extends vertically upward, and its anterior face is closely applied to the 
 deep surface of the permanent hypodermis. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 179 
 
 At the stage represented in Fig. 12, the cavity of the pocket is scarcely 
 noticeable. It should appear, of course, between the second ( r .) and 
 third (p r.) layers, and at the deep end of the infolding a trace of it is 
 visible ( cav .). The second and third layers, however, are quite distinct, 
 and show no indications of fusion. The cavity of the pocket is oblit- 
 erated only by its opposite walls coming in contact, so that even in 
 Fig. 12 a pocket may be spoken of without inconsistency. In stages 
 earlier than that given in Fig. 12, the cavity of the pocket is very no- 
 ticeable, and from its external opening to its deep end it is a continuous 
 open space. 
 
 In a horizontal section of the earliest stage examined, the region just 
 above the external opening of the pocket presents the appearance of a 
 slightly irregular tube cut crosswise (PI. III. fig. 13). The wall of the 
 tube is made up of a single layer of hypodermis, whose deep surface 
 is covered with a delicate basement membrane (fig. 13, mb.). The cavity 
 of the tube is continuous with the pocket of the infolding (fig. 13, cav.). 
 At about half the distance from its opening to its deep end, the pocket is 
 divided in the median plane into a right and left compartment (PL III. 
 figs. 14, 15). Each compartment has the form of a sac flattened from 
 before backwards. The sacs extend dorsally on either side of the median 
 plane, and end blindly. 
 
 One can distinguish, then, in the invagination a common neck, and 
 two symmetrically placed sacs which arise from it. In the sagittal sec- 
 tion (PI. III. fig. 12) already described, the thin ventral third of the 
 infolded hypodermis corresponds to the neck, and the thickened dorsal 
 two thirds to the anterior wall of the sac. The position of the sacs is 
 indicated externally by the areas of pigment already alluded to ; the sacs 
 are destined to become the retinas. The neck soon disappears, but some 
 time before this takes place the outer wall of each sac is thickened still 
 more and becomes more deeply pigmented. The thickened faces form 
 the essential part of the retina, with which, after the closure of the pocket, 
 the posterior thinner layer fuses. 
 
 The three liypodermal layers which enter into the composition of the 
 eye, have received special names. That portion of the permanent hypo- 
 dermis which is directly external to the optic sac, constitutes the first 
 layer. At a later stage it produces the lens, and consequently has been 
 termed by Mark (’87, p. 77) the “lentigen.” By other authors it has 
 been generally designated as the “ vitreous.” Directly under the lentigen, 
 and forming the thick external wall of the optic sac, is the second or 
 retinal layer. Behind this layer the thin internal wall of the sac forms 
 
180 
 
 BULLETIN OF THE 
 
 the third or post-retinal layer. At the sides and blind end of the sac, 
 the two deeper layers, the retina and post-retina, are continuous. 
 
 Granting the optic sacs to have arisen by involution, it is important to 
 notice that of the three layers described the retinal layer is inverted ; i. e. 
 that face which before involution is external becomes after involution 
 internal. A similar inversion in the retinas of the anterior median eyes 
 of Agelena has already been demonstrated by Locy (’86, p. 87). In 
 fact, at this early stage, the only striking difference between the eyes in 
 Agelena and Centrurus is, that in the scorpion the two sacs are united 
 by a common neck, whereas in the spider they are independent involu- 
 tions. It seems scarcely possible that this is an essential difference, and 
 I therefore believe that the median eyes of scorpions, like the eyes of 
 spiders, arise from hypodermal involutions not immediately connected 
 with the formation of other organs. 
 
 Since the above conclusion was arrived at, Kowalevsky and Schulgin 
 (’86, pp. 530-532), who have studied the development of Androctonus, 
 have published in a preliminary communication the results of their work. 
 In their description of the nervous system the development of the eye 
 occupies several paragraphs. On account of the absence of figures their 
 necessarily brief account is somewhat difficult to follow, and in one place 
 I am not sure of their meaning. 
 
 Their statements on the median eyes are substantially as follows. A 
 pair of semicircular depressions occur in the cephalic plate. From the 
 anterior margin of each depression a fold grows down toward the mouth. 
 The closure of each depression by its fold gives rise to a right and left 
 cephalic vesicle. From the region at which the mouth of each vesicle 
 has just closed, a new fold develops. Each of the new folds opens 
 toward the animal’s mouth, and takes on the form of a pocket. The right 
 and left pockets thus formed are the first traces of the median eyes. The 
 authors then describe the connection of the two pockets by a common 
 neck, and the thickening of the retinal layer, in the ventral part of which 
 pigment is deposited. 
 
 As will be noticed, the description summarized has to do with the 
 earliest condition in the formation of the eye. The youngest stage of 
 my material is too advanced to permit me to make positive statements 
 on this subject. The point about which I have had difficulty relates to 
 the description of the brain and eye folds. In describing the formation 
 of the brain vesicles the authors speak of an accessory fold (eine acces- 
 sorische Falte, p. 530). When the development of the median eyes is 
 described, they speak of a new fold (eine neue Falte, p. 531). Finally, 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 181 
 
 in the paragraph especially devoted to the median eye (p. 531), the 
 following occurs : “ Die Mittelaugen werden von der gleiclien Faite * 
 gebildet, welche am Baue der Kopflappen Antheil nimmt, nur mit dem 
 Unterschiede, dass fur den Bau des Hirns die tiefen Theile der Faite 
 verwendet werden, wahrend die Augen Derivate der peripherischen Theile 
 derselben Faite* sind.” After speaking of an accessory fold and a new 
 fold in connections already alluded to, the statement that the cephalic 
 lobe and median eye are derived from the same fold seems to me 
 contradictory. 
 
 The involution of the optic sacs in spiders, as Locy has shown (’86, 
 PI. XI. fig. 70), takes place at a much later period than the formation of 
 the cephalic ganglia, and to all appearances independently of the latter. 
 Whether in scorpions the growth of these two structures is connected or 
 not, is a question for the determination of which I have not yet secured 
 the requisite material. Besides the statements of Kowalevsky and Schul- 
 gin, which are somewhat obscure, there remains as a guide only the 
 analogous case in spiders ; the fact that the later stages in the eyes of 
 spiders are essentially the same as those of the scorpion, lends support to 
 the view that the eyes in scorpions, as in spiders, arise from folds inde- 
 pendently of those concerned in the formation of the brain. 
 
 The most noticeable changes which the pair of sacs undergoes in the 
 further development of the eyes are, first, an obliteration of their cavities, 
 and, second, a considerable change in position. The closure of the sacs 
 is first effected in the region where they unite with the common neck. 
 From this point the fusion of the retinal and post-retinal layers proceeds 
 toward the blind end of each sac, and the neck, becoming detached from 
 the sacs, is slowly withdrawn to form a part of the permanent hypodermis 
 (Pi. II. fig. 11, col.). The line of demarcation between the retina and 
 post-retina at the deep end of the sac is the last trace of the already ob- 
 literated cavities (PL II. fig. 10, cav.). 
 
 The change in position undergone by the eyes is correlated with a 
 change in the form of the animal’s body. In the embryo the region of 
 the prosomatic shield occupies the anterior face of the animal, and there- 
 fore lies in a plane approximately perpendicular to the long axis. The 
 optic sacs are situated near the centre of this region, the plane of their 
 flattening being nearly vertical, and the lines corresponding to the future 
 axes of the eyes being horizontal. As the animal develops, the shield 
 assumes a position more nearly horizontal, till at length it becomes en- 
 tirely so. The axes of the eyes, having shifted through an arc of 90°, 
 * The Italics are not in the original. 
 
182 
 
 BULLETIN OF THE 
 
 then have a vertical direction. The only planes which in both the adult 
 and embryo cut the eyes similarly are those parallel with the sagittal 
 plane. In a horizontal section of a young embryo the eye shows the 
 same relation of parts as one sees in a transverse section of an adult. 
 
 As previously stated, the eye in the embryo consists of three cell layers, 
 lentigen, retina, and post-retina. These three layers are recognizable 
 in the adult eye, and in considering the histology of this structure the 
 three layers will be treated in the order named. 
 
 The lentigen , as Grenacher (’79, p. 40) first clearly demonstrated in 
 spiders, is distinct from the retina, and is directly continuous with the 
 hypodermis. Graber (’79, p. 61) established the existence of a similar 
 condition in the median eyes of scorpions. 
 
 The lentigen results from a modification of the hypodermis directly 
 external to each optic sac. For some time after involution this hypo- 
 dermis consists of undifferentiated cells, whose positions are indicated by 
 their spherical nuclei. About the time when pigment is deposited in 
 the retina, the hypodermis in front of each pigmented area thickens, and 
 the outlines of its cells become visible (PI. III. fig. 15, pr ?*.). This is the 
 first modification in the formation of the lentigen. The thickening of 
 the lentigen increases, and each cell assumes the form of a long pyramid, 
 whose base rests upon a membrane between retina and lentigen, and 
 whose slightly truncated apex reaches the forming lens (PI. II. figs. 9 
 and 1 0). In immature eyes the sides of the lentigenous cells are perpen- 
 dicular to the surface on which they rest. In a transverse section of the 
 head of an adult (PI. I. fig. 2), the cells are curved. About three fourths 
 of the lentigen, extending from the median toward the lateral margin of 
 the eye, has its cells convex toward the sagittal plane ; in the lateral 
 fourth, the cells are concave toward the sagittal plane, and in the small 
 intermediate region they are straight (compare Lankester and Bourne, 
 ’83. pi. X. fig. 8). In a longitudinal section of an adult head (PI. I. 
 fig. 1), the lentigenous cells all appear perpendicular to the Surface on 
 which they rest. 
 
 The nuclei of the lentigen cells, at the first indications of a thickening 
 in the lentigenous region, keep to its deeper parts, and form in the 
 adult eye a continuous line close to the deeper face of the lentigen (PI. I. 
 fig. 2, nl. pr r.). 
 
 The lentigen as a whole is of glassy transparency. In young stages 
 the hypodermis at the edge of the lens nearest the median plane shows a 
 deposit of pigment. This pigmented region in time extends around the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 183 
 
 edge of the lens, and, as Graber has indicated (’79, p. 62), forms in the 
 adult a complete circle, — the iris. In the iris proper the whole of each 
 cell contains pigment granules, while in the adjoining hypodermis the 
 granules are scattered in small groups through the cells, and are especially 
 abundant at their outer ends. 
 
 The lens owes its origin exclusively to the activity of the lentigen. As 
 is well known, it consists of a thickening of the external cuticula. The 
 lentigen bears the same relation to the lens as the hypodermis does to 
 the indifferent cuticula. 
 
 In Centrurus the cuticula at most points on the body consists of three 
 layers. The outermost, first recognized as distinct by Graber (’79, p. 59), 
 is a thin, homogeneous, colorless layer (PI. I. fig. 2, ll). Under this is a 
 second layer of about equal thickness with the first, but usually of a deep 
 yellow color (PI. I. fig. 2, ll'). These two layers together form about 
 one fourth the whole thickness of the cuticula. The third layer (PI. I. 
 fig. 2, ll"), embracing the remaining three fourths, is distinctly laminated. 
 The deepest lamella of this third layer readily takes up borax-carmine. 
 The remaining lamellae are distinguishable from the second layer chiefly 
 by their want of color. The cuticula is very commonly penetrated by two 
 sets of pore-canals (PI. I. fig. 2, can. po. and can. po.'), fine and coarse. 
 
 As the indifferent cuticula passes into the region of the lens, the fol- 
 lowing conditions are noticeable. The external hyaline layer passes un- 
 changed either in thickness or texture over the front of the lens. The 
 second or colored layer becomes perfectly colorless, and by its increased 
 thickness adds to the convexity of the lens. The bulk of the lens, how- 
 ever, is produced by a thickening of the third layer. 
 
 Whereas in the indifferent cuticula only its deepest lamella is colored 
 with borax-carmine, in the lens all parts below the outer homogeneous 
 layer readily take up this dye. A similar condition has been observed 
 in several other local thickenings in the general cuticula, especially on 
 the ventral side of the animal. The conclusion to be drawn from these 
 observations is, that the lens in its composition is more closely related to 
 the last-formed cuticular lamella than it is to the older lamellse. 
 
 The coarse pore-canals never occur in the lens. Grenacher (’79, p. 90) 
 was unable to find fine pore-canals in the lens of Phalangium, although 
 Leydig had previously claimed them to be found in such lenses. Graber 
 stated (’79, p. 60) that all arthropod lenses which he had examined con- 
 tained fine pore-canals. In Centrurus, notwithstanding that many sec- 
 tions of lenses have been examined, fine pores have never been visible, 
 although in the adjoining cuticula they are plainly evident. 
 
184 
 
 BULLETIN OF THE 
 
 Graber (’79, p. 59) raised the question whether the lens consists of 
 only the normal cuticular layers thickened, or contains additional layers. 
 This is a question upon which evidence is not easily obtainable, for 
 it deals with layers which in the lens may be of considerable thickness 
 and yet remain so thin as to be almost imperceptible in the indifferent 
 cuticula. The condition of the lens in young individuals offers some 
 evidence. About the time a young scorpion leaves the mother’s back, the 
 indifferent cuticula appears to consist conclusively of what in later stages 
 corresponds to the external hyaline layer. Careful search has failed to 
 show any subjacent cuticula, and yet in the region of the lens a very per- 
 ceptible layer of stainable cuticula is visible. This seems to indicate 
 that the lentigen has the power of producing cuticula independently of 
 that produced by the indifferent hypodermis. Admitting this, it seems 
 probable that of the many lamellae in the lens some may be peculiar to 
 the lens itself and unrepresented in the adjacent cuticula. 
 
 The separation of the retina and lentigen, as discovered by Grenacher, 
 was further emphasized by Graber’s discovery (’79, pp. 64-67) of a 
 limiting membrane (“ praeretinale Zwischenlamella ”) between them. 
 This preretinal membrane, as Graber showed, is continuous with the 
 “ sclera ” and the basement membrane of the hypodermis. The explana- 
 tion of these structures which is offered by the formation of the eye from 
 a hypodermal sac, has already been discussed by Mark (’87, p. 71). He 
 has claimed that the sclera is the basement membrane of the post-retinal 
 layer, and that the preretinal membrane is the fused basement membranes 
 of the lentigen and retina. The explanation given in the case of the 
 infolded eyes of spiders applies equally "well to the median eyes in 
 scorpions. 
 
 As to the nature of the basement membrane, especially in the region of 
 the sclera and preretinal membrane, two opposing theories have been 
 advanced. Graber (’79, pp. 63, 64) maintains that the basement mem- 
 brane including the sclera is cuticular, not cellular, and as its matrix he 
 claims cells whose nuclei were found both by Grenacher (’79, p. 60, fig. 
 34) and himself (’79, p. 64, fig. 18). For the preretinal membrane 
 Graber (’79, pp. 64, 65) also claims a cuticular nature and states that it 
 contains no nuclei. Lankester and Bourne (’83, p. 189) describe the 
 ommateal capsule or sclera as laminate and devoid of nuclei. Mark (’87, 
 p. 71) believes that the basement membrane with its modifications is a 
 cuticula derived from the basal ends of the hypodermal cells. 
 
 Opposed to these views Schimkewitsch (’84, pp. 8, 9, 11, 12) main- 
 tains that the basement membrane and its modifications are connective 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 185 
 
 tissue, and consequently cellular. His’ first argument is for the basement 
 membrane of the unmodified hypodermis. He shows that this membrane 
 passes off from the hypodermis and invests muscles. Arguing by analogy 
 from Froriep’s conclusion that the sarcolemma of striate muscles in ver- 
 tebrates is connective tissue, he maintains that the investment of the 
 muscles in spiders and the continuous basement membrane are connective 
 tissue. This argument of itself is scarcely convincing, for we do not 
 know that the sarcolemma in vertebrates and arthropods has necessarily 
 the same structure. Schimkewitsch used a second argument, which was 
 more weighty, namely, that in the envelope of the eye (sclera) nuclei had 
 been found. But the figures which illustrate this point are, as Mark 
 (’87, p. 70) has stated, open to criticism. 
 
 In the developing eye in Centrums, the basement membrane appears 
 as a thin sharply defined structure bounding the deep ends of the hypo- 
 dermal cells. It is continuous over the optic nerve, and unites with the 
 membrane investing the brain (PI. III. fig. 16, mb.). In the region of 
 the preretinal membrane it is double, one layer limiting the lentigen, the 
 other the retina (PI. II. fig. 9, mb.). This confirms Mark’s theoretic con- 
 clusion. In the earliest stages studied, mesodermic nuclei occur at inter- 
 vals between the two membranes, except directly over the centre of the 
 eye, where the two membranes are in contact (PI. III. figs. 14, 15, nl. 
 ms d.). Although mesodermic nuclei occur between the two retinas, and 
 also between the retina and the brain, they are never found within the 
 basement membrane of the eye region, as they are within the envelope of 
 the brain. As the two layers of the preretinal membrane unite, the 
 mesodermic cells, instead of being included between them, migrate toward 
 the margins of the eye, and leave the preretinal membrane when com- 
 pleted destitute of cellular elements. In the region of the sclera, how- 
 ever, mesodermic nuclei, often very much flattened and always closely 
 applied to the outside of the membrane, are distinguishable almost up to 
 the adult state (PI. III. fig. 15, PI. II. fig. 9, nl. ms d.). It is, therefore, 
 nearly certain that some of the substance of this mesodermic covering 
 enters into the formation of what is known as the “ sclera.” In the 
 adult sclera, however, no nuclei are visible, and besides it is by no means 
 certain that these mesodermic cells form a continuous investment over 
 the basement membrane, — perhaps nothing more than a network. 
 
 The nuclei in the eye on the right of Schimkewitsch’s Figure 11 
 (PI. III.) are almost identical in appearance with those found in the 
 young eyes of Centrurus, where they appear to occupy the middle of the 
 membrane ; they are in reality outside it, as can be readily demonstrated 
 
186 
 
 BULLETIN OF THE 
 
 when, as sometimes occurs, one finds a place where the mesodermie ele- 
 ment with its nucleus has been loosened from the sclera, and both nucleus 
 and sclera remain uninjured. 
 
 The nuclei which Schimkewitsch has drawn in Fig. 4 (PI. II.) and 
 Fig. 11 (PI. III.) are undoubtedly mesodermie, and represent a thin 
 tissue on the outside of the sclera.* Those in his diagramatic figure 
 (PI. III. fig. 4), if they are, as Schimkewitsch says, identical with those 
 of the other two figures, are mesodermie nuclei drawn on the wrong side 
 of the sclera ; if, on the other hand, their position is correct, they are not 
 the same nuclei as those in Figs. 4 and 11, but, as Mark (’87, p. 70) 
 maintains, the nuclei of the post-retinal layer. 
 
 From what has been said it will be inferred that in the adult neither 
 the sclera nor preretinal membrane contains nuclei. It is conceivable 
 that in some cases mesodermie nuclei might be surrounded in either of 
 these structures. Such, of course, would be exceptional. Of the twenty 
 preretinal membranes studied in section only one has shown nuclei ; but, 
 strange as it may seem, half t of this one contained no less than fourteen. 
 They were uniformly distributed, and always elongated parallel to the 
 striations of the membrane. In position they were appreciably nearer 
 the retina than the lentigen (PI. II. fig. 8, nl. ms d.). This instance 
 shows that the preretinal membrane may at least have a central layer of 
 mesodermie tissue, although the greater part of it is ectodermic cuticula. 
 
 The great thickness of the preretinal membrane in scorpions has al- 
 ready been noticed by Graber (’79, p. 67). In Centrums, as in others, it 
 presents a fibrous laminate appearance, and in specimens treated with 
 potassic hydrate it is slightly swollen and vacuolated. 
 
 At the edge of the preretinal membrane, where its two constituents 
 separate, the one which passes around the retina is much thinner than 
 the one which continues under the hypodermis. This is an indication 
 of the relative amount of substance contributed respectively by the retina 
 and the lentigen in the formation of the membrane. The line which 
 would separate the lentigenous from the retinal part must be drawn 
 somewhat nearer the retina than the lentigen. It is on this line, more- 
 
 * This limitation of the meaning of the word “ sclera ” seems desirable in view 
 of the possibility that a mesodermie covering may be altogether wanting, but it is 
 not intended as a criticism of Schimkewitseh’s statement that the sclera contains 
 nuclei; for the “sclera” as previously understood may evidently be in part meso- 
 dermic, and therefore cellular, as Schimkewitsch has claimed. 
 
 t The remaining half of this membrane was mounted on a second slide, and 
 treated by a method which did not make its nuclei distinguishable. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 187 
 
 over, that one would expect to find mesodermic elements if any were 
 present, and naturally enough it is in this region that the exceptional 
 nuclei previously referred to occur (PI. II. fig. 8). 
 
 The fact that mesodermic tissue is incorporated in the preretinal mem- 
 brane makes it highly probable that the mesodermic cells noticed on the 
 sclera really contribute to that layer, and that the sclera is in part meso- 
 dermic and in part ectodermic. To summarize, then, the preretinal 
 membrane, like all parts of the basement membrane, appears first as an 
 ectodermic cuticula. Mesodermic elements may be included between its 
 two layers, but this is the exception. The most of the membrane is in 
 any event cuticula, of which the greater part is produced by the lentigen, 
 the lesser by the retina. 
 
 The retinal and post-retinal layers. — The intimate connection into 
 which these two layers enter in forming the retina is a sufficient reason 
 for considering them together. Grenacher’s (’79, pp. 39-57) researches 
 on the eyes of arachnids led him to believe that the retina consisted of 
 a number of similar elements, each of which contained a rod-like body, or 
 bacillus, and a nucleus. Each element was, therefore, to be considered a 
 single cell. He also discovered that there were two different types in 
 the disposition of the nucleus and bacillus. Either, as in the anterior 
 median eyes of Epeira (’79, pp. 43-45, fig. 18 A), the bacilli were in 
 front of the nuclei, or, as in the posterior median eye (fig. 18 B), they 
 were behind the nuclei. In the structure of the eye this interesting 
 dimorphism, as Grenacher has termed it, has proved to be a feature of 
 common, if not universal, occurrence with spiders. 
 
 Graber, who based his conclusions on the study of the retina in scor- 
 pions as well as spiders, opposed Grenacher’s views, and claimed to have 
 found in the median eye of Buthus (’79, pp. 71, 72) at least two nuclei 
 to each element, — a large, basal, ganglionic nucleus, and, near the outer 
 end of the element, a smaller apical one. The equivalent of Grenacher’s 
 bacillus lay between these. In the case of the lateral eyes * of Buthus 
 (PI. Y. fig. 5), as well as in the anterior and posterior median eyes of 
 Epeira (PI. VII. figs. 25, 26), Graber figured a third small nucleus 
 directly behind the bacillus. 
 
 This discovery, if corroborated, would invalidate Grenacher’s view of 
 dimorphism in the eyes of spiders, and one would be forced to admit 
 that the retinal elements are multinuclear, and therefore not single cells. 
 Grenacher’s (’80, pp. 415-430) reply to Graber, at least so far as the 
 
 * When the discussion of the lateral eyes is reached, the subject of their nuclei 
 will be considered more at length. 
 
188 
 
 BULLETIN OF THE 
 
 retina in scorpions is concerned, is based on a study of the median eyes 
 only. He (’80, pp. 422-425) shows conclusively that for a ganglionic 
 nucleus Graber has described and figured a body which is not a nucleus. 
 Grenacher, after a careful search for Graber’s middle and anterior nuclei, 
 positively denies their existence. This, as Grenacher says, leaves the 
 retinal elements in scorpions devoid of nuclei ; he then proceeds to show 
 that in the region of Graber’s so-called ganglionic nucleus there exists a 
 true nucleus essentially unlike the latter. Therefore, according to Grena- 
 cher, the retinal elements in scorpions are to be placed in the category 
 to which the anterior median eyes of Epeira belong.* 
 
 Lankester and Bourne (’83, p. 188) agree with Grenacher that each 
 retinal cell contains a single nucleus ; but they also maintain that 
 Graber’s anterior and middle nuclei are to be found in the retina. 
 These nuclei, however, do not belong to the retinal elements proper, but 
 to small intrusive pigment cells. 
 
 The composition of the adult retina in Centrums has been studied by 
 means of sections and maceration preparations. A horizontal section 
 of an adult retina (PI. I. fig. 1 ) presents a concavo-convex outline ; a 
 portion of the convex face occupies the median plane of the body, and is 
 fused to the corresponding part of the opposite retina. The concave face 
 is limited by the preretinal membrane. The concave region is composed 
 of a series of deeply pigmented club-shaped masses, which taper off into 
 the lighter middle region. Behind the lighter area, which occupies fully 
 half of the thickness of the retina, many of the bands and lines of pig- 
 ment become thickened into irregular dark blotches, which make up a 
 poorly defined mottled area. This soon merges in the median plane into 
 the retina of the opposite side, and elsewhere into a densely pigmented 
 zone limited behind by the sclera. This pigmented zone can be traced 
 around the side of the retina till at the edge of the latter it becomes 
 confluent with the pigmented area first mentioned. 
 
 After removing the pigment and staining the section in Grenadier’s 
 alcoholic borax-carmine, the outermost pigmented region (PI. I. fig. 2) is 
 seen to consist of a very granular substance, in which cell-walls can be 
 traced from the preretinal membrane backward to the pointed, rod-like 
 structures, or rhabdoines. The latter cause the lightness of the large 
 
 * Graber (79, p. 69) designated those elements in which the nuclei were behind 
 the bacillus as “ post-bacillar ” ; those in which the nucleus was in front of the bacil- 
 lus, “ pre-bacillar.” Mark (’87, p. 73) has proposed for these terms pre- and post- 
 nuclear, respectively. Since these present advantages over the older terms, they 
 will be adopted in the following pages. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 189 
 
 middle area. This granular substance (PL II. -fig. 4) extends down 
 between the rhabdomes, and merges with a less regularly granular sub- 
 stance behind. The rhabdomes at their deep ends merge imperceptibly 
 into this irregularly granular substance. In the region where the deep 
 ends of the rhabdomes disappear, large slightly granular nuclei occur 
 (Fig. 4, nl. r.). All the nuclei in the retina of Centrums are found in 
 what has been described in the pigmented eye as the mottled area. The 
 nuclei nearest the concave surface of the retina are the largest, and, as 
 has been previously mentioned, are slightly granular. Behind these, in 
 the middle of the nuclear region, smaller oval nuclei (Fig. 4, nl. pig.) 
 occur. Here also nerve fibres are abundant, and those curious bodies 
 mistaken by Graber for nuclei and designated by Lankester and Bourne 
 under the name of phaospheres (Fig. 4, pha sp.). The deepest nuclei 
 (Fig. 4, nl. p r.) in the retina are flattened, and more deeply colored than 
 the rest. They lie upon the internal surface of the densely pigmented 
 zone, previously mentioned, and form a line of separation between that 
 zone and the coarsely granular substance in front. The substance of the 
 deepest zone is almost identical in character with that of the outer portion 
 of the retina, and its granular appearance, like that of the external layer, 
 is largely due to the colorless remains of pigment granules. The smaller 
 anterior and median nuclei of Graber do not exist in Centrums, either 
 in the retinal cells or between them, as claimed by Lankester and Bourne 
 (’83, pp. 192, 193). The latter authors state (’83, p. 192) that the 
 reason Grenacher overlooked these nuclei was that the acid which he 
 used to remove the pigment destroyed them. In the case of Centrums 
 sections depigmented with the 35 % mixture of nitric and hydrochloric 
 acids, with \°J 0 solution of potassic hydrate, or unaffected by depigmenting 
 reagents, but colored with borax-carmine and cut three micromillimeters 
 thick, show no trace whatever of anterior or middle nuclei. The exami- 
 nation of fresh material and of maceration preparations has given the 
 same results. Moreover, since the nuclei in the brain after treatment 
 with \°J 0 potassic hydrate are not to be distinguished from those in the 
 same organ unaffected by that reagent, it seems scarcely possible that the 
 same reagent could destroy nuclei in the retina. It is therefore safe to 
 conclude that at least in the retina of Centrums no nuclei exist external 
 to the band of larger nuclei already described. 
 
 Having shown that the retinal nuclei are limited to the deeper region 
 of the retina, and that these nuclei are of three principal types, we are 
 now prepared to inquire into the cellular composition of the retina. 
 This is best done by means of isolation preparations. In the retina 
 
190 
 
 BULLETIN OF THE 
 
 there are at least three kinds of cells. Two can be readily isolated ; the 
 third has been studied only in sections. 
 
 The retina extending from the line of deepest flattened nuclei to its 
 outer margin breaks up into two very distinct forms of cells, — retinal or 
 nerve-end cells, as Lankester and Bourne (’83, p. 182) have called them, 
 and pigment cells. The retinal cells (PI. II. fig. 5) are elongated and 
 rounded at their outer ends ; they terminate below in nerve fibres. From 
 the rounded external end the calibre is uniform till the region of the 
 rhabdomeres is reached. Here the cells increase in diameter, and then 
 continue for some distance uniform in size. Finally, each cell, enlarging 
 slightly at its deep end, rapidly tapers into a nerve fibre. Throughout 
 its whole extent the retinal cell contains pigment, which is principally 
 concentrated, however, at its rounded outer end. 
 
 The pigment cells (PI. II. fig. 6) at their anterior ends, like the pig- 
 mented tops of the retinal cells, abut against the preretinal membrane. 
 From this they pass backward, and in the region of the rhabdome, where 
 the retinal cells enlarge, they contract to thin fibres, which, after the 
 rhabdome has been passed, again expand into irregular pigment sacs at 
 the deep part of the retina. When isolated, they present the appearance 
 (PI. II. figs. 6, 7) of two sacs of pigment connected by a slender rigid 
 fibre. 
 
 The large round or slightly oval nuclei have been identified as be- 
 longing to the retinal cells (PI. II. fig. 7), and the smaller oval nuclei 
 occupy the deep swollen ends of the pigment cells. It is possible 
 that some of the pigment cells may not be prolonged in front of the 
 rhabdomes, and therefore not possess anterior sacs ; but I have never 
 been able to discover such. The filamentous middle portion connecting 
 the two extremities of the long pigment cells is so constant and char- 
 acteristic in maceration preparations, that pigment cells which do not 
 extend to the front of the retina must form the exception, if in fact they 
 exist at all. 
 
 Another method employed in studying the cells of the retina, and one 
 especially instructive for the region of the anterior zone, was by the aid 
 of sections perpendicular to the retinal cells. The retina has the form of 
 a shallow bowl ; consequently in sections perpendicular to its axis the 
 deeper portions of the retina will lie at the periphery of the section, and 
 its centre will be the region nearest the preretinal membrane. 
 
 Figure 3 represents a portion of a retinal section whose centre, and con- 
 sequently highest portion, is toward the right, and whose periphery or 
 deeper portion is toward the left. The relatively higher portion of the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 191 
 
 section is at that point below the preretinal membrane where the rhab- 
 domeres diminish into simple cell boundaries, and the five cells which 
 make a single group are here easily distinguishable. To the left the 
 rhabdomes are much larger, and have assumed their usual outlines. The 
 rhabdomes have increased in size at the expense of the cells. It will be 
 noticed that each of the cells present belongs to some group of rhab- 
 domeres, and consequently all are retinal cells. 
 
 The section (Fig. 3, a) which was the next external to the one just de- 
 scribed shows practically the same condition, except that, being slightly 
 nearer the front of the retina, the rhabdomes are not quite so distinct, 
 especially in the extreme right, where in one or two groups scarcely any 
 trace of the rhabdomeres can be seen. Nevertheless, all the cells of the 
 former section can be identified, and moreover between the groups in the 
 upper right hand corner an additional cell is noticeable. This cell, which 
 by a comparison of the two sections is seen to be a supernumerary element, 
 is not a retinal (nerve-end) cell ; but since in maceration preparations the 
 outer expanded ends of the pigment cells were always found near the 
 preretinal membrane, there is every reason for considering this such a 
 cell. Moreover, when sections nearer and nearer the preretinal membrane 
 are examined, these additional cells become more numerous, until finally 
 they are with difficulty distinguished from the retinal cells. The anterior 
 sacs of the pigment cells, then, can be demonstrated on sections as well 
 as by maceration. 
 
 The rhabdomes never reach the anterior face of the retina, but fall 
 short of it by the thickness of several sections. This space between the 
 rhabdomes and preretinal membrane corresponds to the anterior zone of 
 deep pigmentation seen in longitudinal sections. The pigment in this 
 region is so dense that the outlines of the cells can be traced only with 
 difficulty. 
 
 The phaospheres, as Lankester and Bourne (’83, pp. 185, 186) have 
 called the curious bodies mistaken by Graber for nuclei, are abundant in 
 the nuclear zone of the retina (PI. II. fig. 4, pha sy>.). They are as small 
 as the oval nuclei around them, and often smaller, but differ from these 
 in containing usually one, and sometimes two, three, or even four highly 
 refractive dots. Lankester and Bourne state that they are usually behind 
 the nucleus of the retinal cell. In isolated cells I have never succeeded 
 iu satisfactorily identifying them, therefore in Centrums I cannot feel sure 
 of their position. In one section only has a phaosphere occurred in a 
 prenuclear position; in all others they have been strictly behind the 
 neighboring nuclei. 
 
192 
 
 BULLETIN OF THE 
 
 As to their nature two suggestions have been made. Lankester and 
 Bourne (’83, pp. 185, 186) imply that they are of the nature of rhab- 
 domes; in this light they are further discussed by Mark (’87, p. 93). 
 Patten (’86, p. 684) is inclined to look upon them as degenerate nuclei. 
 In Centrums the phaospheres, being of nearly the same size as the nuclei, 
 present less favorable opportunities for study than in those scorpions 
 where they are much larger. Those in Centrums stain in much the 
 same way that the surrounding nuclei do, and in fact are to be distin- 
 guished from these mainly through their highly refractive dots. In many 
 cases, however, these dots are not well marked, and it is then difficult 
 to determine whether a given body is a nucleus or a phaosphere. The 
 nuclei are constantly oval in form; the phaospheres are more or less 
 irregular in outline. This irregularity, however, is only noted in phao- 
 spheres which have very refractive dots, and never in those which seem 
 to be transitional in form between nucleus and phaosphere. 
 
 The third type of cell occurs as a single layer of pavement-like ele- 
 ments at the back of the retina (PI. II. fig. 4). It has been correctly 
 stated by Graber (’79, p. 84) that this pavement layer is the matrix of 
 the sclera. Lankester and Bourne (’83, p. 192, pi. X. fig. 8, p) have 
 also observed it in Androctonus, where the cells are relatively much 
 smaller than in Centrums. In sections of Centrums the outlines of 
 these cells are visible, though faint ; in form they are broadly columnar. 
 Their nuclei, as previously stated, take a deep color, are flattened, and 
 are always located at the end of the cell farthest from the sclera. 
 
 This deep layer of cells envelops the convex face of the retina, passing 
 up on its sides till it reaches the edge of the retinal cup (Lankester and 
 Bourne, ’83, pi. X. fig. 8, p). Here, as Graber has shown (’79, PI. Y. 
 fig. 14), it becomes continuous with the retinal layer. Only in the re- 
 gion where the retinas of the - two median eyes fuse does this basement 
 layer fail to cover the deep surface of the retina proper. 
 
 The principal histological changes which take place during the devel- 
 opment of the eye relate to nuclei, the pigment, and the optic nerve. 
 The formation of the optic sacs, the disappearance of their common neck, 
 and the fusion of the post- retinal with the retinal layer has already been 
 described. 
 
 While the eye is yet an ectodermic pocket (PI. III. figs. 12-15), the 
 nuclei are distributed through the whole of the thickened retinal layer ; 
 in the post-retina they form a single row. At this stage the nuclei of 
 the different cells are indistinguishable. Their outlines are round or 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 193 
 
 slightly oval ; their contents, except for a few sharply marked granules, 
 are very transparent. Somewhat later, but before the optic sacs have 
 closed, they are less abundant near the front face of the retina, but other- 
 wise no special arrangement is as yet evident. 
 
 The rhabdomeres play an important part in the future distribution of 
 the nuclei. They first appear as light streaks, which, beginning close to 
 the preretinal membrane, gradually extend backward. With the exten- 
 sion of the rhabdomeres, the nuclei recede to the deeper parts of the 
 eye, and with very few exceptions * never occupy a place in front of the 
 rhabdomeres. 
 
 At about the time the young scorpion is born, the cavity of the optic 
 sac having disappeared, the nuclei of the retinal layer are found to have 
 arranged themselves in two groups. In axial sections of the eye (PI. II. 
 fig. 9) one group forms an irregular line at the base of the rhabdomeres, 
 the other a broad band in the deeper part of the eye. The space sepa- 
 rating these two groups is considerable, and contains only a few scattered 
 nuclei. The deeper nuclei in the broad band, i. e. those nearer the sclera, 
 are to be referred to the post-retinal layer. 
 
 At this stage the nuclei are still undifferentiated, and even after the 
 young scorpion has left the mother’s back it is some time before one can 
 recognize differences between them. It is only in the fully developed 
 adult that a marked differentiation is reached. By this time the nuclei 
 of the retinal cells have become slightly more homogeneous (PI. II. fig. 
 4, nl. r.) and somewhat reduced in size. The nuclei of the post-retinal 
 cells have become much flattened and stain more deeply. These, as 
 well as the nuclei of the pigment cells, are reduced in size, and have 
 become more homogeneous. The columnar “ matrix ” cells previously 
 described, and to which these flattened nuclei belong, constitute the 
 post-retina ; and their transition at the rim of the optic cup into the reti- 
 nal layer is only a preservation of the relation they have sustained to that 
 layer from the time of the original involution. This interpretation of 
 the “matrix” cells has already been maintained by Mark (’87, p. 56). 
 
 The phaospheres appear at a very late date. In young scorpions 
 which have left the mother’s back no trace of phaospheres was discover- 
 able, and it was only in those eyes in which the three forms of nuclei 
 were already distinguishable that the structures were noticed. The time 
 of their appearance — a period of nuclear differentiation — is evidence 
 in favor of their nuclear origin. 
 
 * In only one instance out of the many in which developing eyes have been 
 examined has a nucleus remained in a prebacillar position. 
 
 VOL. xm. — no. 6. 13 
 
194 
 
 BULLETIN OF THE 
 
 That the retinal cells and the post-retinal cells, as well as the pigment 
 cells, contain pigment, has already been stated. Lankester and Bourne 
 (’83, p. 194) were somewhat in doubt whether the retinal cells in the 
 median eyes of scorpions contained any pigment. Patten (’86, p. 728) 
 believes that they do not contain pigment. The evidence furnished by 
 sections perpendicular to the length of the cells (PI. II. fig. 3, gra. pig.) 
 is, I think, conclusive. 
 
 Under the head of “intrusive pigmentary connective tissue,’’ Lankes- 
 ter and Bourne (’83, p. 191) include the pigment cells in Androctonus, 
 and, with less confidence, their so-called intracapsular pavement. The 
 pigment cells proper are considered by them as of mesodermic origin. 
 This they defend by several arguments, but admit that their reasons 
 cannot be regarded as offering a sufficient basis for a final conclusion. 
 
 During early stages in the development of Centrums, mesodermic 
 tissue is often seen making its way into the substance of the brain, and 
 its appearance is characteristic. It penetrates into the nervous system 
 as thin continuous sheets of cells, which in cross-section appear as lines. 
 During the development of the eye no such appearances have been en- 
 countered, and it is fair to conclude that mesodermic tissue has not 
 gained access to the eye by the same means that it has to the brain. 
 
 Lankester and Bourne suggest that it may have entered the eye 
 capsule at the opening for the optic nerve ; but the capsule (sclera) is 
 reflected on to the optic nerve, and, even admitting that mesodermic 
 tissue did gain access here or from the brain, where it undoubtedly exists, 
 one would naturally expect the pigment to appear first in the region of 
 the optic nerve. Contrary to this, as Kowalevsky and Schulgin (’86, 
 p. 531) have shown, — and my own observations confirm theirs, — pig- 
 ment first appears in the front of the retina on its ventral — afterward 
 becoming its anterior — edge, at a point farthest from that where the optic 
 nerve joins the retina. Taking all the evidence into account, it seems 
 that the nerve-end cells, the intracapsular pavement cells (post-retifia), 
 and the pigment cells are alike ectodermic, and that the retina contains 
 no tissue that can be referred to a mesodermic source. 
 
 Th Coptic nerve in the adult scorpion joins the eye at a point on the 
 under side of the eye capsule. From this point bundles of fibres pass 
 anteriorly through the base of the retina in front of the post-retinal 
 layer, and from small secondary bundles are given off single fibres which 
 join the bases of the retinal cells. 
 
 In the youngest stages studied, the optic nerve was already formed, and 
 its fibres (PI. III. fig. 17, n.fbr.) passed over the front of the retina, 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 195 
 
 apparently connecting with the external ends of the retinal cells. At 
 least the fibres disappear here, and cannot be traced into the retina. The 
 optic nerve (PI. III. fig. 16, n. opt.) at this stage emerges from the retina 
 by passing over the rim of the optic cup in a region corresponding to the 
 outer edge of the pocket. The region extends from the dorsal margin 
 half-way down toward the ventral margin of the cup. 
 
 During the further development there is but little change in the 
 point of exit for the optic nerve. It simply shifts from its posterior 
 lateral position in the embryo, to a posterior ventral one in the adult. 
 The change in the course of the intracapsular fibres is much more sig- 
 nificant. 
 
 There is reason to believe that in the embryo the nerve fibres are at- 
 tached to the external ends of the retinal cells (Figs. 14-17). In the adult 
 they certainly emerge from the deep ends of these cells. The steps which 
 connect the earliest with the final condition consist of a migration of the 
 point of attachment for the nerve fibre from the external end of the cell 
 to the deep end. The migration of the fibres takes place at the same 
 time that the nuclei recede into the deeper parts of the eye, and seems to 
 be controlled by the same influence, namely, the growth of the rhabdo- 
 meres. An analogous condition in the eyes of Agelena has been described 
 by Mark (’87, pp. 84-87). There is, however, a difference ; the nerve 
 fibres in Agelena never come to have a post-nuclear attachment to the 
 retinal cell, whereas in the figures of Graber and those of Lankester and 
 Bourne, and certainly in the retina of Centrurus, the nerve fibres emerge 
 from the cell behind the nucleus. Mark (’87, pp. 91, 92) has claimed 
 for th«se facts an important significance, and concludes that they point 
 to a functional condition of the retina before involution. The bearing of 
 this will be further considered under the head of theoretic conclusions. 
 
 The Lateral Eyes. 
 
 The lateral eyes in scorpions, although in some respects more inter- 
 esting than the median eyes, have on the whole received less attention. 
 Grenacher in his two papers previously quoted makes no mention of 
 them ; for our present knowledge of their structure we are indebted to 
 the researches of Graber (79) and of Lankester and Bourne (’83). The 
 results of these inquiries are in so far unsatisfactory that in several es- 
 sential points they are directly opposed to each other. The points upon 
 which there is a conflict of opinion are (1) the origin of the retina, 
 and (2) the presence or absence of a lentigen. 
 
196 
 
 BULLETIN OF THE 
 
 On the question of the origin of the retina in arthropods, two un- 
 reconcilable opinions have been held. Some authors have maintained 
 that the retina was an outgrowth from the brain, and others that it was 
 a modification of the hypodermis. Graber may be taken as a represent- 
 ative of the former school, Grenacher of the latter. The evidence upon 
 which they based their opinions was derived in the two cases from qtiite 
 different kinds of eyes. Grenacher believed, since he had found in 
 eyes like those of the larval Dytiscus a retina which was continuous 
 with the hypodermis, that therefore the retina in the more complex 
 eyes was derived from the same hypodermal source. Graber, arguing 
 from those eyes in which the retina is separated from the hypodermis by 
 a preretinal membrane, maintained that the retina is an outgrowth from 
 the brain, and not derived from the hypodermis. Such an eye as the 
 larval eye of Dytiscus would, even in the absence of other evidence, 
 seriously weaken the force of Graber’s argument. As an explanation of 
 such structures, Graber is inclined to think that the larval eye of Dy- 
 tiscus really possesses a preretinal membrane, with hypodermis in front 
 of it j but that, on account of the thinness of this structure, Grenacher 
 has overlooked it. In other words, Graber considers the arthropod 
 ocellus as a two-layered structure, the outer layer of which is hypoder- 
 mal, and the inner layer, or retina, neural in its origin. 
 
 In Graber’s figures and description of the lateral eye in scorpions, the 
 two essential parts of the median eyes, the lentigen and retina, are rep- 
 resented ; but the lentigen, unlike that of the median eyes, is reduced 
 to a very thin layer of cells. This is perfectly consistent with Graber’s 
 theory ; but whether it represents the actual structure of the eye or not 
 is questionable, since Lankester and Bourne (’83, pp. 182 and 187) ex- 
 pressly state that the lateral eye of Androctonus is composed of a single 
 layer of cells, — a thickening of the superficial hypodermis, — and claim 
 that Graber is incorrect in describing a separate layer concerned in the 
 formation of the lens. 
 
 Since the publication of these papers, Locy’s discovery of the method 
 of development in spiders’ eyes has firmly established the hypodermal 
 origin of the retina. It has also offered a perfectly rational explanation 
 for the presence of Graber’s preretinal membrane. Thus the hypothesis 
 of the neural origin of the retina is no longer tenable. 
 
 The presence or absence of a lentigen and preretinal membrane is, as 
 Mark (’87, p. 55) has stated, important in determining whether a given 
 eye has been formed by involution with inversion or not. Although the 
 hypodermal nature of the retinas in both the lateral and median eyes of 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 197 
 
 scorpions is unquestionable, yet whether the lateral eyes, like the me- 
 dian, have been formed by an involution with inversion, or whether 
 their formation is accompanied simply by a thickening and more or 
 less extensive depression in the hypodermis, is still an open question. 
 Graber’s figure (’79, PI. V. fig. 4), with its preretinal membrane and 
 lentigen, would indicate that the eye arose by involution. Lankester and 
 Bourne’s figures (’83, PI. X. figs. 2, 3, and 4), in which these structures 
 are absent, would favor the explanation that the eye is only a hypodermal 
 thick enfng. 
 
 The position of the lateral eyes in scorpions has already been described. 
 In the adult Centrums each group consists of four eyes, three of which 
 are large and are designated by systematists as “principal” eyes, and 
 the fourth is small and known as an “ accessory ” eye. The larger eyes 
 are arranged in a horizontal line at the antero-lateral angle of the shield ; 
 the small eye is above a point midway between the posterior and middle 
 larger eyes. 
 
 A vertical section through the axis of one of the larger eyes (PI. III. 
 fig. 18) shows at the surface a strongly convex lens (Ins.) beneath which 
 a relatively small retina (r.) appears. The outline of the latter is marked 
 by the basement membrane (mb.), and on its dorsal and ventral edges it 
 is seen to be continuous with the hypodermis (hd.). In an eye from 
 which the pigment has not yet been removed, the whole retina is 
 intensely black. The pigment extends up to the margin of the lens, as 
 figured by Lankester and Bourne (’83, PI. X. fig. 1), and spreads out 
 above and below into the adjacent hypodermis. It is far more abundant 
 in the dorsal hypodermis than in the ventral. 
 
 The lens in the adult eye consists of essentially the same parts as in 
 the median eye, and contains no pore-canals. Its substance except the 
 front hyaline layer is stained throughout by alcoholic borax-carmine. In 
 young individuals (PI. III. fig. 21) the lenses of the lateral eyes, even 
 better than those of the median eyes, show a formation of stainable 
 cuticula ( 11 ") under the hyaline layer (ll) before a similar secretion has 
 taken place from the general hypodermis. 
 
 In the adult eye not the least appearance of a lentigen or preretinal 
 membrane is to be found, even after careful depigmentation. The fact 
 that the pigmentiferous tissue extends up to the lens is of itself sug- 
 gestive of the absence of a lentigen, for in ocelli generally this layer is 
 remarkable for its transparency. When to this is added the fact, that no 
 nuclei exist in the front part of the eye, and that in no place does the 
 basement membrane extend as a preretinal membrane across the front of 
 
198 
 
 BULLETIN OF THE 
 
 the eye, the evidence against the presence of a lentigen is apparently 
 complete. 
 
 The composition of the retina in the lateral eyes is much more diffi- 
 cult to study than in the median eyes. This is due in part to the small 
 size of the lateral retinas, and in part to their almost complete chitinous 
 investment. To make isolation preparations is wellnigh impossible, by 
 far the best results being obtained from the study of sections. 
 
 Graber (’79, PI. Y. fig. 5), believing that the composition of the 
 median and lateral retinas was essentially the same, has figured in the lat- 
 eral eyes of Scorpio retinal elements with three nuclei. Moreover, the 
 retinal elements are grouped, as in the the median eyes, in fives (PI. Y. 
 
 fig. 8). 
 
 Lankester and Bourne (’83, pp. 181-187) claim that the retina con- 
 sists of unicellular elements, or nerve-end cells, as they call them, and of 
 indifferent cells. The indifferent cells occur both between the nerve-end or 
 retinal cells, as “interneural cells,” and around the edge of the retina, as 
 “ perineural cells.” The indifferent cells all contain pigment ; the retinal 
 cells, in their opinion, are probably pigmented on their peripheries. 
 
 In Centrums the nuclei (PI. III. fig. 18, nl. r. and nl. pi n .), as in 
 the median retinas, are limited to the deeper portion and to the periphery 
 of the eye, and Graber’s anterior and median nuclei are not present. The 
 nuclei {nl. r.) belonging to the deep portion of the retina are slightly 
 larger than those {nl. pi n.) on the periphery, and very uniform in size. 
 The fact that in this part of the retina there is only one form of nucleus 
 leads to the conclusion that the retina in Centrums is composed of only 
 one kind of cells, and that here the interneural cells described by Lan- 
 kester and Bourne do not exist. 
 
 Sections perpendicular to the axis of this retina show immediately 
 under the lens the sharp outlines of cells which deeper in the retina have 
 their walls thickened into rhabdomeres. No additional cells, like those 
 in the median eyes, appear in the outermost sections of the retina, and 
 therefore the interneural cells, if present, must be limited to the deeper 
 portion of the retina. The fact that there is no difference in the nuclei 
 of this region leads me to believe that interneural cells are entirely 
 wanting. In Centrums the retinal cells (PI. III. fig. 19) show no 
 tendency to be arranged in groups of five, and the rhabdomeric thicken- 
 ing {rhb m.) takes place on all sides of the cell. This is particularly 
 noticeable in examining the region nearest the lens. In the outermost 
 sections the cells are sharply outlined and their walls are very thin. 
 In the second or third section from the lens, the walls suddenly become 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 199 
 
 thicker around the whole circumference of the cell, and take on a lus- 
 trous appearance. With Kleinenberg’s haematoxylin the substance of 
 the rhahdomeres can be colored, and the line of demarcation between 
 products of the separate cells can be distinguished. This structural con- 
 dition can be traced to the deeper part of the retina, where the cell 
 outlines become indistinct, the rhabdomeres incasing each retinal cell 
 for a half or two thirds of its length. 
 
 Pigment (PI. III. fig. 19, gra. pig.) is uniformly distributed through 
 the retinal cells, as well as the perineural cells to be described later. This 
 is best seen in sections perpendicular to the axis of the eye. Phao- 
 spheres, although present in the median eyes, do not occur in the lateral 
 eyes. The optic nerve (PI. III. fig. 18, n. opt.) emerges from the deep 
 end of the retina, and its course is so oblique to the axis of the eye that 
 a section which shows the retina well seldom shows much of the optic 
 nerve. 
 
 The perineural cells surround the depressed retinal area, and their 
 attenuated ends, especially on the ventral side of the eye, often reach 
 out, even in the adult condition, in front of the retinal cells themselves 
 (PI. III. fig. 18). The positions that the nuclei occupy in the ventral 
 portion of the perineural ring suggest that these cells may at one time 
 have extended far enough to have completely covered the retina, and the 
 fact that in young individuals (PI. III. fig. 21) the retina is largely cov- 
 ered by the perineural cells indicates that in all probability the lens is the 
 product of these cells. In that event the perineural cells are the physio- 
 logical equivalent of the lentigen. The peripheral margin of this lentige- 
 nous ring passes by insensible gradations into the surrounding hypodermis. 
 
 The development of the lateral eyes is referred to by Kowalevsky and 
 Schulgin (’86, p. 531) as follows: “Die Seitenaugen entwickeln sich 
 unabhangig von den Mittelaugen, und bei ihrer Ausbildung nimmt die 
 Yertiefung der obern Schicht der Kopfplatte Antheil. Die Einzelheiten 
 dieses Vorganges sind von uns noch nicht bearbeitet.” This is the only 
 reference which they or other students have made to the development of 
 the lateral eyes. 
 
 The “ ocular areas,” as Lankester and Bourne designate the regions 
 occupied by the lateral eyes, appear in Centrums as pigmented tracts of 
 hypodermis on either side of the head and a little below and behind the 
 median optic sacs. Horizontal sections of the embryo cut these areas in 
 the most advantageous way for a general study ; they show that the 
 whole ocular area is produced by a thickening of the hypodermis. 
 
200 
 
 BULLETIN OF THE 
 
 The horizontal sections shown in Figs. 22-27 (PI. IV.) are arranged 
 to represent the characteristic features of the ocular area of the left side 
 of the head, as one would observe it in passing from a dorsal to a more 
 ventral position. Calling that of Fig. 22 the first section, they are the 
 1st, 3d, 6th, 13th, 16tli, and 21st sections in a series from a single 
 animal. Fig. 22 represents the hypodermis directly above the eyes and 
 at the edge of the ocular area. The extent of this area is indicated by 
 the thickened region. Two sections below this (Fig. 23) the ocular area 
 is more extended, and shows a single simple depression (No. 1). It will 
 be observed that the band of nuclei indicates a more marked depression 
 even than the outline of the hypodermis itself. This simple depression 
 in the hypodermis indicates the position of a lateral eye. The cells 
 which compose the wall of the cup are wedge-shaped ; their nuclei are 
 below the middle of the cells, and those cells which occupy the central 
 portion of the depression are so attenuated at their free ends as scarcely 
 to reach the surface. The basement membrane (mb.) closely invests the 
 deep face of this structure, as it does any ordinary hypodermal thicken- 
 ing. The sixth section, Fig. 24, exhibits a region in which the ocular 
 area is greatly thickened, but it shows no depressions, and the nuclei 
 extend very near to the surface. Fig. 25, seven sections deeper than 
 Fig. 24, presents four cup-shaped depressions (Nos. 2, 3, 4, 5), each essen- 
 tially like the depression previously described. The two central depres- 
 sions (Nos. 3, 4) are the largest ; next in size is the anterior one (No. 2), 
 and smallest of all is the posterior one (No. 5). As in the case of depres- 
 sion No. 1 (Fig. 23) the band of nuclei in the region of each depression 
 forms a much deeper cup than the outer surface of the hypodermis. The 
 basement membrane (mb.), as in Fig. 23, invests only the deep surface of 
 each hypodermal cup. From this plane ventrally the hypodermis gradu- 
 ally becomes thinner, and at the extreme edge of the dorsal shield the 
 indifferent hypodermis is reached. (Compare Figs. 18, 20.) 
 
 The five depressions just described are early stages in the development 
 of the lateral eyes. In the adult Centrums only four eyes are present. 
 Of the five depressions seen in the embryo the most posterior (No. 5) 
 of the ventral four disappears, and three remaining form the “ princi- 
 pal ” lateral eyes. The fourth or “ accessory ” eye arises from the dorsal 
 depression (No. 1), which, even in the embryo, occupies a position above 
 the space between the second and third depressions (Nos. 3 and 4) of 
 the lower row. The presence in the embryo of a rudimentary fifth eye 
 is interesting, in view of the fact that there are five eyes in the adult of 
 Androctonus, as has been shown by Lankester and Bourne. It is proba- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 201 
 
 ble that one of these five eyes in Androctonns is represented by the rudi- 
 mentary eye in Centrums, although this can be definitely settled only by 
 a careful comparison. 
 
 In the embryo the fibres of the optic nerve ( n . opt.) emerge from the 
 base of the retina (PI. IV. fig. 25). This, moreover, is their position 
 throughout the life of the scorpion (PI. III. fig. 18). 
 
 The further changes which affect the form of the optic depressions 
 before they become matured eyes are unessential modifications of the 
 already established plan. At the time of the production of a lens (PI. 
 III. fig. 21) the lentigenous (perineural) cells stretch over from all sides 
 and overtop the retina. The external ends of the lentigenous cells con- 
 tain no pigment (PI. III. fig. 20). 
 
 The basement membrane, from the time when the depressions are 
 formed till the eye is completed, covers the modified hypodermis as it 
 covers a simple liypodermal thickening. There is never any indication 
 of a preretinal membrane, nor, from the structure of the eye, should we 
 expect to find one. In all stages the basement membrane presents the 
 appearance of a single delicate lamella, and at no time is there an addi- 
 tional sheet of mesodermic tissue, as in the median eyes. 
 
 The evidence derived from the anatomy of the adult eye, the absence 
 of a preretinal membrane and permanent lentigen, and the continuity of 
 the retina with the hypodermis, together with the facts derived from a 
 study of the development of the eye, show conclusively that in scorpions 
 the retina of the lateral eye is what Lankester and Bourne have called 
 monostichous, and that this retina, unlike that of the median eyes, is 
 normal, not inverted. 
 
 Theoretic Conclusions. 
 
 The striking similarity in the structure and development of the median 
 eyes in scorpions and the anterior median eyes in spiders has already been 
 indicated. In both cases the retina by a process of involution has be- 
 come inverted. The question whether the retina was functional during 
 the phylogenetic involution of the eye is, as Mark has maintained, an- 
 swered in the affirmative by the phases noted in the development of the 
 optic nerve. At least, the fact that the fibres of the optic nerve are at 
 first attached to the morphologically deep ends of the retinal cells, and 
 only at a later date come to emerge from the opposite end, is most easily 
 explainable on the supposition that the retina was functional before invo- 
 lution. The primitive eye would, then, consist of a single la} 7 er of retinal 
 
202 
 
 BULLETIN OF THE 
 
 cells from the deep ends of which the nerve fibres emerge. Admitting 
 that in the ancestral eye the rhabdomeres were in their usual position, 
 namely, at the outer end of each retinal cell, an inversion of this retina 
 would not only place the optic fibres on the front face of the retina, but 
 the rhabdomeres would come to occupy the deep ends of the cells. The 
 prenuclear rhabdomeres of a normal retina would, therefore, be homolo- 
 gous with the postnuclear rhabdomeres of an inverted retina. The 
 prenuclear rhabdomeres of the median eyes in scorpions must, then, be 
 secondary structures, developed in such a way as to replace functionally 
 the older postnuclear structures.* 
 
 The phaospheres, as Mark (’87, p. 93) has already suggested, may rep- 
 resent the remains of postnuclear rhabdomeres. These are to be regarded, 
 then, in the nature of disappearing organs, and the fact that in some 
 species of scorpions they are present, while in others they are absent, 
 would favor this view. As Mark has stated, the phaospheres, if they 
 represent postnuclear rhabdomeres, should be found only in eyes with 
 inverted retinas. Lankester and Bourne, as previously mentioned, have 
 described them in the lateral eyes of Euscorpius. Mark hesitated, in the 
 case of the lateral eyes, as to -whether he should follow Graber’s observa- 
 tions and consider them triplostichous, with inverted retinas, or whether 
 he should follow Lankester and Bourne and consider them monostichous. 
 In the former case the phaosphere might readily represent postnuclear 
 rhabdomeres ; in the latter, this interpretation would be out of the ques- 
 tion. In Centrums the structure and development of the lateral eyes show 
 conclusively that they are monostichous, and there seems to be small room 
 to doubt that the same is the case with the lateral eyes in Euscorpius. 
 In these eyes, however, Lankester and Bourne claim the presence of phao- 
 spheres. I have had no material from Euscorpius to examine ; but since 
 in Centrums the median eyes contain phaospheres, while the lateral eyes 
 are devoid of them, it is a matter of interest to see whether, upon further 
 investigation, the presence of phaospheres in the lateral eyes of Euscor- 
 pius is confirmed, or whether that genus, like Centrums, has phaospheres 
 in the median eyes only. If they should not be found in the lateral 
 eyes, there would still be reason for considering them the remnants of 
 rhabdomeres ; but if they should be found there, this view would be no 
 longer reasonable. 
 
 The possible relation of the median to the lateral eyes in scorpions 
 has already suggested itself, for in pointing out the probable nature of 
 
 * This relation of the structures of the normal and inverted retina has been fully 
 discussed by Mark (’87, pp. 87-94). 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 203 
 
 the phylogenetic antecedent of the median eyes, a condition has been 
 implied which agrees with the essential features of the lateral eyes. Of 
 all the eyes in spiders and scorpions, the lateral eyes in scorpions are un- 
 doubtedly the least complicated, and they may be looked upon as deviat- 
 ing least from the probable ancestral type. 
 
 Summary of Results. 
 
 Nos. 2-11 refer to the median eyes ; Nos. 12-17 refer to the lateral eyes. 
 
 1. The retinas of the median and lateral eyes are strictly hypodermal in 
 
 their origin. 
 
 2. The median eye is triplostichous, and is formed by an involution of 
 
 hypodermis accompanied with an inversion of the middle layer, 
 which forms the retina proper. 
 
 3. The first layer or lentigen , is modified hypodermis immediately ex- 
 
 ternal to the pocket of involution, and, in addition to secreting 
 the lens, serves a purpose which gave to it its earlier name of 
 “ vitreous.” 
 
 4. The lens is the specialized cuticula produced by the lentigen. It 
 
 differs from ordinary cuticula in containing no pore-canals, and, 
 excepting the external hyaline layer, in being stainable through- 
 out. 
 
 5. The lentigen can produce cuticula independently of the general 
 
 hypodermis. 
 
 6. The second layer, or retina , is inverted, and consists of two kinds of 
 
 cells, — retinal (nerve-end) cells and pigment cells. It contains 
 phaospheres. 
 
 7. The retinal ( nerve-end ) cells contain pigment ; their walls are thick- 
 
 ened into prenuclear rhabdomeres, and a nerve fibre emerges from 
 their deep ends. They are so arranged in groups of five, that five 
 rhabdomeres are united to form one rhabdome. 
 
 8. Each of the pigment cells is reduced to two sacs, connected by a stiff 
 
 fibre. The external sac contains pigment ; the internal, the 
 nucleus and pigment. 
 
 9. The third or post-retinal layer is the “ sclera matrix ” of Graber. 
 
 It becomes intimately fused with the retina. 
 
 10. The fibres of the optic nerve in the embryo emerge from the external 
 ends of the inverted retinal cells ; in the adult, from the opposite 
 ends. 
 
204 
 
 BULLETIN OF THE 
 
 11. The basement membrane is a cuticula produced by the inner ends of 
 
 the hypodermal cells. The preretinal membrane is the united 
 basement membranes of the lentigen and retina. It may or may 
 not contain mesodermic elements. The sclera is the basement 
 membrane of the post-retina. It is usually overlaid with a deli- 
 cate mesodermic tissue. 
 
 12. The lateral eyes are monostichous, and arise from a simple thickening 
 
 and depression of the hypodermis. 
 
 13. A ring of “perineural” cells, forming the margin of the depression, 
 
 secretes the lens, and therefore constitutes the lentigen. Since, 
 owing to subsequent recession, they do not remain interposed 
 between the lens and retina, they have not the double function 
 of lentigen and “ vitreous ” which the outer layer of the median 
 eye has. 
 
 14. The lens has the same structure as in the median eye. 
 
 15. The retinal cells occupy only the deeper portion of the depression. 
 
 There are no interneural cells. Along the external portion of 
 each retinal cell its lateral walls are thickened into rhabdomeres. 
 The nucleus is near the deep end of the cell, and from this end 
 the nerve-fibre emerges. Phaospheres are not present. 
 
 16. The basement membrane (sclera) contains no mesodermic elements. 
 
 There is no preretinal membrane. 
 
 17. The lateral eyes may be fairly taken to represent the ancestral type 
 
 of the median eyes. 
 
 Cambridge, July 1, 1887. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 205 
 
 BIBLIOGRAPHY. 
 
 Graber, V. 
 
 ’79. Ueber das unicorneale Tracheaten- und speciell das Arachnoideen- und 
 Myriapoden-Auge. Arch. f. mikr. Anat., Bd. XVII. Heft 1, pp. 58-93, 
 Taf. 5-7. 1879. 
 
 Grenacher, H. 
 
 ' 79 . Untersuchungen iiber das Sehorgan der Arthropoden, insbesondere 
 der Spinnen, Insecten und Crustaceen. Gottingen : Yandenhock und 
 Ruprecht. 1879. 8 + 188 pp., 11 Taf. 
 
 ' 80 . Ueber die Augen einigen Myriapoden. Zugleich eine Entgegnung an 
 Herrn Prof. Dr. Y. Graber in Cernowitz. Arch. f. mikr. Anat., Bd. 
 XYIII. Heft 4, pp 415-467, Taf. 20, 21. 9 Oct., 1880. 
 
 K Kowalevsky, A., and M. Schulgin. 
 
 ’86. Zur Entwicklungsgeschichte des Skorpion (Androctonus ornatus). 
 Biologisches Centralblatt. Bd. YI. Nr. 17, pp. 525-532. 1 Nov., 1886. 
 
 ' Lankester, E. R., and A. G. Bourne. 
 
 ’ 83 . The minute Structure of the Lateral and the Central Eyes of Scorpio 
 and of Limulus. Quart. Jour, of Micr. Sci., Yol. XXIII., n. ser., pp. 177- 
 212, Pis. 10-12. Jan., 1883. 
 
 V Locy, W. A. 
 
 '86. Observations on the Development of Agelena naevia. Bull. Mus. Comp. 
 Zool. at Harvard Coll., Yol. XII. No. 3, pp. 63-103, 12 pis. Jan., 1886. 
 
 1 ^ Mark, E. L. 
 
 ' 87 . Simple Eyes in Arthropods. Bull. Mus. Comp. Zool. at Harvard Coll., 
 Yol. XIII. No. 3, pp. 49-105, 5 pis. Feb., 1887. 
 
 \ Metschnikoff, E. 
 
 ' 71 . Embryologie des Scorpions. Zeitschr. f. wiss. Zool., Bd. XXI. Heft 2, 
 pp. 204-232, Taf. 14-17. 15 June, 1871. 
 
 V' Patten, W. 
 
 '86. Eyes of Molluscs and Arthropods. Mittheilungen a. d. zool. Station 
 zu Neapel, Bd. YI. Heft 4, pp. 542-756, Pis. 28-32. 1886. 
 
 Schimkewitsch, W. 
 
 ' 84 . £tude sur l’Anatomie de l’fipeire. Ann. des Sci. Nat., 6® ser., Zool., 
 Tom. XYII. Art. No. 1. 94 pp., 8 pi. Jan., 1884. 
 
206 
 
 BULLETIN OF THE 
 
 EXPLANATION OF FIGUEES. 
 
 ABBREVIATIONS. 
 
 a. Anterior. 
 
 can. po. Fine pore-canals. 
 
 can. po/ Coarse pore-canals. 
 
 cav. Cavity of infolding. 
 
 cl. pig. Pigment cell. 
 
 col. Neck of invagination. 
 
 enc. Brain. 
 
 env. em. Embryonic envelop. 
 
 gra. pig. Pigment granule. 
 
 h d. Hypodermis. 
 
 ir. Iris. 
 
 II. O uter hyaline layer of cuticula. 
 IV. Middle 
 
 11". Deep 
 
 Ins. Lens. 
 
 mb. Basement membrane. 
 
 mb. pr r. Preretinal “ 
 mu. Muscle. 
 
 n.fbr. 
 
 Nerve fibre. 
 
 n. opt. 
 
 Optic nerve. 
 
 nl. ms d. 
 
 Nucleus of mesodermic cell. 
 
 nl. pig. 
 
 “ pigment “ 
 
 nl. pi n. 
 
 perineural “ 
 
 nl. p r. 
 
 “ postretinal “ 
 
 nl. pr r. 
 
 “ lentigen “ 
 
 nl. r. 
 
 retinal “ 
 
 P- 
 
 Posterior. 
 
 pha sp. 
 
 Phaosphere. 
 
 pr. 
 
 Post-retina. 
 
 pr r. 
 
 Lentigen. 
 
 r. 
 
 Retina. 
 
 rhb. 
 
 Rhabdome. 
 
 rhb m. 
 
 Rhabdomere. 
 
 scl. 
 
 Sclera. 
 
 1, 2, 3, 4, 5. Lateral eyes. 
 
 All the figures were drawn with the aid of an Abbe camera. Except where 
 otherwise specified, all the preparations were examined in benzol-balsam. 
 
 Figures 1 to 18 represent the structure of the median eyes. Figures 1 to 9 
 illustrate the adult eye. 
 
 PLATE I. 
 
 Fig. 1. A horizontal section of the retinas of the two median eyes seen from the 
 dorsal side. The tissue has been simply hardened and cut, the pigment 
 remaining intact, and no dye being employed. X 195. 
 
 Fig. 2. A transverse section of the retina of the right median eye seen from the 
 posterior face. The pigment has been removed by potassic hydrate 
 and the tissue stained in Grenadier’s alcoholic borax-carmine. X 195. 
 
 PLATE II. 
 
 Fig. 3. The outer face of a frontal section through the retina of a median eye. 
 
 The portion of the figure nearer the right side is close to the centre of 
 the retina, that to the left is nearer the periphery. Colored with Klei- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 207 
 
 nenberg’s haematoxylin. The outline figure on thin paper (Fig. 3a) is 
 taken from the section directly external to the one just described. X 475. 
 Posterior face of a part of a transverse section of the retina described in 
 Fig. 2. X 475. 
 
 A retinal cell isolated in -^% solution of chromic acid, and examined in a 
 mixture composed of equal parts of water and glycerine. X475. 
 
 A pigment cell isolated and examined in the same manner as that shown 
 in Fig. 5. X475. 
 
 A retinal cell with an attached pigment cell. The pigment was removed 
 with a solution of potassic hydrate, and the cell was isolated and stained 
 in Grenadier’s alcoholic borax-carmine. Examined in a mixture of 
 glycerine and water. X 475. 
 
 The posterior lateral portion of a horizontal section of a retina seen from 
 the dorsal side. Partially depigmented with a solution of potassic hy- 
 drate, and subsequently stained with Czoker’s cochineal. In several 
 places the lentigen has been artificially ruptured. X 475. 
 
 Figs. 9 to 17 represent the structure of the median eyes in young scorpions. 
 
 Fig. 9. The posterior face of a transverse section of the right retina in a young 
 scorpion about the age at which it leaves the mother’s back. Depig- 
 mented with potassic hydrate ; stained in Czoker’s cochineal. X 195. 
 
 Fig. 10. The left face of a section through the right eye parallel to the sagittal 
 plane. Pigment unchanged; stained in Grenacher’s alcoholic borax- 
 carmine. This specimen was taken about the time of birth. X 195. 
 
 Fig. 11. Right face of a section from the sagittal plane of an individual somewhat 
 younger than that seen in Fig. 10. In this section the neck of the in- 
 vagination (col.) is seen to reach almost to the retina. In the sections 
 on either side of this the neck appears much reduced, and at no point 
 does it unite with the retina. X 195. 
 
 PLATE III. 
 
 Fig. 12. The right face of a section almost in the sagittal plane of an embryo. The 
 lower part of this section was exactly in the median plane. The upper 
 part was somewhat to the right of that plane. Pigment unaffected ; 
 stained in Grenacher’s alcoholic borax-carmine. X 195. 
 
 Figs. 13 to 17 represent the dorsal faces of a series of horizontal sections in the re- 
 gion of the median eyes. In all the pigment is unchanged, and all have 
 been stained with Grenacher’s alcoholic borax-carmine. The region of 
 the eye extends through forty-four sections. X 195. Beginning with the 
 most ventrally situated and passing dorsally, Fig. 13 represents the 
 seventh. It will be noted that the sections are not strictly horizontal, 
 but that they dip slightly to the left ; consequently in Fig. 13 the wall 
 of the pocket is cut on the right in the thickened or retinal region, but 
 on the left nearer the orifice of the pocket. 
 
 Fig. 14, the fourteenth section, shows the cavity of involution just before it is 
 divided into a right and left compartment. 
 
 Fig. 15, the twenty-third section, shows the cavity divided. 
 
 Fig. 16, the thirty-first section, shows the right compartment reduced in size. 
 
 Fig. 4. 
 Fig. 5. 
 Fig. 6. 
 Fig. 7. 
 
 Fig. 8. 
 
208 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 Fig. 17, the forty-first section, represents the wall of the deep (dorsal) end of the 
 pocket cut tangentially. 
 
 Figs. 18 to 27 inclusive relate to the structure of the lateral eyes. Figs. 18 and 19 
 are from preparations of adult eyes. 
 
 Fig. 18. The anterior face of a vertical axial section of the right eye (No. 2) ; 
 
 compare PI. IV. Depigmented with a solution of potassic hydrate; 
 stained with Grenadier’s alcoholic borax-carmine. X 325. 
 
 Fig. 19. Anterior face of a frontal section. Partially depigmented with 'potassic 
 hydrate; stained with Kleinenberg’s hsematoxylin. X475. 
 
 Figs. 20 to 27 inclusive give the structure of the eyes in young scorpions. Figs. 20 
 and 21 are from material of the same age as Fig. 9. 
 
 Fig. 20. Anterior face of a vertical axial section of left eye (No. 3). The pigment 
 is unaffected, and no dye has been used. The edge of eye No. 1 is seen 
 above. X 325. 
 
 Fig. 21. Anterior face of a vertical axial section of left eye depigmented in potassic 
 hydrate and stained with Grenadier’s alcoholic borax-carmine. X 325. 
 
 PLATE IV. 
 
 Figs. 22 to 27 inclusive are taken from the dorsal faces of a set of sections of the 
 youngest embryos at hand (of the same age as those from which Figs. 13 
 to 17 are taken). The sections have been depigmented with a solution 
 of potassic hydrate and stained in Grenacher’s alcoholic borax-carmine. 
 X 325. The figures represent the left cluster of lateral eyes. Begin- 
 ning dorsally and proceeding ventrally, Fig. 22 is the first section, Fig. 
 23 the third, Fig. 24 the sixth, Fig. 25 the thirteenth, Fig. 26 the six- 
 teenth, and Fig. 27 the twenty-first. 
 
Parker-Eyes in scorpions. 
 
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THE LIBRARY 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
24 
 
 PROCEEDINGS OF THE AMERICAN ACADEMY 
 
 III. 
 
 STUDIES FROM THE NEWPORT MARINE LABORATORY. 
 
 XXL — A PRELIMINARY ACCOUNT OF THE DEVEL- 
 OPMENT AND HISTOLOGY OF THE EYES IN THE 
 LOBSTER. 
 
 By G. H. Parker. 
 
 Presented by Alexander Agassiz, October 10, 1888. 
 
 The following is a brief statement of the results obtained from study- 
 ing the development and histology of the eyes in lobsters. The method 
 in which the optic nerve appears to terminate is so exceptional, that, 
 before making a final publication on this subject, it seems desirable to 
 seek confirmation in the structure of the eyes in other Crustacea. As 
 this will delay the appearance of the paper, and since in other direc- 
 tions definite conclusions have been reached, it seems advisable to 
 publish now an account of my present conclusions. 
 
 The first indication of the optic apparatus in the young lobster is a 
 pair of ectodermic thickenings on either side, and slightly in front of 
 where the mouth is to appear. The superficial part of each of these 
 thickenings gives rise to the retina, and the deep part to the optic 
 ganglion. The ganglionic portion is cut off from the retinal portion 
 by the ingrowth of the basement membrane. In certain regions, how- 
 ever, the basement membrane does not cut the connection between the 
 retina and ganglion. These primitive connections persist in the adult 
 as optic nerve fibres. 
 
 In the eye of an adult lobster each ommatidium consists of at least 
 sixteen cells. Directly under each corneal facet are found two flat 
 lentigenous cells (corneal hypodermis). Under these are four retino- 
 phorae, one for each angle of the corneal facet. The retinophorae 
 are extremely elongated, and extend from the deep face of the corneal 
 hypodermis to the basement membrane. From the corneal hypoder- 
 mis to the spindle the four retinophorae are closely applied to one 
 another. At the distal end of the spindle they separate, passing 
 around that structure as fibres. As they approach the basement 
 membrane they converge slightly, and terminate on the retinal surface 
 
OF ARTS AND SCIENCES. 
 
 25 
 
 of that membrane. Under the centre of each ommatidium the base- 
 ment membrane is considerably thickened, and it is on this thickening 
 that the four retinophorse terminate. 
 
 Each ommatidium has ten pigment cells, — two distal and eight 
 proximal. The distal cells surround the retinophorse in the region of 
 the crystalline cones, and from this region they are continued inward 
 as fibres till they pass through the basement membrane. The eight 
 proximal pigment cells are closely applied to the spindle, the fibres of 
 the four retinophorse passing between them. Seven of these are 
 deeply pigmented ; one is without pigment. The eight cells extend 
 only a short distance in front of the spindle ; the seven pigment cells 
 proper are continued inward as large fibres through the basement mem- 
 brane. In addition to the sixteen cells just described, each omma- 
 tidium has two or three irregular cells filled with a pigment, brownish 
 by transmitted, white by reflected light. These cells envelop the 
 proximal half of the spindle, and extend to the basement membrane. 
 The spindles themselves do not reach the basement membrane. 
 
 The sixteen cells already described are ectodermic in origin. The 
 two or three additional cells may be from either an ectodermic or me- 
 sodermic source, but the evidence thus far gathered points decidedly to 
 their ectodermic origin. 
 
 The basement membrane, as was previously mentioned, has a thick- 
 ening in it under each ommatidium. Around a given thickening there 
 are four openings through the membrane. Each opening, however, is 
 placed between two thickenings, so that in reality only half of each 
 cluster of four openings belongs to a given thickening. There are 
 two classes of openings, one with a single small and four large fibres, 
 and another with one small and three large fibres, passing through. 
 Each thickening has two of each class accompanying it. Of the 
 fourteen large and four small fibres passing through the four open- 
 ings, only one half, or seven large and two small fibres, belong to a 
 given ommatidium. These represent the seven deep and two super- 
 ficial pigment cells. After passing through the basement membrane, 
 these fibrous ends of the pigment cells thicken considerably, and, 
 having grouped themselves in bundles, pass inward, constantly di- 
 minishing in calibre, till they reach the optic ganglion. The optic 
 nerve between the retina and first optic ganglion is composed of 
 these fibres bound together by a small amount of connective tissue. 
 All attempts at isolating any other form of fibres have failed, and it 
 would therefore seem that the fibres of the optic nerve terminate in 
 these nine pigment cells. 
 
Bulletin of the Museum of Comparative Zoology, 
 AT HARVARD COLLEGE. 
 
 Vol. XX. No. 1. 
 
 THE HISTOLOGY AND DEVELOPMENT OF THE 
 EYE IN THE LOBSTER. 
 
 By G. H. PARKER. 
 
 With Four Plates. 
 
\ 
 
 
 
 
V)VL 
 
 
 
 No. 1. — The Histology and Development of the Eye in the Lobster. 
 
 By G. H. Parker. 1 
 
 Table of Contents. 
 
 \1 
 
 Page 
 
 I. Introduction 1 
 
 Methods 3 
 
 II. Histology 4 
 
 1. Corneal Hypodermis ... 6 
 
 2. Cone-cells 10 
 
 3. Distal Retinulae .... 15 
 
 4. Intercellular Spaces of the 
 
 Retina 19 
 
 5. Proximal Retitiulaa .... 20 
 
 Page 
 
 0. Accessory Pigment-cells . . 25 
 7. Innervation of Retina ... 26 
 
 III. Development ....... 31 
 
 1. Plan of the Eye 31 
 
 2. Optic Nerve 43 
 
 3. Differentiation of Ommatidia 45 
 
 4. Types of Ommatidia ... 56 
 
 IV. Bibliography 59 
 
 V. Explanation of Figures ... 60 
 
 Introduction. 
 
 Through the kindness of Mr. Alexander Agassiz it was my privilege 
 to spend the greater part of the summer of 1887 at the Newport Marine 
 Laboratory. During the preceding winter I had been interested in the 
 structure of the eyes in Arthropods, especially in the inversion of the 
 retina in Arachnoids and my instructor, Dr. E. L. Mark, had called my 
 attention to the importance of ascertaining whether the retina in the 
 compound eyes of Crustaceans was inverted or not. At about this time 
 Kingsley (’86 a ) published his preliminary account of the development of 
 the compound eye of Crangon, and claimed that in this crustacean, as 
 in spiders, the retina was inverted. For reasons which I shall mention 
 in the course of this paper, Kingsley’s account did not seem fully sat- 
 isfactory to me, and consequently I decided to study for myself the 
 development of the eye in a crustacean. My visit to the Newport 
 Laboratory offered an excellent opportunity to collect embryological 
 material for such a study. During August and September spawning 
 lobsters were easily obtained, and I therefore determined to study 
 the eye in the lobster, Homarus americanus , Edwards. A series of 
 lobsters’ eggs were collected, and before leaving Newport my observa- 
 
 1 Contributions from the Zoological Laboratory of the Museum of Comparative 
 Zoology, under the direction of E. L. Mark, No. XVII. 
 
 VOL. xx. — no. 1. 1 
 
2 
 
 BULLETIN OF THE 
 
 tions had been carried far enough to satisfy me that the retina in the 
 lobster was a simple ectodermic thickening. On returning to Cam- 
 bridge from Newport, the study of the lobster’s eye was continued in 
 the Embryological Laboratory at Harvard College, under the direction 
 of Dr. Mark. Here I completed the observations on the development 
 of the eye, and studied its histology. In the fall of 1888 a brief pre- 
 liminary account of the results which are now presented in full was 
 published in “ The Proceedings of the American Academy of Arts and 
 Sciences,” Yol. XXIY. pp. 24, 25. 
 
 In procuring at Newport the necessary stages in the development of 
 the lobster I proceeded as follows. 
 
 Female lobsters with eggs were obtained from the fishermen, and 
 kept in floating latticed boxes which were anchored in the small cove 
 beside the Laboratory. A few eggs were taken daily from each lobster. 
 The reagents which I employed in killing the eggs were Kleinenberg’s 
 picro-sulphuric acid, Perenyi’s fluid, a saturated aqueous solution of 
 corrosive sublimate, and hot water. The eggs which were prepared with 
 corrosive sublimate were rendered almost useless by the subsequent 
 formation of a fine precipitate. Those which were killed in Kleinen- 
 berg’s picro-sulphuric acid and in Perenyi’s fluid gave fair results ; 
 the latter reagent left the yolk in good condition for cutting. The 
 best results, however, were obtained by the use of hot water. Eggs 
 which had been prepared in this way could be easily shelled, and the 
 embryos could be readily dissected from the yolk. The separation of 
 the embryo from the yolk proved to be a great advantage, and obviated 
 the necessity of cutting the yolk, a tedious process in an egg as large 
 as the lobster’s. 
 
 In the following account of the development of the lobster’s eye, the 
 stages which it is necessary to describe are taken from different sets of 
 eggs. These sets were from different lobsters, consequently I cannot 
 state with exactness their relative ages. I shall therefore characterize 
 them by their most evident structural peculiarities. Beginning with the 
 earliest stage and proceeding to the later ones, I have lettered them 
 A, B, C, D, E, and F. Set A is in the stage of the “ egg-nauplius ” ; in 
 this set the characteristic three pairs of appendages are easily distin- 
 guishable. In set B the thoracic appendages have begun to form. 
 This stage corresponds very closely to what Reichenbach (’86, Plate III. 
 Fig. 11) has designated in the crayfish as stage H. In stage C the 
 first trace of pigment in the retina is visible. Stage D is from the 
 same series of eggs as stage C, but is seven days older than C. In both 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 3 
 
 stages C and D, the abdomen of the embryo is recurved, and reaches 
 forward covering the space between the optic lobes. Stage E corre- 
 sponds to the time of hatching. Stage F is represented by a young 
 lobster one inch in length. 
 
 The younger stages which follow the hatching of the lobster are 
 obtained with considerable difficulty, and I am under obligations to 
 several of my friends for material which covers this period. For some 
 lobsters in the “Schizopod” stage I am indebted to Mr. Sho Watase. 
 Mr. H. H. Field and Mr. Carl H. Eigenmann kindly collected for me 
 some young lobsters one inch in length. From Mr. F. L. Washburn I 
 received the eyes of several half-grown lobsters, six to eight inches in 
 length. The material which I used in studying the histology of the 
 eye in the adult was very kindly supplied to me by A. T. Nicker- 
 son and Company, of Charlestown, Mass. 
 
 Methods. 
 
 The methods of staining, embedding, etc., which I have employed, 
 are those known to all students of modern histology. In one case, 
 the staining of nerve-fibres, I have used a method which I accidentally 
 discovered while experimenting with Weigert’s haematoxylin. 
 
 In employing this method it is necessary to stain the sections on the 
 slide. The way in which I have stained sections on the slide has already 
 been described (’87, p. 175). Further experience has shown, however, 
 that the successful employment of this method necessitates a careful 
 observance of certain precautions. These I have not sufficiently em- 
 phasized in my former account, and I therefore redescribe the method, 
 calling especial attention to the precautions. The method consists in a 
 cautious use of Schallibaum’s fixative. The fixative which I have em- 
 ployed is composed of clove oil three parts and Squibb’s flexible collo- 
 dion one part. The mixture before being used should be allowed to 
 stand for about a week. After several months it may become ineffective. 
 When working, I usually employ the fixative frequently enough to fol- 
 low its changes, and at the first signs of failure I make a new mixture. 
 If for any reason I have not used the fixative for some time, I test it 
 with a few waste sections before employing it with valuable material. 
 In using it a moderate amount is applied to the slide, and the sections 
 in paraffine are placed on it. The slide and its sections are now sub- 
 jected to a temperature of 58° C. for fifteen minutes. It is important 
 to observe carefully both the length of time during which the slide is 
 heated and the temperature to which it is raised. At the end of fifteen 
 
4 
 
 BULLETIN OF THE 
 
 minutes, the slide, while warm, is thoroughly washed with flowing tur- 
 pentine. This can be applied conveniently from a small wash-bottle. All 
 of the paraffine should be removed from the slide before it becomes 
 cool, otherwise on cooling some paraffine may solidify. This is liable to 
 loosen the film of collodion. The wash of turpentine should be contin- 
 ued not only till the paraffine is thoroughly removed, but till the slide is 
 cool. Then, and not till then, can the turpentine be safely replaced by 
 alcohol, first 95%, then 70%, 50%, and 35%, and finally it can be im- 
 mersed in water. After once having got the slide with its sections into 
 water, the subsequent treatment with alcohol and water seems to have 
 no effect in loosening the sections, although the film of collodion will 
 dissolve easily in ether. I have very generally employed this method 
 of staining for two years, and as it obviates the difficulties which arise 
 from maceration or partial penetration of dyes, I use it in preference to 
 staining in toto. I have lost very few sections by it, and such accidents 
 as I have had were due, I believe, to a neglect of some of the precau- 
 tions which have been mentioned. 
 
 The method of staining nerve-fibres which I have employed consists 
 of a modified use of Weigert’s haematoxylin. The tissue which was 
 stained by this method was for the most part killed in hot water, 
 although I have also successfully stained nerve-fibres which were killed 
 in chromic acid and Kleinenberg’s picro-sulphuric acid. Sections of the 
 optic nerve which had been mounted on the slide and carried into water 
 were treated for about half a minute with an aqueous solution of 
 potassic hydrate t 1 <j%. They were then thoroughly rinsed in distilled 
 water and transferred to Weigert’s hsematoxylin. Here they remained 
 for about three hours at a temperature of 50° C. They were then 
 rinsed again in distilled water, carried through the grades of alcohol, 
 and after being dehydrated with alcohol of about 99%, they were cleared 
 in turpentine and mounted in benzole balsam. Each nerve-fibre when 
 so treated had a distinct blue-gray outline. The sections do not over- 
 stain even when they are kept in the dye for a prolonged period, and 
 there is of course no subsequent decoloring. This method yields fair 
 results when applied to nerves from any part of the lobster’s body, but 
 it is especially successful in treating that portion of the optic nerve 
 which intervenes between the retina and the optic ganglion. 
 
 The Histology. 
 
 The two movable eye-stalks of the lobster are situated one on either 
 side of the rostrum, at the angle which that structure makes with the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 5 
 
 anterior edge of the carapace. The form of the eye-stalk approaches 
 that of a short cylinder terminated by a hemisphere. The cylindrical 
 part of the stalk resembles the general surface of the body in that it is 
 covered with a firm, calcified cuticula. Excepting a portion of the 
 surface next the rostrum, the whole of the hemispherical part during 
 life is black, and covered with a flexible cuticula. The black area de- 
 fines the position of the retina. That portion of the hemispherical 
 surface which is not black, and which faces the rostrum, is covered with 
 a peninsula-shaped piece of inflexible cuticula. A broad isthmus of 
 the same kind of cuticula connects this with the shell of the cylindrical 
 part. The absence of the retina from the peninsula-shaped portion of 
 the hemisphere is due in all probability to the fact that the field of 
 vision for this part of the hemisphere is cut off by the rostrum. The 
 remainder of the hemisphere, that part on which the retina is devel- 
 oped, faces away from the lobster’s body, and its field of vision is not 
 permanently obstructed by any part of the animal. 
 
 A section perpendicular to the surface, and cutting the eye-stalk in a 
 region where the cylindrical and hemispherical parts unite, is shown in 
 Figure 26. The thick, calcified cuticula of the cylindrical part is indi- 
 cated at eta. On the inner surface of this cuticula is a thin hypoder- 
 mis (li d.). The hypodermis is bounded on its inner face by a basement 
 membrane (mb.). The cuticula of the hemispherical part (cm.) is thin 
 and flexible. It can be designated by the name corneal cuticula. 
 (Compare Patten, ’86, p. 544.) Resting on the deep face of the corneal 
 cuticula is the thick cellular layer, named by Lankester and Bourne the 
 ommateum ( omm' .). The proximal face of the ommateum is limited by 
 a basement membrane, which is continuous with that bounding the 
 corresponding face of the undifferentiated hypodermis. The ommateum 
 is continuous with the hypodermis, and in fact can be regarded as a 
 thickening of that layer. Carriere (’85, p. 169) has already pointed 
 out in the eye of Astacus a similar relation between the hypodermis 
 and ommateum, and he believes that this relation holds good for all 
 Decapods. 
 
 On inspecting the external face of the corneal cuticula, one finds it 
 divided into an immense number of square facets, one of which is shown 
 in Figure 2. Although as a rule the outline of the facet is square, it is 
 not invariably so ; for on the margin of the retinal area close to where 
 the ommateum passes over into the undifferentiated hypodermis, the 
 outline often becomes somewhat irregular, and more frequently presents 
 the form of a hexagon than of a square (Fig. 59). The number of facets 
 in each eye of an adult lobster is about 13,500. 
 
6 
 
 BULLETIN OF THE 
 
 In the ommateum the cells are arranged in specialized groups or 
 ommatidia. There is a single ommatidium under each corneal facet, 
 consequently in any given eye the number of ommatidia equals the 
 number of facets. The cellular composition of each ommatidium is 
 best understood from a comparison of longitudinal and transverse sec- 
 tions. Figure 1 represents a longitudinal section through an ommatid- 
 ium. The thick lamellated layer (cm.) at the distal end is the corneal 
 cuticula. Directly below this is a thin layer of cells, the corneal hypo- 
 dermis (cm. hd.). Following on the corneal hypodermis are the cone- 
 cells (cl. con.). They are very long, and extend from the corneal hypodermis 
 inward till their proximal ends disappear in the deep part of the retina. 
 In reality they terminate upon the basement membrane. Their distal 
 ends in the region of the crystalline cones are surrounded by pigment- 
 cells, to which I give the name distal retinulse (rtn } . dst.). These, like 
 the cone-cells, extend to the deeper part of the retina. Here the proxi- 
 mal retinulse and accessory pigment-cells occur. The proximal retinulse 
 are elongated cells ( rtn } . px .), and contain black pigment. They sur- 
 round the rhabdomes (rhb.). The accessory pigment-cells are irregular 
 cells, which fill the space between the deep ends of the proximal reti- 
 nulse. They contain a pigment which is whitish by reflected and yellow- 
 ish by transmitted light. Their nuclei are shown at nl. pig., Figure 1. 
 The x last two kinds of pigment-cells described rest upon the basement 
 membrane (mb.) ; below this membrane the fibres of the optic nerve 
 can be seen ( n.fbr .). 
 
 From this description it will be seen that the ommateum lies between 
 the corneal cuticula and the basement membrane, and is composed of 
 the following kinds of elements : cells of the corneal hypodermis, cone- 
 cells, distal retinulse, proximal retinulse, and accessory pigment-cells. 
 The numbers and positions of these cells are best made out from trans- 
 verse sections. The several kinds of cells will be discussed in the order 
 named. 
 
 The Corneal Hypodermis. 
 
 That the corneal cuticula in Decapods is separated from the cone-cells 
 by an intervening layer of cells is a view which has been held only by 
 recent investigators. Grenacher (’79, p. 123), in his account of the eyes 
 in Decapods makes no mention of such a layer, and leaves one to con- 
 clude that the cone-cells abut against the cuticula. Claus (’86, p. 57) 
 suspected the presence of a corneal hypodermis in Decapods, Schizopods, 
 and Stomatopods, but his search for it was in vain. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 7 
 
 The view that the cuticula and cone-cells are in contact, is strongly 
 contrasted with that maintained by Patten (’86, pp. 626, 642). Ac- 
 cording to this writer, the corneal cuticula is due to the activity of a 
 layer of cells, the corneal hypodermis, which intervenes between the 
 cuticula and the cone-cells. Patten has identified the corneal hypo- 
 dermis in the following genera of Decapods : Penseus, Palsemon, Pagurus, 
 and Galathea. It has also been described by Kingsley (’86, p. 863) in 
 the eyes of Crangon, and by Herrick (’86, p. 43) in the eyes of Alpheus. 
 Carriere (’89, p. 225) has recorded it in the eye of Astacus, and there is 
 now good reason for believing that a corneal hypodermis exists in the 
 eyes of all Decapods. 
 
 Patten’s statement (’86, pp. 665, 666) that the corneal hypodermis 
 “ has been invariably overlooked by Grenacher,” and Kingsley’s asser- 
 tion (’86, p. 863) that the existence of the corneal hypodermis “ was 
 utterly ignored by Grenadier,” are perhaps a trifle too strong. It seems 
 much more probable that Grenacher confused the nuclei of the cone- 
 cells and corneal hypodermis. He evidently never saw both kinds of 
 nuclei in the eye of the same Decapod. In some cases he may have 
 described the nuclei of the cone-cells, in other cases those of the corneal 
 hypodermis. In both instances what he described he took to be the 
 nuclei of the cone-cells. In the eye of Mysis, I believe that he (’79, 
 p. 118) described the nuclei of both the cone-cells and corneal hypoder- 
 mis, although in this case he was of the opinion that both sets of nuclei 
 belonged to the cone-cells. Where only one set is figured, it is difficult to 
 decide whether he has given the nuclei of the cone-cells or of the corneal 
 hypodermis. So far as I am aware, there are always in each ommatid 1 - 
 ium of a Decapod two hypodermal nuclei, and four nuclei in the cone- 
 cells. This numerical relation is sufficient to distinguish the groups of 
 nuclei, but it can only be employed satisfactorily where transverse sec- 
 tions at the proper niveau are given. Unfortunately, in the Decapods, 
 Grenacher did not figure any such sections, and it is therefore difficult 
 to decide in particular cases which kind of nuclei he has described. 
 
 In the lobster a well differentiated corneal hypodermis has already 
 been pointed out (Fig. 1, cm. lid.'). In transverse sections this presents 
 the appearance of squares of granular protoplasm (Fig. 3). Each square 
 contains two nuclei, and is bounded by a membrane. A narrow space 
 filled with granular substance separates the membranes of adjacent 
 squares. From the longitudinal section (Fig. 1) it will be seen that 
 these squares are relatively thin, so that their proportions are somewhat 
 like those of square tiles. The outer face of the tile is flat ; its inner 
 
8 
 
 BULLETIN OF THE 
 
 face is hollowed, however, so that its centre is the thinnest part. In a 
 few cases the corneal hypodermis has appeared as cubical blocks, rather 
 than as tiles. This thickened condition probably indicates an increased 
 functional activity, and the more frequently occurring tile-like condition 
 may correspond to a quiescent stage. 
 
 The two nuclei contained in each square are placed some distance 
 apart, and on one of the diagonals of the square (Fig. 3). Their long 
 axes are approximately parallel to the other diagonal. In a given eye all 
 the squares agree in having the nuclei on parallel diagonals. The pres- 
 ence of two nuclei in a square indicates that the square consists of two 
 cells. Any membrane separating the two cells must necessarily pass 
 between the two nuclei, but all attempts to discover such a membrane 
 have failed. However, for a reason which will be given shortly, I be- 
 lieve that the protoplasm of the hypodermal square is divided by the 
 diagonal which lies between the nuclei. In the centre of each square 
 several oval or round outlines are usually visible (Fig. 3). These are 
 vesicular bodies which occur in the distal ends of the cone-cells, and 
 which can be seen through the very thin corneal hypodermis. 
 
 The corneal cuticula is the result of the activity of the corneal hypo- 
 dermis. Viewed from the surface, the cuticula is divided by narrow 
 bands into square facets (Fig. 2). Each facet is external to a hypoder- 
 mal square. The proximal and distal faces of each facet, as can be seen 
 in the transverse section (Fig. 1), are very nearly flat, the proximal 
 face only being a trifle convex. This convexity, however, is so slight 
 that one cannot attribute to the facet the character of a lens. 
 
 When a piece of corneal cuticula is cleaned by treating it with potas- 
 sic hydrate, # and is then examined in water, the markings which are 
 visible with difficulty in preparations mounted in balsam are easily seen. 
 Each facet in addition to its narrow limiting bands has a faintly marked 
 diagonal band which divides the square into tw r o equal triangles (Fig. 2). 
 In the different facets of a given eye the diagonal bands are parallel. 
 Newton (’73, p. 327, Plate XVI. Fig. 3), in describing the structure of 
 the eye in the lobster, states that each facet is crossed by two diagonals 
 at right angles to each other. This statement I cannot confirm, for, 
 although I have searched with care, I have never succeeded in finding 
 more than a single diagonal in each facet. In the middle of the diagonal 
 there is an irregular hazy patch. This at times has a distinctly marked 
 cross in it. When the cross is present, one of its axes lies in the diago- 
 nal band, the other extends at right angles to the band (Fig. 2). 
 
 Whether all of these markings extend through the substance of the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 9 
 
 cuticula, or whether they are confined to its surface, is difficult to say. 
 The production of the cuticula is such a uniform process that one would 
 naturally expect to find that the marking extended through it, for the 
 successive layers would be similarly marked, and thus bands would be 
 established extending from its deep to its superficial face. Concerning 
 the vertical extension of the bands between the facets there is no ques- 
 tion, for in transverse sections of the cuticula (Fig. 1, x ) they reappear 
 in their proper positions, and extend from one surface to the other. 
 Owing to the roughness of the cut face, they are much less readily de- 
 tected in sections than when viewed from the outer surface of the cutic- 
 ula (compare Figs. 1 and 2). The diagonal band and its central spot 
 have not been observed in transverse sections, even when the plane of 
 section is in the most advantageous position for demonstrating these 
 structures. Notwithstanding their apparent absence, both may be pres- 
 ent, although indiscernible. For even in the superficial view, when the 
 outline of the facet was so readily visible, the diagonal band was only 
 faintly seen. In transverse sections, where the distinct boundary of the 
 facet is visible with difficulty, one should not expect to see the much 
 fainter diagonal. On comparing the diagonal band and the boundary 
 of the facet by focusing through the corneal cuticula, I was unable to 
 distinguish a greater vertical extension in the one than in the other. 
 Since it has been shown that the boundary of the facet extends through 
 the cuticula, this observation supports the conclusion that the diagonal 
 band also extends through it. 
 
 Patten (’86, pp. 626, 627) has described in the facet of Penseus a 
 band which has many resemblances to the diagonal band in the lobster. 
 It is not diagonal, however, but transverse, and divides the square facet 
 into two equal rectangles, in which the sides are in the proportion of 
 one to two. I have already given my reason for believing that the 
 diagonal band in the cornea of the lobster extends through the sub- 
 stance of the cuticula. Patten states that the transverse band in 
 Penaeus is only a superficial structure, and says (’86, p. 627) that in 
 cleaning the cuticula “ when the treatment with caustic potash has been 
 carried to excess, all markings disappear except the contours of the 
 facets.” I have subjected the corneal cuticula of a lobster to a boiling 
 solution of potassic hydrate (75%) for a quarter of an hour, and, al- 
 though the potash completely cleaned the cuticula, the outlines of the 
 facets, the diagonal band, and its spot were as readily visible after this 
 treatment as before. A second and third trial with the same piece of 
 cuticula did not noticeably effect the markings. In this respect, then, 
 
10 
 
 BULLETIN OF THE 
 
 the diagonal band in the lobster is materially different from the trans- 
 verse band in Penseus, and I conclude that in the cornea of the lobster 
 the limiting and diagonal bands are essentially similar in that they both 
 extend through the cuticula. 
 
 In all probability the bands between the facets were produced during 
 the secretion of the cuticula by the interference of the partitions which 
 separate the hypodermal squares. If this be true, it is probable that 
 the diagonal bands represent a like interference. It is important to 
 notice that the diagonal band in the cuticula corresponds to the imagi- 
 nary diagonal which lies between the nuclei of each hypodermal square, 
 never to the diagonal which crosses the nuclei (compare Figs. 2 and 3)i. 
 This diagonal then corresponds to the position in which one would look 
 for a membrane between the pair of hypodermal cells ; and although 
 such a structure has not been observed, the diagonal band in the cornea 
 is a strong indication of its presence. 
 
 Admitting this to be the significance of the diagonal band, it is but 
 natural to expect that, if deeper cells touch the cuticula, they would pass 
 outward between the hypodermal cells. The fact that the hazy patch 
 which lies in the middle of the facet is always on the diagonal band, 
 and directly external to the distal tips of the cone-cells, leads to the 
 belief that this patch marks the place where the cone-cells pass between 
 the cells of the hypodermis and touch the cuticula. I am not of opin- 
 ion that the patch is produced by the secretion of the cone-cells, al- 
 though I have no evidence that the cone-cells cannot produce cuticula 
 at their distal tips. It seems to me more probable that they have given 
 rise to the patch by a series of interrupted interferences with the ac- 
 tivity of the corneal hypodermis. If such be the case, a distinct cross 
 might be produced when the area of interference was definitely circum- 
 scribed. When the area was not so sharply bounded, a hazy patch with 
 indistinct outlines might be the result. 
 
 From the facts which have been presented, I conclude that each hypo- 
 dermal square consists of two flattened cells, triangular in outline, and 
 very intimately applied on their longest sides. 
 
 The Cone-cells. 
 
 One of the most important questions in the anatomy of the cells of 
 the crystalline cones (retinophorm) concerns the relation which these 
 cells bear to the rhabdome. Max Schultze was the first to maintain 
 (’67, p. 407) that the cone-cells and rhabdomes were separate struc- 
 tures. Grenacher’s researches lead to the same conclusion. As an oppo- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 11 
 
 nent of this view, Patten (’86, p. 670) has claimed that the cone-cells 
 and rhabdome were continuous, and in fact that the rhabdome of the 
 compound eye was only an enlargement of the proximal end of the cone- 
 cell. Kingsley (’86, p. 863) in his description of the eye in Crangon 
 supported Patten’s view. 
 
 Of those authors who maintain the separateness of the cone-cells and 
 rhabdome, no one, I believe, has given a fully satisfactory account of the 
 way in which the proximal ends of the cone-cells terminate. Grenacher, 
 in describing the eye in Palsemon said (’79, p. 123) : “Die fein ausge- 
 zogene Spitze dieser Pyramide [the cone-cells] durchsetzt, bevor sie in 
 contact mit der Retinula tritt, zuerst eine in Form eines Hohlcylinders 
 sie umhiillende Pigmentmasse um sich dann in das Vorderende der 
 Retinula eine Strecke weit einzusenken.” A more detailed account was 
 given by Schultze, who, after stating (’68, p. 10) that in some crusta- 
 ceans the cone-cells appeared to terminate a little in front of the distal 
 end of the rhabdome, said that in the crayfish “ geht der Krystallkegel 
 nach unten in vier Spitzen aus, welche sich aus den vier Kanten der 
 Oberflache entwickeln und das obere Ende des nervosen ebenfalls vier- 
 kantigen Sehstabes umschliessen. Die vier Spitzen legen sich dabei an 
 die Kanten des letzteren an und laufen als lange feine Faden auf der 
 Oberflache des Sehstabes herab, diesen umklammernd und mit ihm ober- 
 flachlich verbunden aber durch Maceration isolirbar. Gegen das Ende 
 spitzen sie sich fein zu und verlieren sich auf der Oberflache des Korpers, 
 den sie umfassen.” This account is the most complete of any that I 
 have seen, and yet that Schultze was not fully satisfied that he had seen 
 the proximal termination of the cone-cells is probable from the fact that 
 he says the fibres are lost on the surface of the rhabdome. 
 
 The relation of the rhabdome to the cone-cells, and the way in which 
 these cells terminate in the lobster, is as follows. As in other Deca- 
 pods, each ommatidium in the eye of the lobster contains four crystalline 
 cone-cells. Together these cells form an elongated pyramid, with its 
 base next the corneal hypodermis and its apex on the basement mem- 
 brane (Fig. 1, cl. con.). At the distal end of the ommatidium, in the 
 region which corresponds to the base of the pyramid, the four cells 
 are closely applied to each other. This condition is maintained till 
 the deeper part of the ommatidium is reached. Here the four cells, 
 reduced to fibres, separate and end independently on the basement 
 membrane. 
 
 A transverse section of the distal ends of the cone-cells is shown in 
 Figure 4. On the external faces of each group of four cells there is a 
 
12 
 
 BULLETIN OF THE 
 
 distinct bounding membrane (mb. piph.). This can be called the periph- 
 eral membrane. The four cells in each group are separated one from 
 another by delicate membranes (mb. i cl.), which often show undoubted 
 continuity with the peripheral membrane. These membranes, since they 
 lie between the cone-cells, can be called the intercellular membranes. 
 The distal end of each cell contains coarsely granular protoplasm and a 
 nucleus (Fig. 4, nl. con.). The nuclei usually lie in the external angles 
 of the cells, and do not readily take up coloring matter. The terminal 
 granular protoplasm of the four cells forms a distal cap (Fig. 1, cap.). 
 This cap fills the concavity on the proximal face of the corneal hypo- 
 dermis, and its central distal tip probably passes between the pair of 
 hypodermal cells and touches the cuticula. The spot or cross which is 
 thus probably produced in the centre of each facet has already been 
 described. 
 
 Below the cap of granular protoplasm is the crystalline cone. The 
 firm peripheral membrane of the cap is continued over the cone and 
 proximal part of the cone-cell. The distal end of the cone in cross- 
 section is a square with rounded angles. At this end there is no indi- 
 cation of a division of the cone into four segments corresponding to the 
 four cells. A transverse section midway the length of the cone (Fig. 5) 
 shows no features essentially different from those of the section across 
 the distal end. The proximal end of the cone in cross-section is nearly 
 circular (Fig. 6). On the sides of this end of the cone one often notices 
 small re-entrant angles (Fig. 6, x). The peripheral membrane dips into 
 these. The angles are usually four in number, never more, and occupy 
 positions which indicate the planes of separation between the four cone- 
 cells. In some cases delicate membranes originating from the angles di- 
 vide the substance of the cone into its four constituents (Fig. 6, mb. i cl.). 
 These membranes correspond in position to the intercellular membranes 
 at the distal end of the cone-cells. The substance of the cone is very 
 finely granular. The four constituents of each cone terminate very 
 nearly at one level. In passing in a proximal direction through a series 
 of sections, the substance of the cone is last seen as a thickening which 
 flanks the cell membranes, especially the intercellular membranes. 
 (Compare Figs. 1 and 6.) 
 
 Below the cones the outlines of the four cone-cells are well marked 
 by both peripheral and intercellular membranes (Fig. 7). The inter- 
 cellular membranes are continuous with those seen in the proximal ends 
 of some cones. In this region the cells contain coarsely granular proto- 
 plasm. In passing from the deep ends of the cones to the proximal 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 13 
 
 retinulse, the most striking difference noticed in the cone-cells is a dimi- 
 nution in their diameter. (Compare Figs. 1, 6, and 8.) On a level with 
 the distal ends of the proximal retinulse, the groups of cone-cells still 
 retain their four-parted character (Fig. 9, cl. con.). Each group is easily 
 traced between the retinulse till it approaches the distal end of the 
 rhabdome. The change which here takes place is represented in Fig- 
 ure 12. Of the four ommatidia which are shown in this section, the one 
 indicated at a is cut slightly above the rhabdome. In the case of om- 
 matidia b, c , and d, the plane of section passes through the end of the 
 rhabdome. In each of these three, it will be noticed that the rhabdome 
 is surrounded by four bodies, which correspond in position to the four 
 cone-cells. The four ommatidia which are drawn in Figure 12 are in 
 no way exceptional, but represent a very usual condition. Many such 
 cases have been examined, and whenever the tip of the rhabdome was 
 in the section, it was invariably surrounded by the four bodies previously 
 mentioned. When, on the other hand, the plane of section did not pass 
 through the rhabdome, only the four cone-cells were present. The 
 round bodies at the sides of the rhabdome can be traced from section 
 to section, and I therefore believe them to be fibres. Moreover, there 
 is no break observable in their continuity with the cone-cells, and I 
 therefore further believe that they are the fibrous prolongations of the 
 cone-cells. They have one peculiarity which is worthy of comment. 
 As the cone-cell passes over into the fibre, a considerable diminution in 
 its diameter takes place. This is accomplished at the distal end of the 
 rhabdome, and within a space equal to the thickness of one or at most 
 two sections (7.5-15 //.). Occasionally there is to be seen a group, in 
 which one or two cells have been reduced to fibres, and the remaining 
 ones are as large in transverse section as an individual in ommatid- 
 ium a (Fig. 12). The conclusions which are arrived at from the study of 
 sections are confirmed by isolation-preparations. Figure 28 represents 
 a portion of an isolated group of four cone-cells from a single omma- 
 tidium. In the distal part of the specimen the four cells are intimately 
 bound together, but at the proximal end they appear as four separate 
 fibres. As in the transverse section (Fig. 12), the continuity of the 
 fibrous and thicker portion of the cone-cell, and the rapid reduction of 
 the cone-cell to form the fibre, are plainly seen. The cone-cells are usually 
 somewhat separated before they reach the rhabdome. It can scarcely 
 be said that they touch the rhabdome, although this is the region in 
 which they are nearest to it. As the fibres pass into the deeper part of 
 the retina they are found to lie nearer the periphery of the ommatid- 
 
14 
 
 BULLETIN OF THE 
 
 ium. They lie between the proximal retinulse, but at some distance 
 from the rhabdome (Fig. 14, cl. con.). They still retain, however, the 
 samq relative positions in the ommatidium. Their peripheral location 
 is maintained (Fig. 17) till they are very close to the basement 
 membraue. 
 
 The changes which the fibres undergo as they approach the basement 
 membrane is shown in Figure 19. In this section the plane of cutting 
 was slightly oblique to the basement membrane. Of the three omma- 
 tidia which are here represented, those indicated at c and d are cut at 
 about the same level, and very close to the basement membrane. Om- 
 matidium b is cut farther from the membrane than either c or d. The 
 fibres of the cone-cells are closer to each other in c and d than in b. 
 As this condition is generally true in other sections, it follows that, as 
 we approach the basement membrane, the fibres of the cone-cells con- 
 verge. The convergence is also shown in Figure '21. Of the omma- 
 tidia here figured, b is cut farthest from the basement membrane. In 
 it the four cone-cells (cl. con.) can be seen, and between them a dot 
 which represents the proximal end of the rhabdome (rhb.). Nearer the 
 basement membrane is ommatidium a, in which the four cone-cells can 
 be recognized, and to one side a fibrous area. The rhabdome does not 
 extend as deep as this. The fibrous area represents a region in which 
 the plane of section passes through an elevation on the distal face of 
 the basement membrane. Ommatidium d is still nearer the membrane. 
 The cone-cells are here brought more closely together, and are sur- 
 rounded by the fibrous substance of the elevation. The form of the 
 elevation is now seen to be that of a cross. At x is shown a basal section 
 of the cross-shaped elevation surrounded by four large openings through 
 the basement membrane. The cone-cells are no longer visible at this 
 level, and I therefore believe that without penetrating the basement 
 membrane they terminate in these elevations. This belief is further 
 supported by the fact that in transverse sections of the basement mem- 
 brane the cone-cells distinctly end in the substance of these elevations 
 (Fig. 29, cl. con.). 
 
 The facts obtained from a study of the lobster’s eye support the claim 
 made by Schultze and Grenacher, that the cone-cells and rhabdomes are 
 separate structures. In the case of the crayfish, Schultze, moreover, 
 saw the prolongations of the cone-cells, and traced them into the deeper 
 part of the retina. It is probable that in the crayfish, as in the lobster, 
 the fibrous ends of the cone-cells terminate in the basement membrane, 
 but this Schultze did not see. Such an omission is by no means sur- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 15 
 
 prising; for when we reflect upon the methods at his command, it is 
 remarkable what success he had in tracing the course of the fibres, and 
 in demonstrating the relation of the cone-cells and rhabdome. 
 
 Patten has advanced the view that the cone-cells are provided with 
 an axial nerve-fibre, and that the cone itself is the true perceptive ele- 
 ment. I shall defer the consideration of this topic till I describe the 
 innervation of the retina. 
 
 The Distal Retinulae. 
 
 Surrounding each crystalline cone are two pigment-cells, the distal 
 retinulse. These cells not only surround the cone, but extend as fibres 
 into the proximal part of the retina (Fig. 1, rtrd . dst.). 
 
 The relation which the distal retinulse sustain to the cone can be 
 studied most readily in transverse sections. In a section passing 
 through the distal end of the cone (Fig. 4, ommatidium cl), it will be 
 observed that of the four lateral faces which the cone presents, two, the 
 lower and left-hand ones, are covered by a single retinula (rtn r . dst.). 
 The retinula is thickest at the lower left-hand angle of the cone, and 
 becomes thinner the farther it extends on the two adjacent faces. At 
 the more distant edges of these two faces the retinula terminates. 
 Thus the retinula is composed of a central portion and two blade-like 
 extensions. Each blade covers one face of the cone. The second 
 retinula is essentially like the one just described, but lies at the upper 
 right-hand angle of the cone and covers its upper and right-hand 
 faces. In this way the four faces of the cone are sheathed by a pair of 
 retinulse. 
 
 On inspecting the arrangement of the retinulse in adjoining omma- 
 tidia (Fig. 4), it is evident that they are so placed that the thick end 
 of each blade-like portion is opposite the thin end of the blade of a 
 neighboring retinula, and that, in passing along the space between the 
 cones, as one retinula becomes thicker the other becomes thinner. The 
 delicate membranes which separate the blades consequently extend 
 obliquely across the spaces between the cones (Fig. 4). In the space 
 which is left between the angles of four adjoining cones the mem- 
 branes of the retinulse are very much thickened. That this is a 
 thickening in the membrane of the retinula, and not due to substance 
 produced by the cone-cells, seems probable for two reasons. First, the 
 membrane is often somewhat thickened in regions between two retinulse, 
 and where the cone-cells could not well touch it. Such thickenings are 
 directly continuous with the larger ones already mentioned (Fig. 4). 
 
16 
 
 BULLETIN OF THE 
 
 Secondly, in isolation-preparations the thickenings always remain at- 
 tached to the retinulse ; the cones, on the other hand, are covered with 
 membranes of uniform thickness. The thickening in the membrane 
 is not characteristic of the whole length of the retinula, but is pecu- 
 liar to the region corresponding in level to the distal end of the cone 
 (Fig. 1). 
 
 The foregoing description applies to the structure of the distal 
 retinulse as seen in the plane of Figure 4. This plane passes through 
 the outer ends of the cones. In other regions the retinulse present 
 somewhat different conditions. The relation of the retinulae to the 
 hypodermal squares is shown in a section (Fig. 3) which is slightly 
 more superficial than that just described. The two retinulae which 
 were located at the two angles of the cones here occupy the corre- 
 sponding angles of the hypodermal squares. They do not, however, 
 entirely cover the four lateral faces of the square, as they did those of 
 the cone, but from the angles at which they are located they extend 
 over half of each of the adjoining faces. It follows from this that 
 together they flank only one half of the lateral exposure of the square. 
 The blades are now no longer wedge-shaped in transverse section, nor 
 do they overlap neighboring blades, but each one stretches completely 
 across the space in which it lies. Of the lateral surface of the square, 
 that half which is not sheathed by its own pair of retinulse is covered 
 by the arms of four adjacent retinulse. Consequently, six retinulse in 
 all touch each hypodermal square. Two of these belong to the omma- 
 tidium which is represented by the square ; four belong to adjoining 
 ommatidia. The relation of these will be readily seen by referring to 
 Figure 3. 
 
 In passing from the plane in which each cone is surrounded by its 
 own pair of retinulse to the' one in which the corresponding hypodermal 
 square is surrounded by six retinulse, the blades of the two retinulse 
 proper to the cone undergo a gradual narrowing ; so that, instead of 
 each blade covering the whole of one face of the cone, it covers less and 
 less, and eventually sheathes only one half of the corresponding face of 
 the hypodermal square. As the blades of the retinulse become narrower, 
 they expose the surface of the cone, but this is still kept covered by the 
 retinulse of adjoining ommatidia. In any ommatidium there are four 
 blades which become narrow, consequently there are four regions in 
 which adjacent retinulse touch the cones; and as there is a separate 
 retinula for each region, it follows that four additional retinulse here 
 come in contact with the cone. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 17 
 
 The distal retinulse touch the cuticula along the band which marks 
 the boundaries of the facets. That the retinulse contribute to the 
 formation of the cuticula is very improbable, although I believe that 
 it is largely through their interference that the outlines of the facets 
 are produced. 
 
 The lateral surfaces of each cone are completely enclosed by retinulse ; 
 the pair of retinulse belonging to the cone play the principal part. 
 That portion of each retinula which encloses the proximal two thirds of 
 the cone is densely pigmented (Fig. 1). In transverse sections through 
 the pigmented region one can see that each cone is completely sur- 
 rounded by a pigment band. On a level with the middle of the cone 
 each retinula contains a nucleus (Figs. 1 and 5, nl . dst.). This is im- 
 bedded in pigment. The membranes of the retinuke are less distinct in 
 the pigmented region than near the distal end of the cone. The only 
 membrane which was observed (Fig. 5) was one which corresponds to 
 the thickened membrane shown in Figure 4. At the proximal end of 
 the cone, tha retinulse rapidly contract till they are reduced to fibres 
 (Figs. 6 and 7, rtn'. dst.). The pigment is present for only a slight 
 distance below this level. The fibres of the retinuke are grouped in 
 pairs, and in this relation extend to the proximal part of the retina. It 
 is noticeable that the two fibres which constitute a pair are derived, not 
 from a single ommatidium, but from two adjacent ommatidia. These 
 fibres when seen in longitudinal sections were probably mistaken by 
 Newton (’73, pp. 328, 329) for an investing membrane. At least, in 
 all attempts to demonstrate the existence of such a membrane I have 
 failed, and there is so strong a resemblance between the fibres of the 
 distal retinulse and the structure which Newton figured (’73, Plate 
 XVII. Fig. 15) as the cut edge of an investing membrane, that I am 
 inclined to think them identical. Transverse sections from the proper 
 region would have settled the question whether these bodies were fibres 
 or membranes, but unfortunately Newton has not figured any such 
 sections. 
 
 The pair of fibres in passing from the basal ends of the cones to the 
 proximal retinulse retain the same relative position, and are only slightly 
 reduced in diameter. (Compare Figs. 7 and 8, rtn r . dst.) Deeper than 
 this, they are still identifiable, and can be distinguished from the fibres 
 of the cone-cells by their slightly greater diameter, and by the fact 
 that they are always in pairs. They lie in the space between four 
 ommatidia. (Compare Figs. 12, 15, and 18.) Till within a very short 
 distance of the basement membrane they maintain the condition shown 
 
 VOL. XX. — NO. 1. 2 
 
18 
 
 BULLETIN OF THE 
 
 in Figure 18, rtn /. dst. Beyond this I have not been able to trace 
 them with certainty. The groups are no longer observable, and it is 
 probable that the fibres have separated. I know that in this region the 
 other cells suffer a very considerable rearrangement ; and such being 
 the case, it would be a very difficult matter to identify single fibres, 
 especially fibres as small as these are. I have not found any satisfactory 
 method of staining the fibres so as to distinguish them, as in the case of 
 the fibrous ends of the cone-cells. I can therefore claim to have traced 
 the fibres only to within about 20 \i from the basement membrane. 
 
 As I have already mentioned, the distal face of the basement mem- 
 brane has cross-shaped thickenings on it. In the angles which the arms 
 of the cross make with each other, the basement membrane is perforated. 
 There are consequently around each cross four openings through the 
 membrane. Each opening, however, lies between two crosses, so that in 
 reality only one half of each opening belongs to a given cross, or, if one 
 counts whole openings only, half of the four openings, i. e. two openings, 
 belong to each cross. The crosses correspond in number and position 
 to ommatidia, hence there are also two openings for each ommatidium. 
 In each opening, beside three or four large fibres which will be described 
 later, one finds a single small fibre (Fig. 21, rtn 1 . dst.). That this fibre 
 represents the continuation of the fibrous end of a distal retinula seems 
 probable, for two reasons. First, the diameters of this fibre and of the 
 fibrous part of the distal retinulae are so nearly the same as to be undis- 
 tinguishable. Secondly, the number of fibres which pass through the 
 basement membrane, two for each ommatidium, agrees with the number 
 of distal retin ulse in each ommatidium. I therefore believe that the 
 small fibres which are seen in the openings through the basement mem- 
 brane are the proximal continuations of the fibres of the distal retinulse. 
 If this explanation be true, then it is only natural to expect that, as a 
 pair of fibres approaches the basement, the individual fibres should 
 separate, one passing through each opening. As I have already ex- 
 plained, the fibres, while separated, could be identified only with great 
 difficulty. 
 
 If the fibres pass through the basement membrane, as I believe they 
 do, they terminate only a short distance below it. For at about 15 yu. be- 
 low the membrane all of the fibres are of nearly the same size, i. e. some- 
 what larger than the large fibres which pass through the membrane 
 (Fig. 22). In this region, then, the smaller fibres have either increased 
 to the size of the larger ones, or diminished till no trace of them is left. 
 The fibres here are in groups, however, and these are directly continu- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 19 
 
 ous with the groups which pass through the membrane. Each group of 
 large fibres as it passes through the membrane consists of either three 
 or four individual fibres. If the smaller fibres disappear, the groups 
 below the membrane should consist of three or four fibres also ; if, on 
 the other hand, the smaller fibres increase in size, the deeper groups 
 should consist of four or five fibres. By either method of change there 
 would be groups of four fibres, so that it is the groups of three or five 
 fibres which will be decisive. As a matter of fact, the fibres are very 
 commonly in groups of three, and not in groups of five ; consequently I 
 conclude that the smaller fibres dwindle out a short distance below the 
 basement membrane. 
 
 The distal retinulse have not been identified in many Decapods. 
 Carriere (’85, p. 169) has described them in the eye of Astacus. In 
 Penseus, Patten (’86, p. 634) has observed four cells which belong to 
 the pigmented collar of the retinophora. Two of these, the inner ones, 
 evidently correspond to the distal retinulac of the lobster. They sur- 
 round the cones. The other two, the outer ones, appear to have no 
 homologue in the lobster’s eye. 
 
 The Intercellular Spaces of the Retina. 
 
 In the region of the retina which lies between the proximal ends of 
 the cones and the distal border of the deeper band of pigment, the 
 groups of cone-cells and the pairs of distal retinulse are separated by 
 considerable intervening space (Fig. 1, spa. i cl.'). This space is filled 
 with a fluid which contains a very small amount of albuminoid sub- 
 stance. Patten (’86, Plate 31, Fig. 73, x) has figured a similar fluid- 
 filled space in Penseus. On the application of heat this albuminoid 
 substance in the lobster coagulates and forms larger or smaller vesicular 
 bodies, which vary much in size. They are usually loosely attached to 
 the cone-cells and the fibres of the distal retinulae. They readily take 
 up coloring matter. They have never been observed in fresh retinas 
 when teased in normal salt solution, nor in maceration-preparations. 
 It was probably these bodies which Newton (’73, p. 329, Fig. 15, d) 
 described as the nuclei on the investing membrane. 
 
 In addition to the albuminoid substance which I have described, one 
 occasionally meets with a thin layer of homogeneous material -which 
 lies slightly in front of the rounded ends of the proximal retinulae. 
 This forms a dividing membrane which separates the retina into a prox- 
 imal and distal portion. The membrane is of course pierced in many 
 places. There is an opening in it for each pair of distal retinulse, and 
 
20 
 
 BULLETIN OF THE 
 
 each group of cone-cells. In many cases the membrane has not been 
 observed. It was noticed by Newton (’73, p. 328), and as it is non- 
 cellular it is probably a feeble representative of what Herrick (’86, p. 44) 
 has described as a “ cliitinous ” framework in the deeper part of the 
 retina in Alpheus. 
 
 The Proximal Retinulce. 
 
 The proximal retinulse are pigment-cells which closely invest the 
 rhabdome. With the brownish accessory pigment-cells they constitute 
 the proximal band of brownish black pigment on the distal side of the 
 basement-membrane (Fig. 26, pig. px.). In some cases they appear to 
 terminate distally in rounded knobs, each of which contains a nucleus 
 (Fig. 1, rtn'. px.). In other instances, and these are of frequent occur- 
 rence, their distal ends, in addition to having a swollen nucleated part, 
 are prolonged into delicate fibres (Fig. 30). These fibres when present 
 extend toward the outer surface of the retina, and are applied, not to 
 the cone-cells, but to the fibrous portion of the distal retinulse. The 
 fibres have been traced only a short distance beyond the rounded ends 
 of the cells from which they originate. As the region into which they 
 extend is one readily studied in both sections and maceration prepa- 
 rations, and as these methods of study have given no evidence of fibres 
 other than that of the very short ones already mentioned, it seems fair 
 to conclude that the distal retinulse terminate as fine fibres a short dis- 
 tance in front of their nucleated portions. 
 
 In transverse sections the distal retinulse first clearly appear in the 
 plane represented in Figure 9. Here each group of four cone-cells is 
 surrounded by a circle of seven retinulse. The section from which this 
 figure was drawn is in a slightly oblique plane. In moving from right 
 to left, one passes into deeper and deeper regions. In the more super- 
 ficial part of the section, the right-hand half, each retinula contains a 
 nucleus, which is surrounded by a small amount of pigmented cell-sub- 
 stance. In the deeper part of the section, the left-hand half, the plane 
 is below the region of most of the nuclei, and one sees the seven reti- 
 nulse densely filled with pigment. In the next section (Fig. 10), the 
 retinulse are broader in transverse section. In their expansion they 
 have so far encroached on the space which they surround that it is 
 only large enough to allow the passage of the four cone-cells. The con- 
 traction of the space within the circle of retinulse takes place almost 
 in one plane, as can be seen in the longitudinal section (Fig. 1). In the 
 plane of Figure 10, the retinulse show a tendency to group themselves. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 21 
 
 One can recognize an odd,, usually larger retinula, which occupies the 
 lower right-hand corner of each group. The remaining six retinulse 
 are disposed in pairs. In Figure 11, which represents a plane of section 
 deeper than that shown in Figure 10, the retinulse, although somewhat 
 reduced in thickness, present nevertheless the same method of group- 
 ing as was pointed out iu Figure 10. In this plane one also notices 
 next the odd retinula, a nucleus. This is remarkably constant in its 
 occurrence, both as to position and as to the fact that there is always a 
 single nucleus. When compared with the nuclei of the surrounding retin- 
 ulse, it is found to resemble them very closely. The nuclei of the seven 
 retinulse are characterized by their sharply marked oval outlines, and 
 by the possession of one or two very distinct nucleoli (Fig. 30, nl. px.). 
 In both of these respects the single nucleus agrees so closely with the 
 nuclei of the retinulse, that, were it not for its somewhat smaller size 
 and deeper position, it could not be distinguished from them. The 
 regularity with which it occurs, and its structural peculiarities, incline 
 me to believe that it represents a reduced retinula in which pigment 
 has never been developed. This belief is further supported by the fact, 
 that the additional nucleus is always found next the larger retinula, 
 which from its great size seems to have replaced a second cell. It is 
 therefore probable that each ommatidium of the lobster’s eye possesses 
 eight proximal retinulse rather than seven, and that one of these is 
 rudimentary. 
 
 Below this additional nucleus, the proximal retinulse pass around the 
 rhabdome. In this region they are deeply pigmented, and so completely 
 envelop the rhabdome that I am of opinion that no appreciable amount 
 of light gains access to it except through the cone-cells. The cone-cells, 
 it will be remembered, extend through the central region of each group 
 of proximal retinulse, until they almost impinge on the distal end of 
 the rhabdome. Thus, by excluding the retinulse, they form a trans- 
 parent shaft, which leads to the distal tip of the rhabdome, and by 
 which that structure can receive light. The rhabdome in transverse 
 section has a four-sided outline. Three sides of the rhabdome are occu- 
 pied each by a pair of retinulse. These pairs are composed of the same 
 couples which were previously noticed. (Compare Figs. 10 and 14.) 
 The fourth side of the rhabdome is occupied by the seventh or odd 
 retinula. Thus, again, it is noticeable that this retinula occupies a 
 position where, if perfect symmetry were shown, we should expect two 
 retinulse. The rhabdome in transverse section is broadest midway of 
 its length. In this position the retinulse are small, as if closely pressed 
 
22 
 
 BULLETIN OF THE 
 
 against its sides (Fig. 14). Below its middle transverse plane the 
 rhabdome becomes gradually smaller and smaller, till finally it termi- 
 nates about 15 /x from the basement membrane. As the rhabdome con- 
 tracts in size, the retinulse enlarge. (Compare Figs. 14 and 17.) 
 
 As I have already mentioned, the retinulse are definitely arranged 
 around the rhabdome, and this arrangement persists nearly to its 
 proximal termination ; but between the end of the rhabdome and the 
 basement membrane the retinulse rearrange themselves. This re- 
 arrangement of the retinulse is a step preparatory to their passing 
 through the apertures in the basement membrane, the general struc- 
 ture of which has already been described. It will be remembered that 
 under each ommatidium the distal face of the membrane presents a 
 cross-shaped thickening, and that in each of the four angles which the 
 arms of the cross make with each other there is an opening (Fig. 21). 
 The openings are oval in outline, especially on the distal face of the 
 membrane. One end of a given oval lies in the angle of the cross, 
 and the crosses are so close to each other that the other end of the 
 same oval lies in the angle of a neighboring cross. Each opening then 
 lies in the angles of two adjoining crosses, and through it pass two 
 groups of retinulse, one from each of the two ommatidia to which the 
 crosses correspond. 
 
 The four groups into which the retinulse of a single ommatidium 
 are divided pass one through each of the four surrounding apertures. 
 Three of the groups consist of pairs of retinulse; the fourth group is 
 represented by only a single retinula. Although these groups agree 
 numerically with the groups of retinulse, which, as I have already 
 shown, surround the rhabdome, they are not composed of the same 
 individual retinulse. 
 
 For convenience of comparison, numbers can be assigned to the dif- 
 ferent retinulse. This has been done in ommatidium c (Fig. 15), where 
 the large odd retinula is numbered 1, and the remaining retinulse, pro- 
 ceeding in a circle to the left, are numbered 2 to 7. On this plan of 
 numbering, the four groups of retinulse which have been already indi- 
 cated as surrounding the rhabdome are composed as follows. What 
 may be called the first group is formed of retinulse 2 and 3, the second 
 group contains retinulse 4 and 5, the third retinulse 6 and 7, and the 
 fourth retinula 1. It will be observed (Fig. 15) that the seven retinulse 
 are also divided into four other groups by the fibres of the cone-cells. 
 Three of these groups are composed of pairs of retinulse, the individuals 
 of which lie nearest the angles of the rhabdome. In Figure 15, omma- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 23 
 
 tidium c, these three groups are represented by retinulse 1 and 2, 3 and 
 4, and 5 and 6. The fourth group is composed of the single retinula 
 number 7. 
 
 The four groups thus defined are identical with the ones which pass 
 through the four openings in the basement membrane. In Figure 21 
 the four openings which surround the cross ( x ) can be designated from 
 their positions as the upper, lower, right-hand, and left-hand openings. 
 The upper and lower openings each present the transverse section of 
 four large fibres. The right- and left-hand openings are each occupied 
 by three large fibres. The source of these fibres can be ascertained by 
 comparing Figures 20 and 21. In ommatidium c (Fig. 20) retinulse 
 5 and 6 unite with retinulse 1 and 2 of ommatidium d, and thus consti- 
 tute the four fibres which pass through the upper aperture (Fig. 21). 
 In a similar way, retinulse 1 and 2 of ommatidium c unite with 5 and 6 
 of an ommatidium which lies below c, and pass as four fibres through 
 the lower opening (Fig. 21). Retinulse 3 and 4 of ommatidium c 
 (Fig. 20) unite with retinula 7 of an ommatidium to the left of c, and 
 emerge as three fibres through the left-hand opening (Fig. 21). Retin- 
 ulse 7 of c, and 3 and 4 of b (Fig. 20), unite and form the three large 
 fibres of the right-hand opening (Fig. 21). This plan of distribution is 
 repeated in each ommatidium, and thus brings about the groups of 
 three or four fibres which ocgur in each opening through the basement 
 membrane. The groups of fibres are distinguishable for only a very 
 short distance below the basement membrane. The individual fibres of 
 each group soon separate, and in the deeper part of the optic nerve they 
 never again present this grouping. The description of the termination 
 of the fibres will be deferred to a later part of this paper. 
 
 The relation of the rhabdome to the cone-cells in Homarus has al- 
 ready been described. That they are separate structures, as Schultze 
 and Grenacher have asserted, I believe there can be no doubt. I have 
 seen nothing which favors the view held by Patten, namely, that the 
 rhabdome is an enlargement in the proximal part of the cone-cells. In 
 Homarus the rhabdome has the general form of a spindle. In trans- 
 verse section, however, it is not circular, but square. Its four sides are 
 thrown into ridges, the crests of which extend across the sides at right 
 angles to its longest axis. The inner face of each proximal retinula is 
 thrown into corresponding undulations. The rhabdome and retinula 
 are so adjusted to each other that a crest on one fits into a furrow 
 on the other. (For a similar condition in Penseus, compare Patten, 
 ’86, Fig. 72.) The retinulse and rhabdome are thus intimately bound 
 
24 
 
 BULLETIN OF THE 
 
 together. On comparing opposite faces of the rhabdome, it will he seen 
 that the crests of one side are in the same horizontal plane as those of 
 the other ; but on comparing adjoining faces, it will be observed that the 
 crests of one correspond to the furrows of the other. 
 
 In the rhabdome of the lobster I have not found a complicated system 
 of plates, such as Patten describes in Penaeus. The substance of the 
 rhabdome in the fresh condition is apparently homogeneous, but in har- 
 dened preparations it is finely granular and stratified. The strata are 
 at right angles to the long axis of the rhabdome, and the rhabdomes 
 often break transversely. The stratified condition of the rhabdome, and 
 the close relation which the proximal retinulae bear to it, support the 
 conclusion that the rhabdome is the product of the proximal retinulae. 
 
 In the structure of the rhabdome there is one peculiarity which, 
 although I cannot explain it, requires some comment. In transverse sec- 
 tions the square area of the rhabdome is often divided into four smaller 
 squares by two intersecting lines (Figs. 13 and 43). I made this obser- 
 vation before I had studied the relation of the distal tip of the rhab- 
 dome to the cone-cells, and I concluded then, that, if Patten was correct 
 in believing that the cone-cells and rhabdome were continuous, these 
 four divisions of the rhabdome must correspond to the four cone-cells. 
 I recognized the fact, however, that, if such was the case, the group of 
 cone-cells in the region of the rhabdome was turned through an angle of 
 45° as compared with its position at the surface of the retina (contrast 
 Figs. 4 and 13). After having satisfied myself that the cone-cells and 
 rhabdome were separate structures, I was forced to the conclusion, that 
 the four segments of the rhabdome were independent of the four cone- 
 cells. That there can be no question of the independence of these two 
 structures is shown by the condition of the cone-cells and rhabdome 
 in Mysis. In all Decapods, so far as I am aware, the cone is formed of 
 four cone-cells, and the rhabdome has four segments; in Mysis, how- 
 ever, the cone is formed of only two cells, although the rhabdome has 
 four segments. 1 
 
 I can offer no explanation of the cross lines which occur in the rhab- 
 dome. As Grenacher (79, p. 124) has observed, one might at first take 
 them for the outlines of such parts of the rhabdome as were produced by 
 individual retinulae. There are, however, seven retinulae, and only four 
 segments. Not only do the numbers disagree, but the position of the 
 lines in the rhabdome is difficult to explain. If the lines are related 
 to the retinulae, it would be natural to expect that they would coincide 
 1 My attention was called to this fact by my friend, Mr. H. H. Field. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 25 
 
 with the junction of two retinulse. As a matter of fact, three lines do 
 come between retinulse, but the fourth one abuts against the middle of 
 the large odd retinula (Fig. 34). This relation of the cross-lines and 
 retinulse persists from the earliest stage in the production of the rhab- 
 dome, and although the lines are doubtless formed during this process 
 they show a strange independence of the retinulse. 
 
 An exactly similar relation between the cross-lines and retinulse has 
 been described by Grenacher (’79, p. 124) in Palsemon. I cannot 
 agree with Grenacher in believing that the four lines are due to the 
 fact that only four retinulse are concerned in the production of the 
 rhabdome. The relations of the retinulse and rhabdome are the same in 
 the young lobster (Fig. 58) in which the rhabdome is being produced 
 as in the adult. This fact was not known to Grenacher. It shows, I 
 believe, that the rhabdome is the product of the surrounding seven reti- 
 nulse, and that the problematic lines have some other significance than 
 that of indicating regions of production. 
 
 The Accessory Pigment-cells. 
 
 These cells occupy the open space at the base of the ommatidia. 
 They are characterized by possessing a pigment which, as I have before 
 stated, is brownish by transmitted and whitish by reflected light. The 
 cells are bounded proximally by the basement membrane, and their 
 distal ends rarely reach beyond the middle of the rhabdomes (Fig. 1). 
 They are extremely irregular in form, and seem to fit themselves to a 
 cavity of almost any shape. Their function seems to be that of filling 
 what would be otherwise an unoccupied space, as though to lend solid- 
 ity to the tissue in the base of the retina. (Compare Figs. 13 and 16.) 
 Their nuclei are irregular in form and size (Fig. 18, nl. pig.). Judging 
 from the number of nuclei, two or three cells are associated with each 
 ommatidium. The number, however, is variable. The physical prop- 
 erties of the pigment which these cells contain are very characteristic. 
 Streaks of this pigment, and even whole cells, are to be met with in 
 the open space on the proximal side of the basement membrane. 
 
 Cells similar to those which I have called accessory pigment-cells have 
 been described by Carriere (’85, p. 169) in Astacus, and by Patten (’86, 
 p. 636) in Penseus. It is highly probable that the yellow pigment 
 which Grenacher (’79, Fig. 114) figured in the base of the retina of 
 My sis represents accessory pigment-cells as well as the dark pigment 
 which he described (’79, p. 124, Fig. 117) in Pakemon. 
 
26 
 
 BULLETIN OF THE 
 
 The Innervation of the Retina . 
 
 The study of the termination of the nerves in the retina is of partic- 
 ular importance, since it aifords a means of identifying the perceptive 
 elements. Wherever these elements may be, the ultimate branches of 
 the nerve-fibres must unquestionably lead to them; hence the impor- 
 tance of discovering the termination of the nerve-fibres. 
 
 Students who have investigated the compound eyes of Arthropods 
 have held two opinions as to the position in which the nerve-fibres ter- 
 minate. One school has maintained that the fibres terminate in the 
 crystalline cones, and that therefore these bodies are the perceptive 
 elements. The other school has endeavored to show that the fibres 
 end in the region of the rhabdome, and that for this reason the rhab- 
 dome is the perceptive element. I shall not attempt to give an his- 
 torical account of this subject, but only call attention to the fact, that 
 of late years the majority of writers have expressed the opinion that the 
 rhabdome is the perceptive element, and that the cone is merely dioptric 
 in function. This conclusion has been recently criticised by Patten, 
 who believes the cone to be the perceptive body. 
 
 Many of Patten’s statements are based upon observations which were 
 possible only by his methods of investigation. On this account, as well 
 as for the reason that his paper is the most important recent contribu- 
 tion to the study of nerve-termination in compound eyes, I shall not 
 refer to the older publications, but limit myself to what he has pre- 
 sented in his article on “Eyes of Molluscs and Arthropods,” and to such 
 papers as have appeared since the publication of that work. 
 
 When comparing Patten’s statements on nerve-termination with 
 those of other recent investigators, I was inclined to believe, since 
 Patten used new and probably better methods of study, that his results 
 were more trustworthy. The contrast between his views and those 
 which are more generally accepted is so striking, however, that in begin- 
 ning a study of the nerve-terminations the first step to be taken was 
 necessarily one of confirmation. I was unable to obtain the same 
 species of Crustaceans as Patten had used, but I believe that I was safe 
 in assuming that the difference which may exist between the innerva- 
 tion of the retina in Penseus and Homarus could not be a fundamental 
 one, and that the more important feature which had been demonstrated 
 in one genus could be shown in the other. I consequently prepared 
 sections of the retina of Homarus according to the methods which Patten 
 had recommended, and although I was careful in both preparation and 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 27 
 
 examination of these sections, I was entirely unsuccessful in discovering 
 any trace of nerve-fibres such as Patten had described. I therefore 
 resolved to try his methods of maceration. This I did, following closely 
 his directions as to the strength of solutions, length of time during 
 which reagents should be employed, etc., but my results were again 
 negative. Still adhering to the reagents which he had recommended, 
 especially chromic and sulphuric acids, I varied the time during which 
 the eyes were treated, hoping thereby to obtain a combination more 
 favorable for Homarus. The separation of the elements of the retina 
 was often very successful, but I never saw in any of my preparations 
 systems of nerve-fibres which resembled those figured by Patten. In 
 almost all cases the isolated parts of the retina presented many delicate 
 fibrous projections. These projections might be interpreted as shreds 
 of broken nerve-fibres, although in no case did they show a systematic 
 arrangement. Moreover, they were found on all kinds of tissue. For 
 these two reasons, I believe that they were not broken nerve-fibres, but 
 simply shreds of tissue. The substance of the cones was finely granular, 
 and was never penetrated, so far as I could discover, by any fibres. My 
 results from both sections and isolation preparations were invariably 
 negative ; and as my observations had been made upon somewhat over 
 sixty lobsters’ eyes, I concluded that in Homarus there was no evidence 
 in favor of the method of nerve-termination which Patten had described. 
 As I have previously mentioned, it is highly improbable that the 
 methods of innervation in the retinas of Peneeus and Homarus are fun- 
 damentally different, and since I have found in the retina of Homarus 
 no confirmation of Patten’s views, I am of opinion that he must be mis- 
 taken as to the method of nerve-termination in Penseus. Many of 
 Patten’s figures of the individual nerve-fibres in Penseus (’86, Plate 31, 
 Figs. 69, 70, 71) resemble so closely the fibres which I have seen in all 
 of my isolation preparations, and which, for reasons already given, I am 
 persuaded are not nervous, that I am forced to believe that Patten has 
 mistaken for nerve-fibres shreds of non-nervous tissue. 
 
 My criticism of Patten’s results refers only to those which he obtained 
 from a study of the Crustacea. No one, so far as I am aware, has fully 
 confirmed his views concerning the nerve-terminations in this group. 
 Kingsley (’86, p. 864) claims to have seen the axial nerve-fibre in 
 Crangon, but he was unable to trace its finer ramifications in the cone. 
 He states (’87, p. 57), however, that the method of preparation which 
 he employed was not intended especially for the fibres, and that there- 
 fore it is not surprising that they were not identifiable. Herrick 
 
28 
 
 BULLETIN OF THE 
 
 (’89, p. 1G8) could not distinguish fibres in the cones of Alpheus, al- 
 though, like Kingsley, he admits that his failure may be due to the 
 method which he used. Relative to nerve-terminations, the evidence 
 which the work of these two investigators presents can scarcely be called 
 critical, and I therefore hold to my former conclusion, namely, that the 
 fibres which according to Patten represent the nerve-terminations are 
 in reality not nerve-fibres at all. 
 
 After having reached this conclusion, I was naturally led to look 
 elsewhere for the true nerve-fibres. It occurred to me that, in order to 
 be certain that I was dealing with nerve-fibres, it was safer to begin 
 studying them in regions where their identity was beyond question. I 
 therefore examined maceration-preparations of parts of the larger nerves 
 from the lobster’s body. These nerves were readily resolved into a 
 number of fibres, which in transverse section were enormous when 
 compared with such fibres as the axial nerve-fibre figured by Patten 
 (’86, Plate 31, Pigs. 72, 74, 108). I then studied in a similar way the 
 optic nerve. The fibres in this nerve were smaller than those from 
 the other nerves which I had macerated, but they were much larger 
 than those figured by Patten. Each fibre (Fig. 36) possessed a distinct 
 sheath, and its contents were marked by lines which extended parallel 
 to its long axis. These lines I interpreted as indications of fibrillse 
 which composed the fibre. In addition to the large fibres, the optic 
 nerve, as well as the other nerves, showed, when macerated, the fibrous 
 shreds which I have previously mentioned. They were very insignifi- 
 cant in amount, forming, I should judge, not more than a fraction of one 
 per cent of the whole optic nerve, and I was never able to trace them 
 as continuous fibres for any considerable distance. I believe that here, 
 as in the retina, they arose from an artificial tearing of the tissue. 
 
 At about this time I happened to find the modification of Weigert’s 
 method of straining nerve-fibres, which I have described in the Intro- 
 duction. As soon as I was aware of the results which could be obtained 
 by this method, I applied it to a series of transverse sections of the 
 optic nerve. The series extended from the retina to the optic ganglion, 
 and demonstrated conclusively, I think, that the proximal ends of the 
 seven proximal retinulae, after passing through the basement membrane, 
 as I have described, continued inward as fibres, and finally passed into 
 the substance of the optic ganglion. Figures 21, 22, 23, 24, and 25 
 illustrate steps in the passage from the retina to the ganglion. In Fig- 
 ure 21 the groups of three or four proximal retinulse are seen as they pass 
 through the basement membrane. Figure 22 is taken at a level imme- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 29 
 
 diately below the basement membrane. Here two groups of four fibres, 
 and one of three, can be distinguished. Below this level the groups of 
 three and four fibres are no longer to be recognized. The fibres diminish 
 rapidly in diameter as they leave the retina. Figure 23 represents a 
 transverse section of the fibres at one fourth the distance from the base- 
 ment membrane to the distal face of the ganglion. Figure 24 represents 
 a similar section midway between retina and ganglion. Figure 25 repre- 
 sents the fibres as they enter the ganglion. The same kind of fibres 
 have been identified in the substance of the ganglion. These fibres, I 
 believe, are the true fibres of the optic nerve, and, as I have shown, 
 they connect the seven proximal retinulse of each ommatidium with the 
 optic ganglion. 
 
 The termination of the nerve-fibres in the proximal retinulse is so 
 directly opposed to the method of termination which Patten has de- 
 scribed, that, before making a final statement based on a study of the 
 lobster only, it seemed prudent to seek confirmation in other species. 
 This I have done, and I can now state that the termination of the nerve- 
 fibres in the retinulae has been demonstrated in Eupagurus, Cambarus, 
 and Gammarus. From this I conclude that it is highly probably that 
 in the compound eyes of all Crustacea the nerve-fibres terminate in the 
 retinulze. 
 
 This method of nerve-termination, namely, the direct continuity of 
 the nerve-fibre and the perceptive cell, has also been demonstrated by 
 Watase ( ? 89, pp. 34-36) in the eyes of Limulus. 
 
 There are some interesting phases in the transition from the nerve- 
 fibre to the retinula. On examining the transverse sections of nerve- 
 fibres at a level slightly below the basement membrane, one observes 
 that it is in the form of a transparent cylinder the periphery of which 
 has scattered over it a few pigment granules. As the fibre passes 
 through the basement membrane the pigment increases in quantity, and 
 when the retinula is reached the nerve-fibre is represented by a trans- 
 parent axis in its centre (Fig. 19, ax. n.). The nervous axis is transpar- 
 ent, because it contains no pigment granules, while the peripheral portion 
 of the retinula is densely pigmented. In this way the nerve-fibre proper 
 is continuous as a transparent axial shaft from below the basement 
 membrane to a level in the retina, which corresponds to the middle of 
 the rhabdome. From this level to its distal end the retinula is com- 
 pletely filled with pigment, no trace of a transparent central axis being 
 visible. (Compare Figs. 19 and 14 with Figs. 11, 10, and 9.) The trans- 
 parent nervous axis of each retinula terminates, as I have said before, 
 
30 
 
 BULLETIN OF THE 
 
 on a level with the middle of the rhabdome. When it is cut very close 
 to its distal end, the nervous axis is seen to lie almost next the rhab- 
 dome. I have never seen a case, however, in which the axis and rhab- 
 dome were not separated by a line of pigment granules. In transverse 
 sections which have been thoroughly depigmented, the distal half of 
 each rhabdome is surrounded by a great number of bodies which resem- 
 ble coarse granules (Fig. 32 These bodies are limited almost 
 entirely to the distal half of the rhabdome. At first I supposed that they 
 were the colorless skeletons of pigment granules, but they were easily 
 distinguished from the latter by their larger size and sharper outline. 
 It then occurred to me that, since these bodies were found only about 
 that portion of the rhabdome which was distal to the nervous axis, they 
 might therefore be the cut ends of the finer fibrillae into which that 
 axis had been resolved. If such was the case, these bodies were in re- 
 ality fibres, not granules. In order to determine whether they were 
 fibres or granules, I examined oblique sections of the rhabdome. Fig- 
 ure 33 is taken from one of these sections. The granular body in the 
 figure is the rhabdome, and the transverse bands are the strata in its 
 substance. The projection of the long axis of the rhabdome in this 
 figure would be a vertical line. The lower end of the figure is proximal, 
 the upper end distal. Owing to the fact that the rhabdome is cut 
 obliquely, what is seen at its proximal end belongs on one side of it, 
 and what is seen at its distal end belongs on the opposite side. The 
 first proximal dark band in the substance of the rhabdome is very 
 nearly midway between its ends. At the proximal end of the rhabdome 
 three distinct fibre-like bodies are seen. These are in reality longitu- 
 dinal sections of the greatly compressed retinulse, which have been 
 noticed in transverse sections. (Compare Figs. 15 and 18.) At the 
 distal end of the rhabdome, in place of the three retinulm, there are a 
 great number of fine fibres. The fibres occur only around the distal 
 half of the rhabdome, and I believe that in transverse sections these 
 fibres are represented by the small round bodies previously described. 
 From the distribution of these small fibres and their relation to the 
 distal end of the nervous axis, I am of opinion that they represent the 
 fihrillse of the nerve-fibre. 
 
 The entrance of the fibres of the optic nerve into the proximal reti- 
 nolse is of itself strong evidence that the rhabdome, not the cone, is the 
 perceptive body. This conclusion is further supported by the ultimate 
 distribution of the optic fibrill®. If it be admitted that the rhab- 
 dome is immediately concerned in the perception of light, it is only 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 31 
 
 natural to expect that this structure would be accessible to the light. 
 As I have already shown, the cone-cells form a transparent axis, which 
 leads directly to the rhabdome, and through which light could readily 
 reach that structure. Once having penetrated to the rhabdome, it is 
 probable that, either in the substance of the rhabdome itself, or in the 
 superficial layer of pigment which immediately surrounds the rhabdome 
 and in which the fibrillse are, the light is transformed into that kind of 
 energy which is transmitted by nerve-fibres. 
 
 The Development. 
 
 In discussing the development of the eyes in Arthropods, the more 
 recent investigators have given much attention to the general plan of 
 these organs. One of the objects of their researches has been the 
 reduction of the eyes of these animals to a single structural type. If 
 such a type were found to exist, it would very probably reproduce the 
 essential structural feature which the eye of the ancestral Arthropod 
 possessed. The desirability of ascertaing if there is a common type for 
 the eyes of all Arthropods is evident, for upon the nature of the answer 
 to this question must depend to some extent the conclusions concern- 
 ing the phylogenetic relationship of the group, and its different classes. 
 Possibly in the phylogeny of two classes of Arthropods the eyes may 
 have originated independently. Of course organs independently devel- 
 oped could not be homologous, and they might be so differently con- 
 structed that it would be impossible to reduce them to a common type. 
 Notwithstanding the difficulty in homologizing the eyes of one class of 
 Arthropods with those of another, the homologies among the members 
 of a single class are much more readily determined, and many impor- 
 tant comparisons can be safely made. It would, therefore, seem more 
 prudent to limit investigations to the eyes of a single class until they 
 were well understood, rather than institute comparisons between the 
 eyes of different classes, where, from the limitations of our knowledge, 
 such comparisons must be more or less hazardous. 
 
 The Plan of the Eye. 
 
 The work of different investigators has already suggested several 
 structural types for the compound eyes of Arthropods, but the differ- 
 ences whicn some of these types present are of such a fundamental 
 character, that to accept one is to reject another. It was with the hope 
 of gaining confirmation for some one type that I was led to study the 
 
32 
 
 BULLETIN OF THE 
 
 development of the eye in a Crustacean, and, as I have previously ex- 
 plained, the species most available for such a study was the lobster. 
 
 The results which have been presented in the more recent papers on 
 the embryology of the compound eyes require a brief notice before the 
 development of the eye in the lobster is described. What is said in this 
 connection is purely introductory ; no criticism of the views of different 
 authors will be made until after the development of the lobster’s eye 
 has been described. 
 
 The writers who have thus far published accounts of the development 
 of compound eyes can be grouped under four heads, depending upon the 
 type of eye which their researches indicate. The first type is that 
 represented by the eye of Peripatus. Patten (’86, p. 688, ’87, p. 211) 
 is of the opinion that the compound eyes of Hexapods, as well as Crus- 
 taceans, are constructed upon this plan. Each eye should then consist 
 of a closed vesicle which was produced by an involution of the hypo- 
 dermis. The eye would be composed of three layers, which in the 
 order of their positions are as follows : first, the superficial hypodermis ; 
 second, the outer wall of the vesicle ; and third, the inner wall of the 
 vesicle. Patten is of opinion that in the eye of an adult individual 
 these three layers are modified in the following way : the superficial 
 hypodermis becomes the corneal hypodermis; the outer wall of the 
 vesicle is so far reduced as to be inconspicuous, and the inner wall gives 
 rise to the retina. The retina includes the crystalline cones and the 
 pedicels (rhabdomes). Patten supports these conclusions mainly from 
 theoretical grounds, but he believes that he has found evidence of the 
 existence of this type in the development of the compound eyes of 
 Vespa, Blatta, and the Phryganids. 
 
 The second structural type which I shall mention has been advocated 
 by Kingsley (’87, p. 51) in his description of the development of the 
 compound eye of Crangon. 1 In this type the eye results from a vesicu- 
 lar infolding, as in that which was proposed by Patten, but it differs 
 from the latter in the fate which is ascribed to the two walls of the 
 vesicle. According to Kingsley, the outer wall of the vesicle is not 
 reduced, but gives rise to the retina. In it are developed the crystal- 
 line cones and the pedicels (rhabdomes). The inner wall of the vesicle, 
 instead of forming the retina, as Patten believed, is converted into a part 
 of the optic ganglion. 
 
 The third structural type is that which is presented by Reichenbach 
 (’86, pp. 85-96) in his account of the development of the crayfish. 
 
 1 See the note on page 41. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 33 
 
 Here a hypodermal involution takes place, and a vesicle is produced, 
 but the cavity of the vesicle is soon obliterated. The mass of cells 
 which results from the fusion of the walls of the vesicle now divides into 
 an outer and an inner layer. These two layers do not necessarily cor- 
 respond to the outer and inner walls of the original vesicle. The super- 
 ficial hypodermis and outer layer of the infolded mass fuse, and give 
 rise to the retina. The crystalline cones are developed in that part of 
 the retina which is derived from the superficial hypodermis ; the rhab- 
 domes probably originate in that part which is derived from the outer 
 layer of infolded cells ; the inner layer of cells becomes ganglionic. 
 
 Bobretsky ’s (73) account of the development of the compound eyes 
 in Astacus and Palsemon agrees in its essential features with the de- 
 scription which Reichenbach has given for the eyes in the crayfish. 
 Bobretsky, however, does not describe an involution in the optic disks. 
 As Reichenbach suggests, the fact that Bobretsky did not have the oppor- 
 tunity of studying very early stages may explain his failure to observe 
 the involution. In other respects, the accounts are essentially alike, and 
 there is little doubt that the plan of eye which Bobretsky’s researches 
 indicate is the same as that suggested by Reichenbach’s studies. 
 
 Each of the three structural types which have thus far been described 
 are dependent upon the formation of a hypodermal vesicle. The fourth 
 type is simpler than any of the three preceding ones, in that a vesicle is 
 not necessarily produced, the retina being supposed to originate as a 
 simple thickening in the superficial hypodermis. This type has been 
 advocated by Herrick (’86, p. 43) in his account of the development 
 of Alpheus. The researches of Nusbaum (’87, pp. 171-186) on the 
 development of Mysis indicate the same type. Neither in Grobben’s 
 (79) account of the development of the eyes in Moina, nor in Claus’s 
 (’86, pp. 307-324) description of those in Branchipus and Artemia is 
 any mention made of an involution. These authors might, therefore, 
 be cited as favoring the fourth type of eye, although it is to be observed 
 that the special question of the vesicular origin of the eye is not dis- 
 cussed by them. 
 
 The advocates of the fourth type find support, not only in the embry- 
 ology of the Crustacea, but also in that of the Hexapods. According to 
 Carriere (’85, pp. 181-186), the compound eyes of some Hymenoptera 
 and Lepidoptera develop as simple thickenings of the hypodermis. 
 
 Of the four types which have been mentioned, the one with which 
 the development of the lobster’s eye accords will be seen from the 
 following description. 
 
 VOL. xx.— no. 1. 
 
 3 
 
34 
 
 BULLETIN OF THE 
 
 The first traces of the eyes in the development of the lobster are the 
 optic disks. These disks lie one on either side of the median plane, and 
 are for a considerable time the most conspicuous structures in the an- 
 terior region of the embryo. In the early stages of development the 
 disks face ventrally, but as the head of the animal becomes differen- 
 tiated they come to face almost in the opposite direction, i. e. dorsally. 
 At stage A (Fig. 37) the optic disk, so called, is oval in outline rather 
 than disk-shaped ; its longer axis is transverse to the principal axis of 
 the embryo. From that portion of each disk which is near the median 
 plane a band of tissue extends posteriorly, and connects the disk with 
 the ventral plate of the embryo. The disk and the band with which it 
 is connected to the ventral plate are distinguished from the surrounding 
 tissue by their greater number of nuclei. The disks comprise the tissue 
 from which both retina and optic ganglia develop. 
 
 A section passing through an optic disk in a plane perpendicular to 
 the longitudinal axis of the embryo is shown in Figure 38. In this 
 case the section is from the right optic disk. The left side of the sec- 
 tion is farthest from the median plane ; the right side is near that 
 plane. Since at this stage the disk frees ventrally, and since the ventral 
 edge of the section is uppermost in the figure, it is the posterior face of 
 the section which is presented. To the right and left of the disk one 
 can see the undifferentiated ectoderm with its occasional nuclei. This 
 ectoderm is directly continuous with the tissue of the disk ; in fact, the 
 disk is only a local thickening in the ventral ectodermic layer of the 
 embryo. It is due to its greater thickness that the disk contains more 
 nuclei than the surrounding ectoderm. The deep face of the ectoderm, 
 both of that which is undifferentiated and that which forms the disk, is 
 limited by a delicate but distinct basement membrane (Fig. 38, mb.). 
 
 Some idea of the method of growth in the optic disk can be gained 
 from a study of its nuclei. It will be noticed that the nuclei in the 
 deeper part of the disk are rather small and irregularly grouped when 
 compared with those which are found next its outer surface. These 
 superficial nuclei form an almost regular series, which extends from one 
 margin of the disk to the other. They are usually also characterized by 
 having their long axes perpendicular to the surface of the disk. When 
 they increase by division, their planes of separation are in most cases 
 either parallel or perpendicular to the outer surface of the disk. When 
 the plane of separation is perpendicular to the surface of the disk, the 
 tendency for such divisions is to increase the diameter of the disk. 
 When, on the other hand, the plane of separation is parallel to the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 35 
 
 surface of the disk, the tendency is to thicken it. In order to determine 
 the distribution of these two methods of division, the planes of sepa- 
 ration in the superficial nuclei of four disks were carefully observed. 
 Each disk can be divided into halves by a plane parallel to the sagittal 
 plane of the embryo. That half of the disk which lies near the median 
 plane can be called the proximal half ; that which is farther from the 
 median plane, the distal half. Among the superficial nuclei of the distal 
 halves of the four disks which were examined there were seen thirteen 
 cases of division. In all of these cases the tendency was to broaden the 
 disk. There was no case where the division of the nucleus would have 
 thickened it. In the proximal halves of the disk there were six cases 
 of division observed ; five of these tended to thicken the disk, and one 
 would have broadened it. It is therefore apparent that in the super- 
 ficial layer of each disk the proximal half is becoming thicker, while the 
 distal half is becoming broader. 
 
 The deep nuclei of each disk, those lying below the row of superficial 
 nuclei, divide in different planes. Among these nuclei in the four disks 
 which were examined, seven instances of division were noticed. Five of 
 these were in planes which would have thickened the disk ; the remain- 
 ing two would have broadened it. This part of the disk consequently 
 shows a tendency both to become broader and thicker. Of these two 
 tendencies, that which would thicken the disk is the stronger. 
 
 The method of growth which has been described for different parts of 
 the optic disk can be easily distinguished only in its earlier stages. The 
 subsequent changes which affect the structure of the disk render it 
 rather difficult to follow in detail the growth of the disk as a whole ; but 
 in general the broadening of the distal superficial region, the thickening 
 of the proximal superficial part, and the broadening and thickening of 
 the deeper parts are continued. 
 
 The most important changes in the differentiation of the optic disks 
 are the following : first, the separation of the retina and optic ganglion 
 by the formation of the basement membrane ; second, the production of 
 the optic nerve ; and third, the differentiation of the ommatidia. These 
 three changes will be considered in the order named. 
 
 The first step in the differentiation of the retina and optic ganglion 
 occurs at stage B. Figure 39 represents a section from the optic disk of 
 an embryo of this stage. The plane of section in this case corresponds 
 to that of Figure 38. The chief difference observable between the disk 
 at stages A and B is its greater thickness in the older embryo. Not only 
 has the disk thickened, but it has spread laterally, so that the small 
 
86 
 
 BULLETIN OF THE 
 
 angle which is seen on the outer margin of the disk in stage A (Fig. 
 38, x), and which indicates a tendency bn the part of the disk to grow 
 over the adjacent ectoderm, is represented in stage B by a very acute 
 angle (Fig. 39, x), while the disk itself forms an actual fold, which 
 covers a portion of the undifferentiated ectoderm. 
 
 The separation of the retina and its ganglion is accomplished by the 
 production of a basement membrane. This is gradually developed in 
 the substance of the disk between the regions of the retina and the optic 
 ganglion. In order to distinguish the newly formed membrane from 
 the original basement membrane which bounds the under surface of the 
 disk, I shall speak of the former as the intercepting membrane. The in- 
 tercepting membrane (Fig. 39, mb. i cpt.) takes its origin from the base- 
 ment membrane on a line a little within the lateral edge of the disk. 
 From this position it extends at this stage as a delicate lamella for a 
 short distance through the tissue of the disk. The direction of its course 
 is approximately parallel to the outer surface of the disk, and it divides 
 the distal portion of the disk into two masses, one of which is superficial, 
 the other deep. The superficial mass is the first portion of the retina 
 to be differentiated, and, as I have previously stated, the growth in this 
 region is chiefly lateral. The deep mass is the beginning of the optic 
 ganglion. The intercepting membrane does not extend so far as to 
 separate the superficial portion of the disk from the deeper part in the 
 proximal half. This condition is what one might expect, since in the 
 proximal region of the disk the superficial nuclei divide in such planes 
 as to thicken the disk, and a membrane which would separate the deep 
 and superficial parts would be a source of interference in the process of 
 thickening. 
 
 The intercepting membrane is not an involution of the original base- 
 ment membrane, but is produced by the activity of the ectodermic cells 
 between which it lies. There is no reason for doubting, I believe, that 
 at this early stage (B) it is strictly an ectodermic secretion. At this 
 stage it is fan-shaped, the handle of the fan being formed by that part of 
 the membrane which is in contact with the original basement membrane. 
 The plane of the fan is parallel with the outer surface of the disk. In 
 sections which are either anterior or posterior, but parallel to that shown 
 in Figure 39, portions of the intercepting membrane are often visible, 
 and may be apparently unconnected with the basement membrane. 
 This is due to the fact that the plane of section cuts the anterior or 
 posterior edge of the fan without including the handle. 
 
 The chief difference between the intercepting membranes at stages 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 37 
 
 B and C is the greater extension of the membrane at stage C. (Com- 
 pare Figs. 39 and 41.) The retina is now much more completely 
 separated from the ganglion than formerly. The superficial and deep 
 portions of the proximal part of the disk are, however, still continuous. 
 
 In stage D (Fig. 45) an important step is taken in the development 
 of the membrane. It splits at this stage into two layers, one of which 
 adheres to the retina, and the other to the optic ganglion. The retinal 
 and ganglionic layers of the membrane have probably been distinct from 
 the time of their formation ; but until stage D is reached, the double 
 nature of the membrane is not apparent. Occasionally in stage C one 
 notices at the point where the intercepting and basement membranes 
 unite a re-entrant angle (Fig. ±1, x). This in some cases extends a 
 short distance between retina and ganglion, and doubtless represents the 
 first step in the separation of the components which form the intercept- 
 ing membrane. 
 
 In stage E (Fig. 4G) the membrane has completely severed the super- 
 ficial from the deep ectoderm, and the separation of its retinal and gan- 
 glionic constituents extends over a broader area than in the previous 
 stage. 
 
 The subsequent changes in the intercepting membrane consist in a 
 complete separation of its retinal and ganglionic portions. This separa- 
 tion is effected by the withdrawal of the optic ganglion from the super- 
 ficial ectoderm. In the adult, the ganglionic membrane remains relatively 
 thin, but the retinal membrane becomes much thickened. This mem- 
 brane, which has already been described as the basement membrane of 
 the adult retina, is not uniformly thickened, but presents local eleva- 
 tions, each of which is in the form of a cross. The four apertures which 
 pierce the membrane in the angles of the cross-shaped elevations, and 
 the relation which adjoining crosses and apertures bear to one another, 
 have already been described. The proximal face of the basement mem- 
 brane is nearly flat (Fig. 29). The cross-shaped elevations occur on its 
 distal face. The substance of the membrane is apparently homogeneous, 
 and contains no traces of cells or nuclei. The fact that its substance is 
 alike throughout favors the idea that it has been derived from a single 
 source. From stage E to the adult condition a few mesodermic cells 
 have been noticed next its proximal face (Fig. 1, cl. ms d.). These cells 
 are not intimately attached to it, and I am of opinion that they con- 
 tribute little or nothing to its composition. 
 
 From the foregoing account I draw the following conclusions concern- 
 ing the growth of the basement membrane of the eye. In its earliest 
 
38 
 
 BULLETIN OF THE 
 
 stages the basement membrane is strictly ectodermic in origin. In the 
 adult condition it is much thicker than in its early stages, and the 
 greater part of its substance has probably come from the ectoderm, al- 
 though mesodermic cells rest against its proximal face, and possibly may 
 contribute to its formation. 
 
 From the description which I have given, it is evident that the optic 
 disks are thickened regions in the superficial ectoderm, and that these 
 disks are cut by an intercepting membrane into two parts, one deep, 
 the other superficial. The deep part is converted into the optic gan- 
 glion ; the superficial part becomes the retina. So far, then, as the 
 development of the retina in the lobster is concerned, it supports the 
 view that the compound eye in Crustaceans is developed from a simple 
 thickening of the ectoderm. 
 
 In describing the formation of the retina and optic ganglion in the 
 lobster, I have made no mention of an involution. Both Reichenbach 
 and Kingsley have described an infolding in the formation of the eyes, — 
 the former in Astacils, the latter in Crangon, — and it is therefore only 
 natural to look for a similar condition in the eyes of Homarus. Any 
 evidence of an involution in the production of the lobster’s eyes is to 
 be sought in the early stages of development. I regret that in the very 
 early stages my material is deficient, and I have not grounds enough to 
 warrant the statement that no involution occurs. All that I can state 
 is, that in all stages which I have examined I have not been able to find 
 any evidence of an involution. The youngest individual which I have 
 studied was one in which the optic disk was about two thirds as thick as 
 that represented in Figure 38. Excepting its thinness and the smaller 
 number of its nuclei, it presented essentially the same appearance as the 
 one which is figured. It will be noticed that near the centre of the 
 disk in Figure 38 there is a space devoid of nuclei. It occurred to me 
 that such a space might represent the last traces of an involution, and I 
 therefore plotted carefully the nuclei in five pairs of disks, some of which 
 were less mature than the disk shown in Figure 38. The result of the 
 plotting was that the light space which is seen in Figure 38 proved to 
 be an individual peculiarity, and I did not find in the arrangement of 
 the nuclei in the other disks any evidence of an involution. 
 
 The plotting of the nuclei, however, brought to light the method by 
 which the disks increased in size. This has already been described, and 
 offers, I believe, an explanation of the fact that in some cases, as for 
 instance in the crayfish, the formation of the eye is attended with an 
 involution, while in other instances, as in Alpheus, no involution is 
 present. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 39 
 
 It will be remembered that the superficial layer of nuclei in the optic 
 disk of the lobster was divided into two regions. The one farther from 
 the median plane has been called the distal region ; the one nearer, the 
 proximal region. The broadening of the distal region produced the 
 retina. By a proliferation of its cells the proximal region resulted in 
 the formation of the optic ganglion. It is my opinion that this prolifer- 
 ation of cells represents what is produced in the case of some Crusta- 
 ceans by an involution, and that either an involution or proliferation, or 
 possibly a combination of both processes, occurs in the eyes of all Crus- 
 taceans. Whichever process characterizes the development of a given 
 eye, it must be borne in mind that the involution or proliferation is 
 connected with the formation of the ganglion only, and takes no part in 
 the production of the retina. The latter is a simple thickening in the 
 ectoderm ; the optic ganglion is developed either as a proliferation or 
 involution of the ectoderm which lies close beside the retiiia. The 
 results at which various investigators have arrived, different as they 
 may at first appear, can be harmonized, I believe, by this interpreta- 
 tion of the origin of the retina and optic ganglion. 
 
 In his account of the development of the eye in the crayfish, Reichen- 
 bach (’86, p. 85) has described an ectodermic involution, which occurs 
 nearly in the centre of the optic disk. That portion of the disk which 
 is farther from the median plane than the region of involution gives 
 rise to the retina, so that the region of involution in the crayfish 
 occupies a position which corresponds to the region of proliferation in 
 the lobster. Not only do the two regions correspond, but the masses 
 of tissue which are developed from each have certain peculiarities com- 
 mon to both animals. In the case of the crayfish the mass of tissue 
 which results from the involution becomes divided into an outer and an 
 inner wall. These two walls are separated from each other by a band 
 of nuclei, which are larger and lighter in color than the surrounding 
 nuclei. In the lobster the ganglionic tissue which arises by proliferation 
 is divided into an outer and an inner part. The separation is effected 
 by a band of nuclei, which in position and structure resemble the band 
 figured by Reichenbach. (Compare Reichenbach, ’86, Plate XII. Fig. 
 174, and Plate III. Fig. 41 of this paper.) The similarity presented by 
 the bands of nuclei in the lobster and crayfish supports the conclusion 
 that the involution in the crayfish and the proliferation in the lobster 
 are homologous structures. 
 
 An objection to this comparison might be raised on the ground that, 
 according to Reichenbach’s statement (’86, p. 93), the involution in the 
 
40 
 
 BULLETIN OF THE 
 
 case of the crayfish gave rise to a mass of tissue which formed on the 
 one hand the deeper part of the retina, and on the other a portion of 
 the optic ganglion, while the cells which arise by proliferation from this 
 region in the lobster produce only a part of the optic ganglion. But, 
 as Patten (’87, pp. 208, 209) has shown, Reichenbach himself was not 
 certain that any part of the infolded ectoderm contributed to the for- 
 mation of the retina. Reichenbach found it difficult to locate exactly 
 the position which the developing rhabdomes occupied. On page 92 in 
 his account he describes a part of the outer wall (Ausserwand) of the in- 
 folded ectoderm, and states his belief that in it the rhabdomes develop ; 
 in fact, he describes certain red bodies which he says are without doubt 
 the rhabdomes themselves. On page 96 he admits that the layer in 
 which he supposed the rhabdomes originated may be a layer of nerve- 
 fibres. Granting this interpretation, it is no longer possible to consider 
 the previously described red bodies as rhabdomes. Reichenbach does not 
 make this last statement, but his description implies it when he states 
 that, although the region of the red bodies may not be the region of the 
 rhabdomes, yet the rhabdomes doubtless originate in a somewhat more 
 superficial part of the outer wall. Apparently he has not identified the 
 rhabdomes in their new position ; at least, he makes no such statement 
 in his text or description of plates. Since he has also admitted that the 
 red bodies may not be rhabdomes, I cannot see that he has positively 
 identified any structure as a rhabdome. Such being the case, it is diffi- 
 cult to understand on what grounds he can maintain the assertion that 
 the rhabdomes develop in the outer wall. If this assertion cannot be 
 defended, then it is possible that they may develop in the superficial 
 hypodermis. This would be analogous to the condition presented in the 
 lobster. 
 
 If the rhabdome in the eye of the crayfish is developed, as I believe 
 it is, in the superficial hypodermis, and not in the outer wall of the in- 
 folded hypodermic, the objection which was suggested in homologizing 
 the involution in the eye of the crayfish with the proliferation of cells 
 in the eye of the lobster has no weight. 
 
 In the development of the eye of Crangon, according to Kingsley’s 
 (’86 a ) account, there is also an optic invagination. If what I have at- 
 tempted to show in regard to the eyes of the crayfish be true, then this 
 invagination in Crangon should be connected with the formation of the 
 ganglion only. This, as I have already stated, is not the view held by 
 Kingsley, for he maintains that the outer wall of the invaginated pocket 
 gives rise to the retina, and only the inner wall is concerned in the pro- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 41 
 
 duction of the ganglion. This is fundamentally different from the con- 
 dition found in the lobster. The difference, however, is due, I believe, to 
 the fact, that in describing the later stages of development Kingsley has 
 pointed out a cavity which he believed to be the cavity of the invagi- 
 nation, but which in reality is not. The cavity which he has marked oc 
 in his Figures 3, 4, and 5, is unquestionably the cavity of involution, 
 but the space marked oc in his other figures is, in my opinion, a part of 
 the body cavity. 
 
 My reason for this belief is as follows. The cavity of an involution 
 such as is found in the anterior median eyes of spiders or the median eyes 
 of scorpions is, when it has lost its connection with the exterior, a closed 
 ectodermic sac. That such a cavity should be occupied by migrating 
 mesodermic cells seems to me extremely questionable, for in order to 
 enter the cavity it would be necessary for the cells to penetrate one 
 wall of the vesicle. This of course is not impossible, but it is not borne 
 out by analogy with the Arachnoids. The cavity which Kingsley has 
 marked oc in Figure 7 is occupied by several mesodermic nuclei, and 
 what is more important, perhaps, is that it is apparently connected with 
 other cavities in the embryo. These cavities also contain mesodermic 
 tissue. Excepting Figures 3, 4, and 5, the cavities marked oc I believe 
 to be homologous, and I further believe that they represent, not the 
 cavity of involution, but the space which intervenes between the infolded 
 pocket and the superficial ectoderm. This is a part of the embryonic 
 body cavity, and is of course readily accessible to mesodermic cells. 
 The fate of the real cavity of involution is not so easily discovered. 
 Probably, as in the case of the crayfish, it is obliterated in the mass of 
 tissue from which the retinal ganglia arise. 1 
 
 If the interpretation which I have suggested in the preceding para- 
 graph be admitted, the development of the eye in Crangon is essentially 
 the same as in the lobster and crayfish. The proliferation in the optic 
 disks of the lobster is represented by an involution in the disks of 
 Crangon. The cavity of the involution disappears in Crangon, and its 
 walls give rise to ganglionic tissue. The part of the superficial ecto- 
 
 1 Since this paper was written, I have received a copy of the third part of 
 Kingsley’s studies on the development of Crangon. As the following quotation 
 will show, Kingsley (’89, p. 20) has materially changed his views as to the forma- 
 tion of the retina : “ I may say here that I am inclined to believe that I fell into 
 error in my account of the development of the Compound Eye of Crangon, and 
 that the invagination or inpushing which I there described as giving rise to the 
 ommatidial layer of the eye, in reality gives rise to the ganglion of the eye which 
 in the adult is contained within the ophthalmic stalk.” 
 
42 
 
 BULLETIN OF THE 
 
 derm which is immediately external to the ganglionic involution thickens 
 and produces the retina. 
 
 The mode of development which Patten has suggested for the com- 
 pound eye of Arthropods has not received any support, so far as I am 
 aware, from the embryology of the Crustacean eye. The only observa- 
 tions which go to confirm Patten’s opinion are those of his own on 
 Vespa, Blatta, and the Phryganids. Possibly the compound eyes of 
 Crustaceans may be developed upon a different plan from those of Hexa- 
 pods. Certainly the evidence which Patten has given for the Peripatus- 
 like type of the compound eye of Hexapods has not been found in any 
 of the Crustacea. According to Patten’s view of the origin of the com- 
 pound eyes, the corneal hypodermis should arise on the sides of the 
 optic area, and spread over the retina until the latter is entirely cov- 
 ered. This constitutes the closing of the shallow vesicle. When I 
 come to describe the differentiation of the ommatidia, I believe it can be 
 shown beyond a doubt that in the lobster the corneal hypodermis arises, 
 not by any lateral growth from the edge of the optic area, but by a 
 simple process of delammation. The cells of the corneal hypodermis are 
 the differentiated superficial cells of that thickening in the hypodermis 
 which produces the retina. Thus the plan which Patten has suggested 
 for the compound eyes in Arthropods is not supported by the evidence 
 derived from the development of the lobster. Patten himself (’87, 
 p. 202) admits that in Vespa he did not see the closure of the hypo- 
 dermis over the retina, and that in Blatta and the Phryganids (’87, 
 pp. 208 and 211) the process is very obscure. 
 
 The researches of Carriere point to a type of compound eyes for the 
 Hexapods which is similar to that exhibited by the lobster. It is possi- 
 ble that the vesicular origin of the compound eyes of Hexapods, owing 
 to their obscure method of formation, may have been overlooked by 
 Carriere, but I am inclined to believe, after a consideration of both sides 
 of the question, that the evidence favors the simpler method of origin, 
 and therefore that the compound eyes of Hexapods as well as Crusta- 
 ceans arise as simple hypodermal thickenings. 
 
 In Mysis, according to Nusbaum (’87, pp. 171-185), the develop- 
 ment of the eye follows essentially the same course as that which I have 
 described in the lobster. The optic disks are formed and the ganglionic 
 and retinal portions are differentiated in the same manner in both cases. 
 The development of the retina is slightly complicated in Mysis by the 
 fact that, instead of lying in one plane, or very nearly so, the retina is 
 strongly folded on itself, so as to give the appearance of two lamellae, an 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 43 
 
 internal and an external one. The retinal elements are differentiated 
 earliest in the internal lamella. As the eye becomes more distinctly sep- 
 arated from the body, the internal and external lamellae are unfolded so 
 that they are no longer distinguishable as separate parts. The retina is 
 developed from the thickened layer of hypodermis. So far as Nus- 
 baum’s observations extended, the ganglion is produced without an 
 involution. 
 
 The development of the compound eye in Alpheus has been stud- 
 ied by Herrick (’86, ’88, and ’89). The course of development is 
 almost identical with that of the lobster. 1 The optic disks after they 
 thicken are cut by a basement membrane into a ganglionic and a reti- 
 nal portion. There is no involution connected with the formation of 
 the eye. 
 
 In the introduction to the development of the lobster’s eye, mention 
 was made of four structural types which the work of different investi- 
 gators indicated as possible plans for the compound eyes of Crustacea. 
 I have given reasons for excluding three of these. The type of eye 
 which Patten has advocated is unsupported by the embryology of the 
 Crustacea. Reichenbach and Kingsley misinterpreted structures in the 
 eyes which they studied, and were consequently led to erroneous con- 
 clusions. If the interpretations which I put on the work of these two 
 investigators be admitted, all studies on the development of the com- 
 pound eyes of Crustacea point to one conclusion, namely, that in these 
 eyes the retina originates as a thickened layer of hypodermis, and is not 
 modified by any form of involution. The involution when present is 
 connected with the formation of the optic ganglion only. In the pro- 
 duction of the ganglion, the involution can be replaced by a proliferation 
 of the cells. 
 
 The Optic Nerve. 
 
 The development of the optic nerve 2 is intimately connected with the 
 formation of the intercepting membrane. Before the formation of this 
 
 1 I have had an opportunity of examining Dr. Herrick’s unpublished plates on 
 the development of Alpheus, and Dr. Herrick has kindly looked over my figures 
 of the lobster. The correspopdence between the method of growth in the eye of 
 Alpheus and Homarus is certainly very close. The few differences that were 
 noticed were such as might be expected between different species. I take this 
 opportunity of thanking Dr. Herrick for his kindness in extending to me the use of 
 his plates. 
 
 1 The optic tracts of a lobster consist of four principal parts. The first of these 
 is the retina, from which nerve-fibres lead to the optic ganglion. These three 
 
44 
 
 BULLETIN OF THE 
 
 membrane the retina and ganglion is one continuous mass of cells 
 (Fig. 38). When the intercepting membrane is formed, the retina and 
 ganglion are apparently separated (Fig. 39). I am of opinion, however, 
 that this separation is only apparent, and that in reality the two struc- 
 tures are still in connection. At least in this stage and in stage C 
 (Fig. 41) nuclei are frequently found lying directly across the mem- 
 brane. These nuclei present so normal an appearance, and their oc- 
 currence is so frequent, that I cannot believe that their position is due 
 to accidental displacement. My only way of accounting for the place 
 which they occupy is by supposing that the basement membrane is 
 perforated where they are found. The membrane was probably pro- 
 duced in the form of a net, through the meshes of which the retina and 
 ganglion retained their original connection. Either this is true, or it 
 must be admitted that the retina and ganglion were first connected, 
 then separated, and finally reconnected ; a supposition which seems to 
 me unnecessary as well as improbable. From stage C to the adult con- 
 dition the retina and ganglion are unquestionably connected by nerve- 
 fibres. If the conclusion arrived at concerning the origin of the optic 
 nerve is correct, it follows that the optic nerve cannot be properly 
 described as an outgrowth either from the retina or from the ganglion, 
 but it must be considered as a remnant of the original connection which 
 existed between retina and ganglion. Patten (’87, p. 196) has described 
 essentially the same method of origin for the optic nerve of Vespa. The 
 formation of the optic nerve from tissue which represents the original 
 connection of a portion of the optic ganglion with the superficial ecto- 
 derm is doubtless a reproduction of the method by w T hich that nerve 
 arose phylogenetically. 
 
 The fact that the fibres of the optic nerve are from the outset attached 
 to the proximal ends of the retinulse is of significance in determining 
 the plan of the eye. Of the four structural types of compound eyes 
 which have been suggested, that which Kingsley has presented involves 
 the inversion of the middle or retinal layer. Mark (’87, pp. 91, 92) 
 has shown that when the retina is inverted, as in the anterior median 
 eyes of spiders, the nerve-fibres are at first attached to the morphologi- 
 cally deep ends of the retinal cells, and that the attachment afterwards 
 migrates toward the other ends of these cells. From the fact that in 
 
 parts, retina, nerve, and ganglion, are contained in the eye-stalk. The fourth part 
 is a second bundle of nerve-fibres which connect the optic and cephalic ganglia. 
 In using the term optic nerve I refer to that collection of fibres which unites the 
 retina and optic ganglion. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 45 
 
 Crustaceans the nerve-fibres are always attached to the proximal ends of 
 the retinulae, it can be argued that the retina in this group has never 
 been inverted, but retains its original position, and that any structural 
 plan which involves the inversion of the retina is therefore probably 
 wrong. 
 
 The Differentiation of the Ommatidia, 
 
 In the development of the compound eye in the lobster, the deposi- 
 tion of pigment and the differentiation of the ommatidia take place 
 at about the same time. These changes occur at stage C. (Compare 
 Figs. 39, 41, and 42.) At this stage (Fig. 41) it will be observed that the 
 retinal layer is thickest at the lateral margin of the disk (the extreme 
 left in Fig. 41). The retina becomes thinner as one proceeds from the 
 margin toward the median plane (from left to right in the figure). 
 The thickest part of the retina, it will be recalled, was the part first to 
 be separated from the ganglion by the intercepting membrane. As it 
 was the first part of the retina to be separated, so it is the first part in 
 which the ommatidia are differentiated and pigment is deposited. 
 
 The first steps in the differentiation of the ommatidia are seen in 
 Figure 42. Here the nuclei in the thicker part of the retina have sep- 
 arated into two bands, one distal (y), and one proximal ( x ). The distal 
 band, as I shall presently show, can be further separated into a super- 
 ficial and deep layer. These two layers are close to the outer surface 
 of the retina, and approximately parallel with it. ^ 
 
 The arrangement of the nuclei which make up the distal band is most 
 easily observed when the retina is viewed from the surface. The group- 
 ing of these nuclei, from the time when the ommatidia are differentiated 
 to the adult condition, is so characteristic that the fate of the individual 
 nuclei can be easily traced. For the sake of simplicity I shall therefore 
 designate the different nuclei, from their first appearance in groups, by 
 the names of the cells in which they are ultimately found, although it 
 is to be borne in mind that in the early stages the cell walls are not as 
 yet differentiated. 
 
 Viewed from the surface, the superficial layer of the distal band of 
 nuclei presents the appearance which is shown in Figure 43. In this 
 layer there are two kinds of nuclei, one elongated, the other roundish. 
 The elongated ones are always in pairs. They ultimately become the 
 nuclei of the corneal hypodermis. These hypodermal nuclei arise from 
 among the superficial nuclei of the distal band, and do not originate, as 
 Patten believed (’86, p. 645), from a fold which grows over the retinal 
 
46 
 
 BULLETIN OF THE 
 
 area. Of course the number of pairs of hypodermal nuclei equals the 
 number of ommatidia. In the earliest stages in which the ommatidia 
 have been seen there were always three or four pairs of hypodermal 
 nuclei, so that it is probable that the first step in differentiation is the 
 simultaneous production of three or four ommatidia. 
 
 The second kind of nuclei in the superficial layer are roundish and 
 arranged in circles of six around each pair of hypodermal nuclei. The 
 circles are so combined that each nucleus plays a part in three circles ; 
 and as there is one circle for each ommatidium, it follows that only one 
 third of each nucleus belongs to a given ommatidium, or, if one esti- 
 mates the nuclei as units, only one third of each circle of six nuclei, or 
 two nuclei, belong to an ommatidium. These nuclei represent the cells 
 which in the adult have been called the distal retinulse, and of which 
 there was a single pair to each ommatidium. 
 
 The deep layer in the distal band of nuclei lies directly below the 
 superficial layer (Fig. 42). The nuclei which constitute this layer are 
 all of one kind, and are arranged in groups of four (Fig. 44). They 
 represent the cells of the crystalline cones. The centre of a given group 
 of cone-nuclei is directly below the centre of a pair of hypodermal nuclei. 
 At this stage the cone-cells can be observed as elongated pyramids, 
 which lie with their bases in the region of their four nuclei, and their 
 apices extending into the deeper part of the retina (Fig. 42). 
 
 The nuclei of the proximal band show at this stage no special arrange- 
 ment. They migrate to the deeper part of the retina, and there undergo 
 further change. Between them and the basement membrane the pig- 
 ment is deposited. 
 
 The most noticeable changes which the retina as a whole now under- 
 goes are two. First, it thickens until it is throughout nearly as thick 
 as in the region where the ommatidia were first differentiated. (Com- 
 pare Figs. 45 and 46.) Second, the number of ommatidia greatly in- 
 creases. These two changes, the thickening of the retina and the 
 production of new ommatidia, go hand in hand and spread over the 
 general surface of the eye, from the region in which the first ommatidia 
 appear. It is worthy of notice that the new ommatidia are constructed 
 from the undifferentiated cells which immediately surround the area of 
 ommatidia already formed. Cells once incorporated in a given omma- 
 tidium never in any way contribute to the formation of other ommatidia. 
 Moreover, in the differentiation of an ommatidium no cells are left 
 between it and the neighboring ommatidia, so that ommatidia once ad- 
 joining each other remain so. In other words, new ommatidia are not 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 47 
 
 produced between old ones, but only on the edge of the ommatidial 
 area. 
 
 The changes in the ommatidia themselves can be studied most readily 
 in longitudinal and transverse sections of the retina. Figure 47 is an 
 enlarged drawing of that portion of the retina which is marked by a 
 bracket in Figure 46. It will be noticed that in this stage, E, the nuclei 
 are limited for the most part to the distal half of the retina. The 
 middle of the retina is occupied by a band of pigment, which gradually 
 fades as it approaches the basement membrane. The space between the 
 basement membrane and the ganglion is relatively narrow, and is occu- 
 pied chiefly by nerve-fibres. Returning now to the nuclei in the distal 
 half of the retina, it is to be observed that the general arrangement 
 which was pointed out in the last stage also persists here. The nuclei 
 of the distal retinulse are still in circles of six^ (compare Figs. 47 and 
 48, nl. dst.), and are very close to the external surface of the eye. In 
 the centre of each circle is seen a round pink body, the tip of the cone- 
 cells (Fig. 48 con.). Directly below the level of the nuclei in the distal 
 retinulse are the pairs of nuclei belonging to the corneal hypodermis 
 (Figs. 47 and 49, nl. cm.). Each pair of corneal nuclei surrounds the 
 distal end of the cone. 
 
 Below this level one meets the four nuclei of the cone-cells (Figs. 47 
 and 50, nl. con.). From the side view (Fig. 47) it will be seen that the 
 groups of cone-cells are spindle-shaped in outline, and have their nuclei 
 arranged in a transverse plane at the thickest part of the spindle. The 
 nuclei which in stage C formed the proximal band are scattered in this 
 stage between the deep ends of the cones (Fig. 47, nl. px.). They are 
 not definitely arranged. In order to estimate the number of nuclei in the 
 proximal band for each ommatidium, I counted these nuclei as seen in a 
 series of tangential sections. In the outermost section in which the prox- 
 imal nuclei occur there were six nuclei around each cone. These nuclei, 
 however, were arranged in circles similar to the circles of the corneal 
 nuclei ; consequently for each ommatidium there were only two nuclei 
 in each circle of six. The remaining nuclei were all embraced in the 
 two succeeding deeper sections. In each of these two sections there 
 were about nine nuclei around each cone. The arrangement of these 
 nuclei was extremely irregular, and it was consequently difficult to 
 estimate the number of nuclei which belonged to one ommatidium. 
 Most of the nuclei were situated between three cones, therefore, about 
 one third of the nuclei around a given cone can be considered as belong- 
 ing to the ommatidium represented by that cone. As there were in 
 
48 
 
 BULLETIN OF THE 
 
 these two sections about eighteen nuclei around each cone, it follows 
 that one third of this number, or six, represents approximately the num- 
 ber of nuclei for each ommatidium. If then the deeper sections con- 
 tain six nuclei for each ommatidium and the outermost section two, the 
 total number of proximal nuclei for each ommatidium must be eight. 
 
 I do not mean to imply that this estimate can be insisted upon as abso- 
 lutely invariable ; but I wish to show that, as these nuclei represent the 
 proximal retinulae in an adult lobster’s eye, and as there are eight 
 such retinulse and about eight of these nuclei to an ommatidium, the 
 change which the eight embryonic cells undergo in becoming adult 
 retinulse is chiefly that of arrangement, and certainly does not involve 
 any considerable increase in numbers. 
 
 At stage E, which was the one last described, the young lobster was 
 about to escape from the egg-shell. The next stage, F, is that of a 
 lobster about one inch in length. At this stage the optic lobes are 
 represented by optic stalks, and the distal rounded end of each stalk is 
 occupied by the retina. Figure 51 represents a longitudinal section of 
 a single ommatidium from this stage. The distal end of the ommatid- 
 ium is covered with a well developed corneal cuticula. The cuticula 
 is marked out into hexagonal corneal facets (Fig. 52). The facets of 
 course indicate the arrangement of the ommatidia. This arrangement 
 at the first differentiation of ommatidia was such that hexagonal and 
 not square facets would have resulted if a cuticula had been then 
 produced. 
 
 Directly below each corneal facet is a pair of crescentic nuclei, those 
 of the corneal hypodermis (Fig. 53, nl. cm.). These nuclei have in all 
 preceding stages shown a tendency to become elongated and crescentic 
 in outline, but it is in this stage that this peculiarity reaches its highest 
 development. Between each pair of hypodermal nuclei the distal end 
 of the cone-cells is usually seen (Fig. 53, con.). The four cone-cells 
 with their nuclei occur immediately below the corneal hypodermis 
 (Figs. 51 and 54, nl. con.). The cone itself is already formed in part, 
 and lies below the nuclei of the cone-cells. Proximally the four cone- 
 cells can be traced to near the middle of the retina ; here they are no 
 longer distinguishable. The proximal end of the cone itself terminates 
 as four processes, one on the outer lateral wall of each cone-cell (Fig. 56). 
 Surrounding the cone about midway on its length are the nuclei of the 
 distal retinulse (Figs. 51 and 55, nl. dst.). In transverse sections of 
 stage F, these nuclei show the same grouping in circles of six as they 
 showed in previous stages. The outlines of the retinulse are not visible. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 49 
 
 The substance in which the nuclei lie contains a few granules of pig- 
 ment. Below the proximal end of the cone the nuclei of the proximal 
 part of the retina can be seen. These are not so definitely arranged 
 that they can be counted. They occur on several planes. Fortunately, 
 their cells in this stage have very distinct outlines, and a short distance 
 below the nuclei one can see with perfect clearness the seven retinulae 
 which surround each rhabdome (Fig. 57, rtn'.px.). Occasionally, as is 
 seen in Figure 57, the nucleus of the retinula occupies a position in its 
 cell as deep as the plane of this section. The proximal retinulae contain 
 a few pigment granules. On account of the great similarity of the 
 groups of proximal retinulae, I have not been able to plot and super- 
 impose sections with certainty ; and as the nuclei of the proximal retin- 
 ulae are placed at different levels I have not succeeded in identifying 
 an eighth nucleus, which, it will be remembered, was pointed out in 
 the histology of the adult eye as probably representing a degenerate 
 retinula. 
 
 At this stage the first trace of the rhabdome appears (Fig. 57, rhb.). 
 It is a cylindrical thickening in the centre of each group of proximal 
 retinulae, and extends from a short distance below the crystalline cone 
 very nearly to the basement membrane. From its earliest appearance 
 it is divided into four segments, which bear the same relation to the sur- 
 rounding retinulae as they do in the adult. (Compare Figs. 58 and 34.) 
 Nothing has been observed in the development of the rhabdome which 
 indicates the significance of the four lines by which the segments of the 
 rhabdome are separated. 
 
 Owing to a lack of distinctness in the tissue near the basement 
 membrane, I have been unable to identify the individual retinulae in 
 that region. What I have observed is, that fibres pass from the retina 
 through the basement membrane and into the optic ganglion. Pre- 
 sumably these fibres are from the proximal ends of the retinulae, and are 
 grouped as I have described in the retina of mature lobsters. 
 
 At this stage the only observation which I have made bearing on the 
 question of nerve termination is as follows. In each proximal retinula 
 near the rhabdome there are one or two fibres which extend nearly the 
 whole length of the retinula. In transverse sections they of course 
 appear as dots (Fig. 58, /6r\), and might be mistaken for the remains 
 of pigment granules were it not for their sharper outlines and the regu- 
 larity of their arrangement. I am of opinion that they are the first indi- 
 cations of nerve-fibrillae, which, as I have pointed out in the section on 
 Histology, lie in the adult eye next the rhabdome. 
 
 VOL. xx. — no. 1. 4 
 
50 
 
 BULLETIN OF THE 
 
 The cells which I have thus far described in the differentiation of the 
 ommatidia are unquestionably ectodermal in origin. In stage F certain 
 nuclei appear which may have another origin. It will be recalled that 
 in stage C (Fig. 41), although the intercepting membrane is well devel- 
 oped, the retina and optic ganglion are still closely applied to each 
 other. In stage D (Fig. 45) the retina and ganglion have separated 
 enough to form an intervening space of considerable extent. In stage 
 E (Fig. 46) this space not unfrequently contains several nuclei. These 
 are smaller than the ectodermic nuclei in the retina, and of about the 
 size of those in the ganglion. Their chromatine differs from that of 
 both the retinal and ganglionic nuclei, in that it has the form of very 
 distinct particles which give the nucleus a decidedly granular appearance. 
 Moreover, these nuclei, which I believe to be mesodermic in origin, are 
 variable in shape, whereas the different kinds of ectodermic nuclei pos- 
 sess characteristic forms (Fig. 47). In stage F the space between the 
 retina and ganglion also contains a few mesodermic nuclei, and similar 
 ones are noticeable in the base of the retina (Fig. 51, nl. pig.). The 
 latter are different from the nuclei of the proximal retin ulae (Fig. 51, 
 nl. px.), and resemble so closely the nuclei which in the adult have been 
 described as belonging to the accessory pigment cells, that I believe 
 them to be the nuclei of those cells. 1 
 
 From stage F to that of the fully grown lobster the changes in the 
 retina are, with one exception, rather insignificant. The parts of the 
 retina increase considerably in size, especially at the distal end of 
 the ommatidium, and additional pigment is deposited in the distal and 
 proximal retin ulae. The only change of importance which the eye 
 undergoes before full maturity is reached is a rearrangement of the 
 ommatidia. It will be remembered that up to stage F the ommatidia 
 were so arranged in relation to each other that the resulting corneal 
 facets were hexagonal in outline. In the adult lobster the facets are 
 square. The change from the six- to the four-sided facet is effected, 
 I believe, by a partial slipping of rows of ommatidia on each other. 
 Imagine, in Figure 55, a row of ommatidia extending in the direction 
 of the arrow and on either side, and parallel to this another row. Only 
 one ommatidium in each of these two lateral rows is given in the figure. 
 
 1 When the preliminary notice of this paper was written, I was somewhat in 
 doubt as to the origin of the accessory pigment-cells. I had not then had the op- 
 portunity of studying stage F, and I was of opinion that these cells were probably 
 of ectodermic origin. I believe that the evidence which is now at hand indicates 
 that they are cells derived from the mesoderm. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 51 
 
 Suppose the individual ommatidia of the three rows to be arranged in 
 reference to one another as the four in Figure 55 ; i. e. the ommatidium 
 of one row covering the open spaces between two ommatidia in the 
 adjoining row. Under these conditions hexagonal facets would be pro- 
 duced. But now imagine the middle row to move in the direction of 
 the arrow through the distance of half the thickness of an omrpatidium. 
 After the completion of this movement, the ommatidia of the middle 
 row are directly opposite the ommatidia of the adjacent rows. With 
 this arrangement the ommatidia can be grouped in square blocks of 
 four, nine, etc. This is the grouping which obtains in the adult retina, 
 and the facets resulting from it are square in outline. In the more 
 primitive arrangement, that with hexagonal facets, the ommatidia can 
 also be grouped in blocks of four, nine, etc. These blocks, however, 
 are never square in outline, but lozenge-shaped (Fig. 55). 
 
 The process of rearranging the ommatidia is accomplished at a period 
 in the growth of the young lobster much later than that at which the 
 nerve-fibres have arranged themselves in relation to the openings in 
 the basement membrane. Such being the case, one would expect to 
 find in the deeper part of the adult retina traces of the more primitive 
 arrangement. This is seen, I believe, in the lozenge-shaped outline 
 which groups of four ommatidia present in the deeper part of the retina. 
 A good instance of this is to be seen in Figure 14, although it is ap- 
 parent in almost all of the transverse sections which include the proxi- 
 mal retinulae. One might say that the ommatidia were rooted to the 
 basement membrane when the hexagonal system prevailed, and that the 
 rearrangement fully affected only their free distal ends. 
 
 The movement suggested as a means of rearranging the oftnmatidia 
 not only explains the new position of the ommatidia, but also accounts 
 for the situation in which the nuclei of the distal retinulse occur. In 
 Figure 55 each ommatidium is surrounded b}^ a circle of six nuclei. 
 These belong to the distal retinulse. Instead of describing them as 
 being in circles of six, it might be said that there is one nucleus at each 
 corner of every cone. Thus, cone x (Fig. 55) has nuclei 1, 2, 3, and 4 
 at its four corners, and cone y has in a corresponding way nuclei 5, 6, 7, 
 and 8. If the cones were to move as I have already described, and 
 were to carry their surrounding nuclei with them, the result would be 
 that nucleus 5 would come to lie next to nucleus 2, and 7 next to 4, and 
 so on. In other words, a pair of nuclei would occupy each space be- 
 tween the adjoining angles of four adjacent cones. This is the position 
 which the nuclei of the distal retinulse occupy in the retina of an adult 
 lobster (Fig. 5). 
 
62 
 
 BULLETIN OF THE 
 
 Several investigators have already described the development of the 
 ommatidia in the compound eyes of the higher Crustaceans. The differ- 
 ent accounts disagree especially in two particulars ; first, as to the source 
 of the different structures in the retina, and, secondly, as to the number 
 and arrangement of the cells in an ommatidium. In describing the 
 development of the retina, I have already discussed the first difference, 
 and need here only recall my conclusion ; namely, that the retina, includ- 
 ing the corneal hypodermis, crystalline cones, retinulse, and rhabdome, 
 originates as a simple hypodermal thickening, and that no part of it is 
 derived from the deeper ectoderm, which becomes the central nervous 
 system. As to the number of cells which constitute an ommatidium, it 
 will be recalled that in the lobster there are at least sixteen in each 
 ommatidium ; two in the corneal hypodermis, four cone-cells, two distal 
 retinulse, eight proximal retinulse one of which was rudimentary, and a 
 small, but variable number of accessory pigment-cells. The last are 
 probably mesodermic in origin ; all of the others are derived from the 
 ectoderm. 
 
 The several accounts of the corneal hypodermis given by various 
 authors differ principally in the number of cells which are said to be 
 found in. each ommatidium. Reichenbach (’86, p. 91) and Isusbaum 
 '(’87, p. 179) state respectively that in Astacus and Mysis there are 
 four hypodermal cells in each ommatidium. Nusbaum’s statement 
 is further supported by Grenacher (’79, p. 118), who describes four cells 
 under each facet in Mysis. Herrick (’89, p. 167) has found two hypo- 
 dermal cells in the ommatidium of Alpheus. Patten states that the 
 number of corneal cells in the ommatidia of all Decapods which he has 
 examined ?s two. 
 
 All the direct evidence that I have seen points to the conclusion that 
 the ommatidia of the Decapods possess two cells in the corneal hypo- 
 dermis. Reichenbach’s observation directly opposes this view. I have 
 not had the opportunity of examining the same species as Reichenbach 
 did, but I have studied a representative of the fresh-water crayfishes, 
 Cambarus Bartoni , and there is no question that in the ommatidium 
 of this species only two corneal cells are present. The nuclei of these 
 two cells lie in the angles of the hypodermal squares, each one directly 
 above a nucleus of a cone-cell. When viewed from the surface, it is 
 difficult to say whether there are two or four hypodermal nuclei, be- 
 cause the four nuclei of the cone lie so near to the hypodermis and 
 resemble its nuclei so closely. It seems to me possible that in 
 his surface view Reichenbach (’86, Fig. 226) may have drawn with 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 53 
 
 the hypodermal nuclei those of the cone-cells, thus giving four instead 
 of two. 
 
 The four nuclei in the corneal hypodermis of each ommatidium, as 
 described by Nusbaum in the case of My sis, had already been seen by 
 Grenadier, who further stated that the nuclei were arranged in pairs at 
 two levels. Grenacher does not describe nuclei in the two segments of 
 the crystalline cone. In the retina of Mysis stenolepis, the four nuclei 
 which were described by Grenacher are easily identified, but I cannot 
 agree with Grenadier’s statement (’79, p. 118) that all four belong to 
 the same category. The more superficial pair unquestionably belong to 
 the corneal hypodermis, but the deeper pair, I feel confident, are the 
 nuclei of the crystalline cone. The conclusion to be drawn from this 
 interpretation of the work of Reichenbach, Nusbaum, and Grenacher is, 
 that in Decapods and Schizopods each ommatidium possesses two cells 
 in the corneal hypodermis and only two. 
 
 Concerning the number of cone-cells in the ommatidium of Decapods, 
 different writers very generally agree. Reichenbach, Kingsley, Herrick, 
 and Patten all state that there are four, cone-cells in the Crustaceans 
 which they have studied. 
 
 In the cone-cells the number four, according to Grenacher, is charac- 
 teristic not only of Decapods, but, excepting the Schizopods, it is the 
 distinguishing feature of the Podophthalmata. In Mysis, Grenacher 
 states that the cone has two, instead of four segments. I have studied 
 the eyes of Mysis stenolepis , Smith, and my observations confirm this 
 statement. Notwithstanding Grenacher’s assertion, Nusbaum (’87, 
 p. 179) claims that the nuclei of the cone-cells in Mysis are grouped 
 in fours. I am confident that there are only two nuclei in the adult 
 cone, and having seen no evidence of a suppression of two nuclei, I 
 must consequently side with Grenacher in his belief that the cone of 
 Mysis is composed of only two cells. 
 
 The differentiation of the retinulae into distal and proximal groups is 
 more complete in the lobster than in the majority of other Decapods 
 studied. As a result of this incomplete differentiation, it is often 
 impossible to get exact statements from the descriptions of different 
 authors concerning the number and position of the retinulse. 
 
 Reichenbach (’86, p. 93) maintains that in Astacus, after the four 
 cone-cells, and as he believes the four hypodermal cells, were differ- 
 entiated, the remaining cells became pigment-cells (retinulae). Essen- 
 tially the same account is given by Nusbaum (’87, p. 180) for Mysis. 
 Kingsley (’87, p. 53) states that, in addition to the corneal hypodermis, 
 
54 
 
 BULLETIN OF THE 
 
 each ommatidium in Crangon at first consists of four vertical series of 
 nuclei, each series containing five nuclei. This would give a total of 
 twenty cells for each ommatidium. After deducting four, the number 
 of cone-cells, from twenty, the original number of cells, there remain 
 sixteen cells to be accounted for, presumably as pigment-cells. I have 
 examined the eye of an adult Crangon vulgaris, and I find in it, as in 
 the lobster’s eye, two distal retinulae and seven proximal retinulae. This 
 can scarcely be reconciled with Kingsley’s account, unless one admits 
 an extensive suppression of cells. Such a suppression seems to me 
 scarcely probable, and I am therefore inclined to believe that there 
 has been some error in Kingsley’s method of counting. Possibly 
 the series of nuclei were so placed that they were shared by neigh- 
 boring ommatidia, and did not all belong to one ommatidium. This, 
 however, could only be settled by re-examining the early stages of 
 Crangon. 
 
 Herrick (’86, p. 44) describes seven retinulae in the ommatidium of 
 Alpheus, and makes the statement that they do not possess nuclei. He 
 then describes some undifferentiated ectodermic cells, the nuclei of 
 which can be seen in the space between the cones. As this is the 
 position which in the young lobster is occupied by the nuclei of the 
 proximal retinulae, and as Herrick has not identified any nuclei for 
 the proximal retinulae in Alpheus, I am inclined to regard the deeper 
 nuclei of this group as belonging to these cells. The more superficial of 
 the nuclei described by Herrick are apparently arranged in circles of 
 six around each cone. (Compare Herrick, ’86, Figs. 1 and 2.) As in the 
 early stages of the lobster this arrangement was characteristic of the 
 nuclei in the distal retinulae, it is possible that these superficial nuclei 
 in Alpheus may represent the distal retinulae. 
 
 Keichenbach (’86, p. 92), Kingsley (’87, p. 52), Nusbaum (’87, p. 179), 
 and Herrick (’89, p. 167) describe an ingrowth of mesodermic tissue 
 between the retina and ganglion, and in My sis, according to Nusbaum 
 (’87, p. 180), these cells, as in the lobster, give rise to what I have 
 called the accessory pigment-cells. 
 
 From the investigations which have been summarized in the preced- 
 ing pages, it is difficult to draw any general conclusion concerning the 
 number of retinulae in the ommatidia of the higher Crustacea. Reichen- 
 bach and Nusbaum make no statement as to the number of these cells 
 in Astacus and Mysis. Kingsley’s enumeration of them in Crangon 
 seems to be erroneous. Admitting the undifferentiated ectodermic 
 nuclei in Alpheus to be the nuclei of the retinulae, Herrick’s statement 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 55 
 
 that there are seven retinulse in the ommatidia of this Crustacean coin- 
 cides fairly with the results obtained from the lobster. 
 
 From the histological evidence of the adult, on the other hand, writers 
 are generally agreed that the ommatidia of Decapods possess seven 
 proximal retinulse. It is probable, however, that this statement requires 
 some qualification. It will be recalled that, in describing the proximal 
 retinulse of the lobster, I referred to an additional nucleus which ap- 
 parently represented an eighth rudimentary retinula. I have identified 
 this eighth nucleus in the ommatidia of Cambarus, where, as in the 
 lobster, it lies in a plane different from that of the other seven nuclei. 
 If the eighth nucleus should be present in approximately the same 
 plane as the other nuclei, it could be identified only with great difficulty. 
 It is my belief that it often occurs in this position, and probably 
 for this reason it has generally escaped the attention of investigators, 
 for I am of opinion that it is present in the ommatidia of all Decapods. 
 When, therefore, the statement is made that the ommatidia of a certain 
 Decapod contains seven proximal retinulse, the probabilities are that 
 the ommatidium in reality contains eight proximal retinulse, one of 
 which is rudimentary. 
 
 Concerning the Schizopods, Grenacher states (’79, p. 119) that in 
 Mysis there are more than four proximal retinulse, but how many more 
 he is not certain. In Mysis stenolepis my own observations have shown 
 me that there are certainly seven pigmented proximal retinulse. This 
 number agrees with the number of functional retinulse in Decapods, and 
 my opinion is that in Mysis, as in Decapods, an eighth rudimentary 
 proximal retinula may be expected. 
 
 The presence of distal retinulse in the ommatidia of Decapods seems 
 to have generally escaped attention. Patten and Carriere, however, 
 describe these cells in Penseus and Astacus, and the fact that they are 
 easily recognized in the eyes of Homarus, Cambarus, and Eupagurus 
 inclines me to the belief that they form a constant element in the om- 
 matidia of all Decapods. They have also been seen in the retina of 
 Mysis. In the eyes of this genus their nuclei occupy the position indi- 
 tated by d in Grenacher’s Figure M2. It is of interest to observe that 
 their permanent position in Mysis is an early and transitory one in the 
 lobster. (Compare Grenacher, ’79, Fig. 112 d, with Figs. 48 and 55 of 
 this paper.) In both the Decapods and Mysis the number of distal 
 retinulse is two. 
 
56 
 
 BULLETIN OF THE 
 
 The Types of Ommatidia. 
 
 With the conclusions arrived at in the foregoing account as a basis, 
 an ommatidium can be constructed which will serve as a type for the 
 ommatidia of all Decapods. Omitting the accessory pigment-cells, this 
 typical ommatidium would be composed of sixteen cells as follows : 
 two cells in the corneal hypodermis, four cone-cells, two distal retinube, 
 and eight proximal retinulse. In a similar way a typical ommatidium 
 can be constructed for the Schizopods. This type would differ from 
 that proposed for the Decapods, in that its crystalline cone would be 
 formed of two, instead of four cells. 
 
 The ommatidia of the Schizopods and Decapods, as the development 
 of the lobster shows, are closely related. The arrangement of the om- 
 matidia by which hexagonal facets are produced is permanent in Mysis, 
 and temporary in the lobster. The distal retinulse are grouped in the 
 adult Mysis in a way which is reproduced only in the early stages of 
 the lobster. In Mysis the outline of the nuclei in the corneal hypoder- 
 mis, as seen in sections tangential to the retina, is strikingly crescen- 
 tic. This form is temporarily assumed by the corresponding nuclei in 
 the young lobster (Fig. 53). Thus it is evident that the ommatidia of 
 the lobster pass through a stage in which they closely resemble the 
 permanent condition of the ommatidia in Mysis. For this reason, I 
 believe that the ommatidium of Mysis represents a type ancestral to 
 that of the lobster. 
 
 The only important difference which exists between the ommatidium 
 of Mysis and that of the lobster in its early stages is that in the latter 
 the cone consists of four cells, while in Mysis it is composed of only two. 
 If one admits that the ommatidium of Mysis is the forerunner of that 
 of the lobster, this difference in the number of cone-cells can easily be 
 explained on the supposition that the cells which form the cone in 
 Decapods divided once more than those in Schizopods. If these two 
 types of ommatidia are thus ^elated, it is only natural to expect that 
 this process of cell-division may connect the ommatidium of Mysis 
 with that of some lower Crustacean. 
 
 The ommatidia in the eyes of Amphipods are constructed upon a 
 type which seems thus connected with that of Mysis. In Gammarus, 
 for instance, I have found that the ommatidia possess an undifferen- 
 tiated corneal hypodermis, a crystalline cone of two cells, and five retin- 
 ulse. If one desires to convert this type into that of Mysis, or through 
 Mysis into that of the Decapods, the necessary change merely involves 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 57 
 
 the division and differentiation of cells. The two cone-cells in the 
 simpler type would remain unaffected in Mysis ; in the Decapod they 
 would divide once, thus producing a cone of four cells. By division, 
 the five retinulse of the Amphipod would become the ten retinulse of 
 the higher type. These ten retinulse are differentiated into two sets, 
 eight proximal retinulse which surround the rhabdome, seven of them 
 possessing nerve-fibres, and tw 7 o distal retinulse which envelop the cone, 
 but have no nervous connections. If the ten retinulse of the higher 
 type were developed from the five retinulse of the lower type, it follows, 
 since all of the five retinulse possessed nerve-fibres, that those of the 
 ten retinulse which do not possess nerve-fibres must be considered as 
 degenerate. The degenerate cells of the higher type of ommatidium 
 are consequently the eighth proximal retin ula and the two distal retin- 
 ulse. The structural condition of these three cells favors this view. 
 The eighth proximal retinula is identifiable only through its nucleus. 
 Each distal retinula, as I have previously described, possesses a proxi- 
 mal fibre. This fibre passes through the basement membrane with the 
 nerve-fibres of the proximal retinulse, but it is very much smaller than 
 these fibres, and terminates without reaching the optic ganglion. This 
 condition is easily interpreted as one of degeneracy. 
 
 The process by which distal and proximal retinulse were probably differ- 
 entiated from unmodified retinulse is easily suggested. A characteristic 
 distinction between the ommatidia of the higher and lower Crustacea, 
 as, for instance, between Gammarus and Homarus, is that in the former 
 the cone and rhabdome are very close to each other, whereas in the 
 latter the cone proper and rhabdome are separated by considerable 
 space. Without attempting to assign physiological reasons, the state- 
 ment may be made that it seems necessary that the sides of the cone 
 should be sheathed with pigment. In the ommatidia of Gammarus this 
 is accomplished by the distal ends of the five retinulse ; these reach be- 
 yond the rhabdome and envelop the cone. They thus form a funnel, in 
 the large central cavity of which the cone rests, while the neck of the 
 funnel is occupied by the rhabdome. Imagine now the division of 
 these five retinulse into ten, and. the separation of the cone from the 
 rhabdome. It is easy to understand how two of the ten retinulse may 
 retain their connection with the cone, while the remaining eight adhere 
 to the rhabdome. The two retinulse connected with the cone would lose 
 their nervous function and become simple pigment-cells. The retinulse 
 which remain attached to the rhabdome would continue as perceptive 
 cells. The separation of the rhabdome and cone not only offers an 
 
58 
 
 BULLETIN OF THE 
 
 explanation of the way in which the distal and proximal retinuke have 
 arisen, but it also accords well with the fact that the proximal retinuke 
 end distally in fine fibres which stretch toward the cone, and are ap- 
 plied to the fibres of the distal retinulse. 
 
 If what has been said of the growth of ommatidia be true, the course 
 which the development of these structures takes in the case of the 
 lobster must be somewhat different from that by which they arose 
 phylogenetically. In the lobster the division of the nuclei is entirely 
 completed before the ommatidia are differentiated. Consequently, after 
 differentiation has occurred, no further cell-division ensues among the 
 elements of an ommatidium. This fact, however, is by no means a 
 serious objection to the view which I have expressed, for of the two 
 processes, cell-division and the differentiation of ommatidia, it is only 
 necessary to imagine that in successive generations the differentiation 
 was retarded until a stage was reached in which all cell-division was 
 accomplished before the differentiation of ommatidia began. It is of 
 interest to observe, however, that in the lobster the planes of division 
 among the nuclei, which eventually enter into the formation of omma- 
 tidia, correspond in direction to the plane in which the nuclei of the 
 ommatidium in Gammarus would divide, should this ommatidium be 
 converted into that of the lobster. Thus the plane of nuclear division 
 which was so characteristic of the distal superficial part of each optic 
 disk in the lobster may have a phylogenetic significance. 
 
 In how far the ommatidia of all the Crustacea can be brought into 
 relation by a process of cell-division such as I have outlined, and what 
 constitutes the simplest form of an ommatidium, are questions which 
 require careful and extensive comparative study, and which I am not 
 able to discuss here. Suffice it to say, that between the ommatidia of 
 the higher and lower Crustacea there is reason to conclude that such a 
 relation as I have pointed out probably exists. 
 
 Cambridge, October 1, 1889. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 59 
 
 BIBLIOGRAPHY. 
 
 
 
 Bobretsky, N. 
 
 ’ 73 . Development of Astacus and Paleemon. Kiew, 1873. (The substance 
 of this paper, so far as it refers to the eye, is known to me through the 
 abstract in Balfour’s Comparative Embryology, Vol. II. pp. 397, 398. 
 London, 1881.) 
 
 Carriere, J. 
 
 ’ 85 . Die Sehorgane der Thiere vergleichend-anatomisch dargestellt. Miin- 
 chen u. Leipzig : R. Oldenbourg. 1885. 6 + 205 pp., 147 Abbildg. u. 
 1 Taf. 
 
 ’ 89 . Bau und Entwicklung des Auges der zehnfiissigen Crustaceen und der 
 Arachnoiden. Biologisches Centralblatt, Bd. IX. No. 8, pp. 225-234. 
 15 June, 1889. 
 
 Claus, C. 
 
 
 
 ’ 86 . Untersuchungen iiber die Organisation und Entwickelung von Branchi- 
 pus und Artemia nebst vergleichenden Bemerkungen iiber andere Pliyl- 
 lopoden. Arbeit. Zool. Inst. Wien, Tom. YI. Heft 3, pp. 267-371, 
 12 pis. 1886. 
 
 Grenacher, H. 
 
 ’ 79 . Untersuchungen iiber das Sehorgan der Arthropoden, insbesondere der 
 Spinnen, Insecten und Crustaceen. Gottingen : Yandenhock u. Ruprecht. 
 1879. 8 + 188 pp., 11 Taf. 
 
 Grobben C. 
 
 ’ 79 . Die Entwickelungsgeschichte der Moina rectirostris. Arbeit. Zool. Inst. 
 Wien, Tom. II. Heft 2, pp. 203-269, 7 pis. 1879. 
 
 Herrick, F. H. 
 
 ’ 86 . Notes on the Embryology of Alpheus and other Crustacea and on the 
 Development of the Compound Eye. Johns Hopkins Univ. Circulars, 
 Yol. YI. No. 54, pp. 42-44. Dec., 1886. 
 
 ’ 88 . Development of Alpheus. Johns Hopkins Univ. Circulars, Yol. YII. 
 No. 63, pp. 36, 37. Feb., 1888. 
 
 ’ 89 . The Development of the Compound Eye of Alpheus. Zool. Anzeiger, 
 Jahrg. 11, No. 303, pp. 164-169. 25 March, 1889. 
 
 Kingsley, J. S. 
 
 ’ 86 . The Arthropod Eye. Amer. Naturalist, Yol. XX. No. 10, pp. 862-867. 
 Oct., 1886. 
 
 ’ 86 a . The Development of the Compound Eye of Crangon. Zool. Anzeiger, 
 Jahrg. 9, No. 234, pp. 597-600. 11 October, 1886. 
 
60 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 Kingsley, J. S. 
 
 ’ 87 . The Development of the Compound Eye of Crangon. Journ. of Mor- 
 phology, Yol. I. pp. 49-67, 1 pi. 1887. 
 
 ’ 89 . The Development of Crangon vulgaris. Third Paper, with Plates I., 
 II., III. Bull. Essex Inst., Yol. XXI., pp. 1-42. 1889. 
 
 Mark, E. L. 
 
 ’ 87 . Simple Eyes in Arthropods. Bull. Mus. Comp. Zool. at Harvard Col- 
 lege, Yol. XIII. No. 3, pp. 49-105, 5 pis. Eeb., 1887. 
 
 Newton, E. T. 
 
 ’ 73 . The Structure of the Eye of the Lobster. Quart. Jour, of Micr. Sci., 
 Yol. XIII., New Series, pp. 325-343. 1873. 
 
 Nusbaum, J. 
 
 ’ 87 . L’Embryologie de Mysis chameleo (Thompson). Arch. Zool. exper., 
 ser. 2, Tom. Y. pp. 123-203. 1887- 
 
 Parker, G. H. 
 
 ’ 87 . The Eyes in Scorpions. Bull. Mus. Comp. Zool. at Harvard College, 
 Yol. XIII. No. 6, pp. 173-208, 4 pis. Dec., 1887. 
 
 ’88. A Preliminary Account of the Development and Histology of the Eyes 
 in the Lobster. Proc. Amer. Acad. Arts and Sci., Yol. XXIY. pp. 24, 25. 
 
 Note. — Copies of this paper were distributed in November, 1888, although the volume in which it 
 
 appeared was not issued till December, 1889. 
 
 Patten, W. 
 
 ’86. Eyes of Molluscs and Arthropods. Mittheilungen a. d. zool. Station 
 zu Neapel, Bd. YI. Heft 4, pp. 542-756, Pis. 28-32. 1886. 
 
 ’ 87 . Studies on the Eyes of Arthropods. 1. Development of the Eyes of 
 Yespa, with Observations on the Ocelli of some Insects. Journ. of Mor- 
 phology,- Yol. I. pp. 193-227, 1 pi. 1887. 
 
 Reichenbach, H. 
 
 ’86. Studien zur Entwicklungsgeschichte des Elusskrebses. Abhandl. 
 Senckenb. Naturf. Ges, Bd. 14, Heft 1, pp. 1-137. 1886. 
 
 Schultze, M. 
 
 ' 67 . Ueber die Endorgane des Sehnerven im Auge der Gliedertkiere. Arch. 
 
 f. mikr. Anat., Bd. III. pp. 404-408. 1867 • 
 
 ’68. Untersuchungen iiber die zusammengesetzten Augen der Krebse und 
 Insecten. Bonn : Max Cohen & Sohn. 32 pp., 2 Taf. 1868. 
 
 Watase, Sho. 
 
 ’ 89 . On the Structure and Development of the Eyes of the Limulus. Johns 
 Hopkins Univ. Circulars, Yol. VIII. No. 70, pp. 34-37. March, 1889. 
 
Parker. — Lobster Eye, 
 
 EXPLANATION OF FIGURES. 
 
 / 
 
 All the figures were drawn with the aid of an Abbd camera. Unless otherwise 
 stated, the figures are from specimens stained with Grenacher’s alcoholic borax- 
 carmine and mounted in benzol-balsam. Where sections have been depigmented 
 the reagent employed was an aqueous solution of potassic hydrate (see Par- 
 ker, ’87, p. 175). Figs. 1 to 36 refer to the histology of the adult lobster’s eye. 
 Figs. 37 to 59 inclusive deal with the development of the eye. 
 
 PLATE I. 
 
 All figures on this plate illustrate the histology of the lobster’s eye. 
 
 Fig. 1. A longitudinal section of an ommatidium. This is a composite figure, its 
 parts having been drawn to one scale from various sections, and after- 
 wards combined. The distal retinula on the right side of the cone 
 contains its natural pigment; that on the left side has been depig- 
 mented. The letter x indicates the position of the band which limits 
 the corneal facet. The numbers to the right of the figure correspond 
 to the numbers of the following figures of transverse sections, and 
 mark the level at which the latter were taken. X 105. 
 
 The remaining figures on this plate (Nos. 2-25) are transverse sections 
 either of ommatidia or bundles of nerve-fibres. In each case the sections were 
 studied and drawn from their distal faces. Figs. 2 to 20 are magnified 375 
 diameters ; Figs. 21 to 25, 575 diameters. 
 
 “ 2. A corneal facet. This specimen was cleaned in a boiling solution of 
 potassic hydrate and studied in water. 
 
 “ 3. Four pairs of cells from the corneal hypodermis. Around the lower right- 
 hand pair of cells the outlines of the six surrounding distal retinulae are 
 indicated in part by dotted lines. 
 
 “ 4. Four groups of cone-cells. The ommatidia to which the cone-cells be- 
 long are indicated by the letters a, b, c, and d. These letters are used 
 to indicate corresponding ommatidia in deeper sections. 
 
 “ 5 to 20 represent transverse sections through ommatidia in the various regions 
 indicated as follows : — 
 
 “ 5. The middle region of four cones. Completely depigmented. 
 
 “ 6. The proximal ends of four cones. The re-entrant angle on the surface is 
 indicated at x. Completely depigmented. 
 
 “ 7. Slightly below the proximal ends of the cones. 
 
 “ 8. Beyond the distal ends of the proximal retinulae. 
 
 “ 9. The distal ends of the proximal retinulae. 
 
 “ 10. The thick distal portion of the proximal retinulae. 
 
 “ 11. The proximal retinulae immediately above the distal termination of the 
 rhabdome. 
 
 “ 12. At the distal ends of the rhabdomes. Completely depigmented. 
 
Parker. — Lobster Eye. 
 
 Fig. 13. The rhabdome between its distal end and middle. In this section the ac- 
 cessory pigment-cells of each ommatidium are apparently separated 
 by an intervening space from those of neighboring ommatidia. This 
 space is probably the result of shrinkage and subsequent rupture. 
 
 “ 14. In the same plane as that shown in Fig. 13. Partially depigmented. 
 
 “ 15. The middle of the rhabdome. The distal retinulae of ommatidium c have 
 been numbered. Completely depigmented. 
 
 “ 16. The proximal end of the rhabdome. The accessory pigment-cells in this 
 section have probably been ruptured, as in Fig. 13. 
 
 “ 17. In the same plane as that shown in Fig. 16. Partially depigmented. 
 
 “ 18. The proximal tip of the rhabdome. Completely depigmented. 
 
 “ 19. The proximal retinulae close to the basement membrane. Partially de- 
 pigmented. 
 
 “ 20. A diagram of Fig. 19. The proximal retinulae of the different ommatidia 
 have been numbered to correspond with those of ommatidium c in 
 Fig. 15. The retinulae of ommatidium c are tinted pink. 
 
 “ 21. Transverse section of nerve-fibres as they pass through the basement 
 membrane. The line y z shows the plane of section for Fig. 29 ; x 
 indicates the cross-shaped thickening in the basement membrane. 
 Completely depigmented and stained in Weigert’s haematoxylin. 
 (See page 4.) x 575. 
 
 “ 22, 23, 24, and 25 represent transverse sections of the fibres in the optic nerve. 
 
 Weigert’s haematoxylin. X 575. The planes at which these sections 
 were taken are as follows : — 
 
 “ 22. Directly below the basement membrane. The fibres are in groups of 
 threes and fours. 
 
 “ 23. At one fourth the distance from the basement membrane to the optic 
 ganglion. 
 
 “ 24. At half the distance between the membrane and ganglion. 
 
 “ 25. At the surface of the ganglion. 
 
 ABBREVIATIONS. 
 
 ax. n. 
 
 Nervous axis of retinula. 
 
 mb. pi ph. 
 
 Peripheral membrane. 
 
 cap. 
 
 Protoplasmic cap of cone- 
 
 n.fbr. 
 
 Nerve-fibre. 
 
 cl. con. 
 
 Cone-cell. [cell. 
 
 nl. con. 
 
 Nucleus of cone-cell. 
 
 cl. ms d. 
 
 Mesodermic cell. 
 
 nl. cm. 
 
 “ corneal hypodermis. 
 
 con. 
 
 Cone. 
 
 nl. dst. 
 
 “ distal retinula. 
 
 cm. 
 
 Corneal cuticula. 
 
 nl. pig. 
 
 accessory pigment-cell. 
 
 cm. h d . 
 
 Corneal hypodermis. 
 
 nl. j ox. 
 
 “ proximal retinula. 
 
 eta. 
 
 Cuticula. 
 
 ornm 1 . 
 
 Ommateum. 
 
 enc. 
 
 Brain. 
 
 pig. dst. 
 
 Distal band of pigment. 
 
 fbi't. 
 
 Fibrilla3. 
 
 pig. px. 
 
 Proximal band of pigment. 
 
 gn. opt. 
 
 Optic ganglion. 
 
 r. 
 
 Retina. 
 
 h d. 
 
 Hypodermis. 
 
 rhb. 
 
 Rhabdome. 
 
 mb. 
 
 Basement membrane. 
 
 rtn'. dst. 
 
 Distal retinula. 
 
 mb. i cpt. 
 
 Intercepting membrane. 
 
 rtn r . px. 
 
 Proximal retinula. 
 
 mb. i cl. 
 
 Intercellular membrane. 
 
 spa. i cl. 
 
 Intercellular space of retina. 
 
 The other abbreviations which occur on the plates are explained in the descrip- 
 tion of the figures with which they are found. 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Parker — Lobster Eye. 
 
 PLATE II. 
 
 Figs. 26 to 36 deal with the histology of the lobster’s eye ; Figs. 37 to 39, with its 
 
 development. 
 
 Fig. 26. Vertical section through that portion of the eye-stalk where the transi- 
 tion from the undifferentiated hypodermis to the ommateum is accom- 
 plished. The open space x is due to shrinkage. X 31. 
 
 “ 27. A group of the four cone-cells of an ommatidium and the attached cor- 
 neal hypodermis. Isolated and studied in Muller’s fluid. X 365. 
 
 “ 28. Proximal end of a group of four cone-cells where they separate as fibres 
 to pass around the rhabdome. Isolated and studied in Muller’s fluid. 
 X 365. 
 
 “ 29. Transverse section of the basement membrane. The distal face is upper- 
 most; the proximal face below. The section is taken in a plane 
 which would be represented in Fig. 21 by the line z y. x 575. 
 
 “ 30. A rhabdome and its seven surrounding proximal retinulae. At the distal 
 end the free tips of the seven retinulae can be seen. At the proximal 
 end the rhabdome and the four groups of retinulae which pass through 
 the basement membrane are visible. Isolated and studied in chromic 
 acid -£$%. X 200. 
 
 “ 31. An individual proximal retinula. Isolated and studied in chromic acid 
 
 M- * 200 . 
 
 “ 32. Transverse section of a rhabdome and its surrounding cells. The plane 
 of section is about half-way between the middle and distal end of the 
 rhabdome. Completely depigmented with potassic hydrate. X 4G0. 
 
 “ 33. Oblique section of a rhabdome from the same series of sections as Fig. 32. 
 The upper end of the figure is distal ; the lower, proximal. X 460. 
 
 “ 34. Transverse section of a rhabdome and its surrounding retinulae. The 
 nervous axis of each retinula is distinctly stained. Completely de- 
 pigmented ; stained with Kleinenberg’s alum-haematoxylin. x 460. 
 
 “ 35. Longitudinal section of a bundle of nerve-fibres extending through the 
 basement membrane toward the optic ganglion. Depigmented ; 
 Weigert’s haematoxylin. X 575. 
 
 “ 36. Optic nerve-fibre. Isolated and studied in Muller’s fluid. X 575. 
 
 “ 37. Superficial view of a left optic lobe. Enough of the right lobe is drawn 
 to indicate the position of the median plane (x y). x is anterior; y is 
 posterior. Stage A (see page 2). x 280. 
 
 “ 38. Posterior face of a section from a right optic disk cut transversely to the 
 longitudinal axis of the embryo (see page 34). At x is an angle formed 
 by the growth of the retinal cells over the undifferentiated ectoderm. 
 Stage A. x 280. 
 
 “ 39. A section from the optic disk of an embryo in stage B. The plane of 
 cutting corresponds to that in Fig. 38. In this figure x indicates, as 
 in the preceding one, the angle between the undifferentiated ectoderm 
 and the growing retina. X 280. 
 
Parker - Lobster eye. 
 
 Pi. I 
 
 j 
 
 rtridst. 
 
 cm /id 
 
 nl.dst. 
 
 rtridst. 
 
 mbpiph- 
 
 mb.icl 
 
 rtridst 
 
 mb 
 cl ms d 
 n.fbr 
 
 B.Meisel. lith 
 
Parker - Lobster eye. 
 
 Pl. II. 
 
 rfn'dsf 
 
 Hri.px 
 
 Omni: 
 
 
 f>U].px. 
 
 £Q i 
 
 ^ '^ef © « 
 
 rfnpr. 
 
 ax.ft. 
 
 B.Meisel, lith.. 
 
LIBRARY 
 
 or THE 
 
 UNIVERSITY OF ILLINOIS 
 

 
 
 
 
 
 
 ' 
 
 
 * 
 
 
 
 . 
 
 
 
 
 
 
 ' 
 
 
 
— Lobster Eye. 
 
 PLATE III. 
 
 All figures on this plate illustrate the development of the lobster’s eye. 
 
 40. A section through the left optic lobe and left half of the supra-oesophageal 
 
 ganglion. The plane of section is tangential to that part of the sur- 
 face of the egg on which the embryo rests. The position of the 
 median plane is indicated at xy. The surfaces tinted with deeper pink 
 in the figure represent areas containing nuclei in the specimen ; those 
 in lighter pink, areas in which no nuclei were present. The optic lobe 
 is divided into two parts by a band of large, faintly colored nuclei, 
 which, with the smaller surrounding nuclei, are shown in the figure. 
 To the right of the nuclei the broad tinted marginal area represents 
 the retina, r. The remainder of the optic lobe gives rise to the optic 
 ganglion. Stage C (see page 2). X 280. 
 
 41. Posterior aspect of a transverse section of a right optic lobe. The plane 
 
 of section corresponds to that in Fig. 38 ; a: is the angle which indi- 
 cates the separation of the retinal and ganglionic constituents of the 
 intercepting membrane. Stage C. X 280. 
 
 42. This figure is taken from a region which corresponds to the left-hand por- 
 
 tion of Fig. 41. Although from the same set of eggs the embryo from 
 which Fig. 42 was drawn was somewhat more advanced than that 
 from which Fig. 41 was taken. At x the proximal band of retinal 
 nuclei can be seen ; at y the distal band is shown. Stage C. X 460. 
 
 43. The superficial layer from the distal band of retinal nuclei ; seen from 
 
 the external surface of the retina. Stage C. X 460. 
 
 44. The deep layer of the distal band of nuclei. These are seen in opti- 
 
 cal section somewhat within the outer face of the retina. Stage C. 
 X 460. 
 
 45. A transverse section of an optic lobe from a lobster at stage D. The 
 
 plane of section corresponds to that of Fig. 38. As in Fig. 40, the 
 deeply tinted areas were nucleated ; the lighter areas were without 
 nuclei. X 145. 
 


 
 
 
 
 
 
 
Parker. — Lobster Eye. 
 
 PLATE IV. 
 
 All figures on this plate, except Fig. 59, illustrate the development of the 
 
 lobster’s eye. 
 
 Fig. 46. A transverse section of an optic lobe at stage E (see page 2). The plane 
 of section and the method of coloring the figure are the same as in 
 Fig. 45. x 145. 
 
 “ 47. An enlarged drawing of that portion of the retina which is in brackets in 
 Fig 46. Stage E. x 460. 
 
 “ 48. A view of the external surface of the retina. The distal ends of four 
 ommatidia are seen. Stage E. x 460. 
 
 “ 49. A transverse section of four ommatidia in the region of the hypodermal 
 nuclei. (Compare Fig. 47.) Stage E. X 460. 
 
 “ 50. A transverse section of four ommatidia in the plane which the nuclei of 
 the cone-cells occupy. Stage E. X 460. 
 
 “ 51. Longitudinal section of a single ommatidium. Stage F. X 460. 
 
 “ 52. Four corneal facets seen from the external surface. Stage F. X 460. 
 
 “ 53 to 58 represent transverse sections of four ommatidia at Stage F. The 
 numbers on the left side of Fig. 51 indicate the heights at which these 
 sections were taken, and correspond to the numbers of the following 
 figures. In Figs. 53 to 58 the magnification is 460. 
 
 “ 53. A transverse section in the region of the corneal hypodermis. 
 
 “ 54. A transverse section through the region in which the nuclei of the cone- 
 cells occur. 
 
 “ 55. A transverse section in the same plane as the nuclei of the distal 
 retinulae. 
 
 “ 56. A transverse section of the proximal ends of two cones. 
 
 “ 57. A transverse section through the rhabdomes and proximal retinulae. 
 
 “ 58. A transverse section of a rhabdome from Fig. 57. Fig. 58 was drawn 
 with a higher magnification than Fig. 57 in order to show the relation 
 of the proximal retinulae to the segments of the rhabdome. X 640. 
 
 “ 59. A corneal facet from near the periphery of the retina in an adult lobster. 
 
 The hexagonal outline is noteworthy. This specimen was cleaned in 
 boiling potassic hydrate and examined in water. X 280. 
 

 Parker - Lobster eye. 
 
 Pl. iv. 
 
 G.H.P del. 
 
 B.Meisel, lith 
 
Bulletin of the Museum of Comparative Zoology, 
 
 AT HARVARD COLLEGE. 
 
 VOL. XX. NO. 5. 
 
 THE EYES IN BLIND CRAYFISHES. 
 
 By G. H. Parker. 
 
 With One Plate. 
 
 CAMBRIDGE, U. S. A.: 
 PRINTED FOR THE MUSEUM. 
 November, 1890 . 
 
A 
 
 s/t 
 
 No. 5. — The Eyes in Blind Crayfishes. By G. H. Parker. 1 
 
 In the fall of 1888 Mr. Samuel Garman placed at my disposal several 
 crayfishes 2 which had been collected by Miss Ruth Hoppin in the caves 
 of Jasper County, Missouri. The specimens were given to me with the 
 suggestion that I should ascertain the extent to which their eyes had 
 degenerated, for, judging from external appearances, these organs had 
 become as rudimentary as the eyes of the blind crayfish, Cambarus 
 pellucidus, Tellk., from Mammoth Cave. In order to establish compari- 
 sons it was desirable to study the eyes in C. pellucidus, and for this 
 purpose specimens of this species were kindly furnished me from the 
 collections in the Museum of Comparative Zoology. These specimens, 
 as well as those collected by Miss Hoppin, were preserved in strong- 
 alcohol. My study of this material was carried on in the Zoological 
 Laboratory of the Museum, under the direction of Dr. E. L. Mark. 
 
 Notwithstanding the general interest which zoologists have shown in 
 the blind crayfishes there have been very few publications on the minute 
 structure of the eyes of these animals. The earliest contribution to this 
 subject was from Newport, who, in discussing the ocelli of Anthophora- 
 bia., incidentally described the structure of the eye in Cambarus pellu- 
 cidus. According to Newport’s account (’55, p. 164), the eyes in this 
 species would seem to be only partially degenerated, for although the 
 retinal region is not pigmented, the corneal cuticula is nevertheless 
 divided into irregular facets, or “ corneales,” as they are termed, “ and 
 the structure [hypodermis] behind these into chambers to which a small 
 but distinct optic nerve is given.” 
 
 The second investigator who studied the eyes of blind crayfishes was 
 Leydig (’83, pp. 36 and 37). The material which was accessible to him 
 was unfortunately so poorly preserved that it was of little value for his- 
 tological purposes. He nevertheless satisfied himself that the cuticula 
 in the corneal region was not facetted. He also quoted from an abstract 
 
 1 Contributions from the Zoological Laboratory of the Museum of Comparative 
 Zoology, under the direction of E. L. Mark, No. XX. 
 
 2 These crayfishes had previously been submitted to Dr. Walter Faxon for 
 determination. They have since been described by him as a new species, under 
 the name of Cambarus setosus, an account of which will be found in Mr. Garman’s 
 recent paper (’89, p. 237) on “Cave Animals from Southwestern Missouri.” 
 
 VOL. XX. — NO. 5. 
 
154 
 
 BULLETIN OF THE 
 
 4 
 
 of Newport’s paper, to the effect that the eye is “ ohne Hornhaut, Pig- 
 ment und Nervenstabe.” The phrase “ohne Hornhaut ” means, I be- 
 lieve, that a facetted cornea is not present ; at least this seems to be the 
 interpretation placed on it by Leydig, for the quotation is shortly fol- 
 lowed by this sentence : “ Dort wo man eine gefelderte Cornea zu suchen 
 hatte — am Gipfel des Kegels — zeigt sich die Haut von der gewohn- 
 lichen Beschaffenheit.” There was greater reason for Ley dig’s regret 
 that he could not consult Newport’s original paper than Leydig himself 
 appreciated ; for, although he probably had no reason to consider the 
 abstract incorrect, if his quotation from it is exact, it differs at least in 
 one respect from Newport’s account. Newport described the cornea as 
 facetted ; Leydig’s quotation from the abstract states that it was not 
 facetted. I have been unable to discover where this abstract was pub- 
 lished, but, since Leydig quotes directly from it, the probabilities are 
 that the discrepancy between his quotation and Newport’s actual state- 
 ment is to be attributed to an error in the abstract. Aside from this 
 difficulty, it must be borne in mind that Leydig and Newport in their 
 observations on the cornea by no means agree ; for while Newport really 
 describes the cornea as facetted, Leydig states from his own observa- 
 tions that it is without facets. According to Leydig, then, the eye of 
 C. pellucidus is more completely degenerated than the observations 
 of Newport would lead one to suppose. 
 
 The latest account of the eyes in blind crayfishes forms a part of 
 Packard’s paper on “ The Cave Fauna of North America” (’88, pp. 110 
 to 113). Newport and Leydig studied C. pellucidus; Packard had the 
 opportunity of studying not only this species, but also C. hamulatus, 
 Cope and Packard, from Tennessee. In both species according to Pack- 
 ard the cornea was without facets, and the hypodermis was not thick- 
 ened in the retinal region, but an optic nerve and ganglion were present. 
 The results obtained by Packard thus confirm those given by Leydig. 
 
 From this brief historical review it will be observed that one of the 
 principal questions concerning the eyes of blind crayfishes deals with 
 the extent of their degeneration. This change has not only affected the 
 finer structure of the retina, but it has also altered the shape of the 
 optic stalk. I shall therefore begin with a description of the external 
 form of the stalks. 
 
 The optic stalks of blind crayfishes are not only proportionally smaller 
 than those of crayfishes which possess functional eyes, but they have in 
 the two cases characteristically different shapes. In crayfishes with 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 155 
 
 fully developed eyes the stalk is terminated distally by a hemispherical 
 enlargement ; in the blind crayfishes it ends as a blunt cone. This 
 cone-shaped outline is especially characteristic of C. pellucidus (Fig. 2). 
 It will be observed that in this species the optic nerve (n. opt.) termi- 
 nates in the hypodermis immediately below the blunt apex of the cone. 
 In C. setosus (Fig. 1) the termination of the optic nerve is also at the 
 apex of a blunt cone. In this case, however, the axis of the cone does 
 not coincide with the axis of the stalk, as it does in C. pellucidus, but 
 the two axes meet each other at an angle of about forty-five degrees, 
 and in such directions that the conical protuberance at the distal end of 
 the stalk is directed forward and outward from the median plane of the 
 animal. The protuberance is rather more blunt in C. setosus than in 
 C. pellucidus (compare the regions marked r. in Figs. 1 and 2). 
 
 Through the kindness of Dr. Walter Faxon I was enabled to examine 
 two specimens of C. hamulatus. In this species the stalks also termi- 
 nate in blunt cones. They are not so pointed as in C. pellucidus, but 
 approach the more rounded form of C. setosus. 
 
 The three species, C. pellucidus, C. hamulatus, and C. setosus, are the 
 only blind crayfishes thus far known in North America, and, as they 
 agree in having a conical termination to the optic stalks, a peculiarity 
 not observable in crayfishes with functional eyes, it may be concluded 
 that the conical form is characteristic of the stalks in blind crayfishes. 
 Unquestionably, this conical shape is coupled with the degenerate con- 
 dition of the retina. 
 
 In describing the finer anatomy of the eye it will be more convenient 
 to begin with the condition found in C. setosus. Figure 1 is drawn from 
 a longitudinal horizontal section of the optic stalk in this species. The 
 plane of section passes through the region where the optic nerve and 
 hypodermis are in contact. This region (Fig. 1, r.) corresponds to the 
 retina of other crayfishes. The optic stalk is covered with a cuticula 
 (Fig. 1, ct), which is of uniform thickness and which resembles the 
 cuticula of the rest of the body. In this respect the stalk differs from 
 that of decapods with well developed eyes, for in these, although much 
 of the stalk is covered with ordinary cuticula, the retinal region is pro- 
 vided with a thin flexible cuticula. This has been named by Patten the 
 corneal cuticula; it cannot be said to be differentiated in C. setosus. In 
 optic stalks with functional retinas the corneal cuticula is usually 
 facetted, but in C. setosus no indication of facets is discoverable. 
 
 The undifferentiated condition of the cuticula leads one to antici- 
 pate a simple condition in its matrix, the hypodermis. The latter is a 
 
156 
 
 BULLETIN OF THE 
 
 continuous layer of cells (Fig. 1 , hd.) with its distal face applied to the 
 cuticula and its proximal face bounded by a fine but distinct basement 
 membrane (mb.). The layer is throughout very nearly uniform in thick- 
 ness ; at least it is not thicker in the region of the retina than at many 
 other places, and the slight variations in its thickness are not in signifi- 
 cant regions. The only feature of the retinal hypodermis which would 
 suggest that it was unlike the rest is the somewhat closer crowding of 
 its cells. This manifests itself in the arrangement of the nuclei in two 
 or three irregular rows, instead of a single one. In other respects the 
 nuclei of the retinal region and the surrounding hypodermis are essen- 
 tially similar. 
 
 The optic nerve (Fig. 1, n. opt.) consists of a poorly defined bundle 
 of nerve-fibres which extend from the optic ganglion to the hypodermis. 
 The nerve-fibres are doubtless intimately connected with the cells in the 
 hypodermis, for the basement membrane is interrupted where the nerve 
 and hypodermis are in contact. It is probable that the basement mem- 
 brane is reflected from the hypodermis to the optic nerve, although I 
 have not been able to observe this with clearness. 
 
 Recent investigations support the conclusion that the retina in the 
 Crustacea is derived from the hypodermis. In C. setosus that portion 
 of the hypodermis from which the retina would be derived is scarcely 
 distinguishable from other parts of the same layer. The retina in this 
 species, therefore, has so completely degenerated that it has at last 
 returned to the condition of almost undifferentiated hypodermis. 
 
 That the optic nerve still retains its connection with the retinal area 
 is, on the whole, not so significant a condition as one might at first sup- 
 pose. It is probable that the optic nerve arises in this species as it 
 does in the lobster. I have elsewhere (Parker, ’90, p. 43) attempted to 
 show that in the lobster it is not an outgrowth from either the optic 
 ganglion or the retina, but that, as the ganglion was differentiated from 
 the hypodermis, the optic nerve remained as a primitive connection be- 
 tween these two structures. So long, then, as an optic ganglion should 
 be differentiated one might expect an accompanying optic nerve ; but 
 the nerve would be present as a passive connection between hypodermis 
 and ganglion, rather than as a structure which had retained that posi- 
 tion by virtue of its continued functional importance. 
 
 The foregoing account of the eye in C. setosus is based upon obser- 
 vations on three individuals of this species. Two of these measured, 
 from the tip of the rostrum to the end of the telson, 6 cm. ; the third, 
 4.2 cm. In the three individuals the eyes presented essentially the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 157 
 
 same condition. Figure 1 is taken from one of the larger individuals. 
 In this specimen the cuticula was somewhat thinner and the hypoder- 
 mis rather thicker than in the other two. This I believe was due 
 to the fact that the animal had recently moulted. 
 
 So far, then, as the eye of C. setosus is concerned, although the optic 
 ganglion and optic nerve are present, the retina has undergone a com- 
 plete degeneration, and is now represented by a layer of undifferentiated 
 hypodermal cells. 
 
 The eyes of Cambarus pellucidus present a somewhat different condi- 
 tion from that described in G. setosus. A longitudinal horizontal sec- 
 tion of the optic stalk of C. pellucidus is shown in Figure 2. The outer 
 surface of the stalk is covered with a cuticula (ct) of uniform thickness, 
 and there is no indication of facets. Excepting at the apex of the 
 stalk, the hypodermis (Ad.) is composed of a remarkably uniform layer 
 of cells. As in C. setosus, it is bounded on its deep face by a deli- 
 cate basement membrane (mb.). Both an optic ganglion (gn. opt.) and 
 nerve ( n . opt.) are present, the latter being connected with the hypo- 
 dermis. In all these respects C. pellucidus resembles C. setosus, but 
 when the retinal part of the hypodermis in the two species is compared 
 a striking difference can be seen. The retinal hypodermis in C. se- 
 tosus (Fig. 1, r.) is, as we have seen, substantially like the remaining 
 hypodermis of the optic stalk. The retinal hypodermis in C. pelluci- 
 dus (Fig. 2, r.) is much thicker than the hypodermis of the stalk. With 
 this thickened region of the hypodermis the optic nerve is connected, 
 and there is no question, therefore, that this thickening represents the 
 rudimentary retina. Omitting minor details, the form of the thick- 
 ening is that of a plano-convex lens, the curved surface of which is 
 applied to the concave inner face of the cuticula at the distal end of the 
 stalk. The optic nerve is attached to the central part of the flat face 
 of the thickening. 
 
 When the retinal thickening is carefully studied by means of radial 
 sections, one can see that it differs from the neighboring hypodermis 
 not only in thickness, but also in the fact that it contains two kinds of 
 substance : a protoplasmic material uniform with that of the rest of the 
 hypodermis, and a number of relatively large granular masses (Fig. 3, 
 con.). These granular masses contain two, three, four, or sometimes five 
 nuclei, and nuclei are also to be found scattered through the undiffer- 
 entiated protoplasmic substance. The nuclei in the granular masses 
 are slightly smaller than those in the surrounding portion of the hypo- 
 
158 
 
 BULLETIN OF THE 
 
 dermis ; they are, moreover, round in outline, while the other nuclei are 
 usually somewhat elongated. The same features can be observed in 
 tangential sections (Fig. 6). Here, however, the outlines of the larger 
 nuclei no longer appear oval, since these nuclei are now cut in a 
 plane at right angles with their elongated axes. The nuclei in the 
 hypodermis which adjoins the retinal thickening resemble the larger 
 oval nuclei of the thickening. Nowhere in the adjoining hypodermis 
 have the granular masses with their smaller nuclei been observed. It 
 is therefore clear, that in C. pellucidus the retinal hypodermis is dis- 
 tinguished from the neighboring hypodermis, not only by its greater 
 thickness, but also by the fact that it is composed of two kinds of sub- 
 stance, each with its special form of nucleus. Since the protoplasmic 
 material of the retinal region contains nuclei which resemble those of 
 the surrounding hypodermis, it is probable that this material represents 
 hypodermis which has remained unmodified after the differentiation 
 of the granular bodies. As shown in Figure 3, the granular bodies are for 
 the most part limited to the deeper portion of the retinal thickening, and 
 the oval nuclei occupy the more superficial part. If these oval nuclei 
 represent undifferentiated hypodermal cells, it is only natural that they 
 should occupy a superficial position, for it is there that the function of 
 such cells, namely, the secretion of cuticula, could be most advanta- 
 geously carried on. In tangential sections of the retinal thickening, 
 both the nuclei of the undifferentiated hypodermis and the outlines of 
 the cells to which they belong are distinguishable (Fig. 5). These cells 
 when compared with those from the hypodermis of the sides of the stalk 
 (Fig. 4) are seen to be much smaller than the latter. Like those from 
 the sides of the stalk, however, they present no definite grouping. 
 This accords with the fact that the cuticula presented no special mark- 
 ings, such as facets, etc., for such markings could of course result only 
 from some special grouping of the secreting cells. 
 
 It is difficult to say what the granular bodies with their contained 
 nuclei are. Doubtless they represent some element in the retina of the 
 functional eye reduced by degeneration to this form. The ommatidium 
 or structural unit in the retina of a crayfish consists of five kinds of 
 cells. These are as follows : first, two cells in the corneal hypodermis, 
 lying next the cuticula ; second, four cone-cells directly below the j 
 corneal hypodermis; third, two pigment-cells, the distal retinulse, 
 flanking the cone-cells ; fourth, seven pigment-cells, the proximal reti- | 
 nuke, surrounding the rhabdome ; fifth, a few yellowish accessory pig- ; 
 ment-cells limited to the base of the retina. Excepting the accessory 1 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 159 
 
 pigment-cells, all the cells in an ommatidium are ectodermic in origin ; 
 the accessory pigment-cells are probably derived from the mesoderm. 
 Of these five kinds of cells, the granular bodies probably do not repre- 
 sent the accessory pigment-cells, for in fully developed eyes the latter 
 lie on both the distal and proximal sides of the basement membrane, 
 whereas the granular bodies are found only on the distal side of that 
 structure. The granular bodies, then, more likely represent one of the 
 four remaining elements, all of which naturally occur only on the distal 
 side of the membrane. It is not probable that the granular bodies 
 represent the cells of the corneal hypodermis, for these produce the cu- 
 ticula of the retinal region, and if they have any representatives, those 
 representatives must be the distal layer of unmodified hypodermal cells 
 already indicated in the retinal thickening. The position of the granular 
 bodies, therefore, precludes their representing corneal hypodermis. If 
 then the granular bodies are not accessory pigment-cells nor corneal 
 hypodermis, they must be either distal or proximal retinulac or cone- 
 cells. In a previous paper I have given reasons for considering the 
 proximal and distal retinulse as both originating from a commou group 
 of cells, the retinulse. These are essentially sensory in function, as con- 
 trasted with the cone-cells, which are merely dioptric. The question 
 then narrows itself to this: Are the granular masses clusters of dioptric 
 cone-cells or sensory retinulse 1 
 
 In determining to which of these two groups of cells the granular 
 masses belong, the relation which the latter sustain to the fibres of 
 the optic nerve would doubtless be of great importance, for the nerve 
 fibres in fully developed eyes are known to terminate in the retinulse, 
 not in the cone-cells. Unfortunately, the histological condition of my 
 material was such as to preclude the possibility of determining this 
 question. 
 
 The fact that each granular mass contains several nuclei clearly indi- 
 cates that it consists of several cells. The number of cells in each 
 mass, judging from the number of nuclei, varies from one to about five, 
 the more usual number being three or four. When one compares the 
 condition of intimate fusion which the cells of each mass present with 
 the normal condition of the retinulse and cone-cells, the masses must 
 certainly be admitted to resemble more closely the cone-cells. More- 
 over, the number of cells iu each mass, although variable, is nearer to 
 that of the closely united cone-cells than to that of the retinulae. Not 
 only do the number of cells involved and the intimacy of their fusion 
 favor the idea that each mass represents a degenerate cone, but the 
 
160 
 
 BULLETIN OF THE 
 
 granular substance of the mass also closely resembles the granular ma- 
 terial of a cone. For these reasons it seems probable that the granular 
 nucleated masses in the retinal region of C. pellucidus are the degen- 
 erate representatives of the cones in normal eyes. 
 
 The fact that, of all the ectodermic elements of the retina, only the 
 granular nucleated masses continue to be differentiated, throws them 
 into strong contrast with the surrounding structures. The retention of 
 these masses may mean that on account of their extreme differentiation 
 they have had time to respond only incompletely to the influence of 
 degeneration ; or it may imply that phylogenetically they were among 
 the earliest retinal structures differentiated. Admitting them to be 
 degenerated cone-cells and merely dioptric in function, one can scarcely 
 conceive how they could have been differentiated before the sensory 
 cells which they serve. But even if they cannot be regarded as more 
 primitive structures than retinulse, their retention still may be signifi- 
 cant, as an indication that the ommatidia of primitive crustaceans con- 
 tained cone-cells as well as retinulae. 
 
 Former studies have led me to believe that the difference in the 
 ommatidia of various crustaceans could be explained on the assump- 
 tion that the number of elements has been gradually increased from 
 lower to higher forms by cell-division. The simplest conceivable rep- 
 resentative of an ommatidium in the Crustacea might then be a sin- 
 gle cell. This would be of course a sensory cell ; by its division, 
 the more complicated ommatidia might subsequently be derived from it. 
 In such an event, the cone-cells must be modified sensory cells; but 
 the fact that these cells persist in so rudimentary a retina as that of 
 C. pellucidus points rather to the conclusion, that they are probably 
 almost as old, phylogenetically, as the retinulae themselves, and that 
 primitive ommatidia consisted of at least two kinds of cells, sensory 
 cells or retinulae, and cone-cells, derived not from degenerated sensory 
 cells, but from the undifferentiated hypodermis. m 
 
 As I have already shown, the results which Newport, Leydig, and 
 Packard arrived at are not always in agreement. This might be ex- 
 plained by the fact that the organ under consideration is a degenerated 
 one, and consequently subject to considerable individual variation. This 
 supposition, however, is not supported by anything I have observed. 
 The preceding account of the eye in C. pellucidus is based upon the 
 examination of three individuals. These were respectively 6.5 cm., 
 5.6 cm., and 4.4 cm. long. Figure 2 was drawn from the optic stalk of 
 the shortest individual. In all essential features the eyes of the two 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 161 
 
 other crayfishes presented the same condition as that shown in Figure 2. 
 In the specimen 5.6 cm. in length, the granular bodies were less dis- 
 tinct than in the other two, but they were nevertheless recognizable, 
 and the retinal thickening was as pronounced in this as in either of the 
 other specimens. The fact that these three individuals show so little 
 variation leads me to believe that the condition of the eye in the blind 
 crayfish is not so variable as I at first supposed it would be. The same 
 constancy is also true of C. setosus. Hence it seems to me improbable 
 that the differences between Newport’s observation and those of the 
 later investigators are due to individual variations in the specimens 
 studied. The fact that Newport’s work was done before the - develop- 
 ment of present methods of research offers, I believe, a more natural 
 explanation of some of his results, than the supposition of individual 
 variations. That the methods of his time were imperfect is evident 
 from the fact that Newport himself seems to have overlooked the gan- 
 glion of the optic stalk, a structure readily discoverable by means of serial 
 sections. (Compare Newport’s Figure 13 [’55, p. 102] with Figure 2 in 
 this paper.) Ley dig’s observations, so far as they extend, are fully con- 
 firmed by my own. Packard’s account differs from mine in only one par- 
 ticular, but that is of considerable importance ; he states that there is 
 no retinal thickening in the two species studied by him. This difference 
 may possibly be due to individual variations in the crayfishes. Unfor- 
 tunately, Packard does not state the number of specimens which he 
 examined, and consequently one is uncertain how much weight to give 
 to his general statements. 
 
 The conclusions to be drawn from the foregoing account may be 
 summarized as follows. In both species of crayfishes studied, the optic 
 ganglion and nerve are present, and the latter terminates in some way 
 not discoverable in the hypodermis of the retinal region. In C. setosus 
 this region is represented only by undifferentiated hypodermis, com- 
 posed of somewhat crowded cells, while in C. pellucidus it has the form 
 of a lenticular thickening of the hypodermis, in which there exist multi- 
 nuclear granulated bodies. These I have endeavored to show are 
 degenerated clusters of cone-cells. If Packard’s observations are correct, 
 the retina in C. pellucidus may be reduced in some individuals as much 
 as it is in C. setosus, which I have studied, but my own examinations 
 do not render this view probable. 
 
 Cambridge, February 24, 1890. 
 
162 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 BIBLIOGRAPHY. 
 
 Leydig, F. 
 
 ’83. Untersuchungen zur Anatomie und Histologie der Tliiere. Bonn, 
 Emil Strauss, 1883. 174 pp., 8 Taf. 
 
 Newport, G. 
 
 ’55. On the Ocelli in the Genus Anthopliorabia. Trans. Linn. Soc., Lon- 
 don, Yol. XXI. pp. 161-165, Tab. X., Figs. 10 to 15 incl. Read, 
 April 19, 1853. 
 
 Packard, A. S. 
 
 ’88. The Cave Fauna of North America, with Remarks on the Anatomy of 
 the Brain and Origin of the Blind Species. Mem. Nat. Acad. Sci., Yol. 
 IY. Pt. 1, pp. 1-156, 27 Pis. Read, Nov. 9, 1886. 
 
 Parker, G. H. 
 
 ’90. The Histology and Development of the Eye in the Lobster. Bull. Mus. 
 Comp. Zool. at Harvard Coll., Yol. XX. No. 1, pp. 1-60, 4 Pis. 1890. 
 Garman, S. 
 
 ’89. Cave Animals from Southwestern Missouri. Bull. Mus. Comp. Zool. 
 at Harvard Coll., Yol. XVII. No. 6, pp. 225-240, 2 Pis. Dec., 1889. 
 
Parker. — Blind Crayfishes. 
 
 EXPLANATION OF FIGURES. 
 
 ABBREVIATIONS. 
 
 con. cone. 
 
 mb. 
 
 basement membrane. 
 
 ct. cuticula. 
 
 ill. con. 
 
 nucleus of cone-cell. 
 
 gn. opt. optic ganglion. 
 
 nl. hd. 
 
 nucleus of hypodermis. 
 
 hd. hypodermis. 
 
 n. opt. 
 
 optic nerve. 
 
 r. retina. 
 
 The specimens from which the following figures were taken' were killed and 
 preserved in strong alcohol, and stained in Czocher’s alum-cochineal. The cray- 
 fish from the optic stalk of which Figure 1 was drawn was 6 cm. long. That from 
 which the remaining figures were made was 4.4 cm. long. 
 
 Fig. 1. A longitudinal horizontal section through the right optic stalk of Cam- 
 barus setosus, Faxon. The histological detail is given in the hy- 
 podermis only. The optic ganglion and the optic nerve are tinted. 
 Between these structures and the hypodermis the space is filled with 
 a loose connective tissue. X 65. 
 
 “ 2. A longitudinal horizontal section through the right optic stalk of Cam- 
 barus pellucidus, Tellk. This drawing was made in the same manner 
 as Figure 1. X 65. 
 
 “ 3. An enlarged drawing from the distal end of the section which immediately 
 follows that from which Figure 2 is taken. This figure shows the details 
 in the retinal enlargement of the hypodermis. The space between 
 this enlargement and the cuticula was artificially produced. X 275. 
 
 “ 4. Tangential section of the hypodermis from the side of an optic stalk of 
 Cambarus pellucidus. X 275. 
 
 “ 5. Tangential section of the superficial portion of the retinal thickening in 
 the eye of Cambarus pellucidus. X 275. 
 
 “ 6. Tangential section of the deep portion in the retinal thickening of the eye 
 of Cambarus pellucidus. This section is taken from the same series 
 as the one from which Figure 5 was drawn. X 275. 
 
Parker.- Blind Crayfishes. 
 
 Pl.I 
 
 GJ.P del. 
 
 B.MeiselM.Boston. 
 

 
 THE LIBRARY 
 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
 
Bulletin of the Museum of Comparative Zoology, 
 AT HARVARD COLLEGE. 
 
 VOL. XXI. No. 2. 
 
 THE COMPOUND EYES IN CRUSTACEANS. 
 
 By G. H. Parker. 
 
 CAMBRIDGE, U. S. A.: 
 PRINTED FOR THE MUSEUM. 
 May, 1891. 
 
 I 
 
No. 2. — The Compound Eyes in Crustaceans . By G. H. Parker. 1 
 
 Table of Contents. 
 
 Page 
 
 I. Introduction . 45 
 
 II. The Retina 47 
 
 III. Arrangement of the Ommatidia 60 
 
 IV. Structure of the Ommatidia . . 66 
 
 1. In Amphipoda 68 
 
 2. In Phyllopoda 73 
 
 3. In Copepoda 77 
 
 4. In Isopoda 84 
 
 5. In Leptostraca 98 
 
 Page 
 
 6. In Cumaceae 99 
 
 7. In Schizopoda 99 
 
 8. In Stomatopoda .... 104 
 
 9. In Decapoda 108 
 
 V. Ommatidial Formula . . .115 
 
 VI. Innervation of the Retina . .116 
 
 VII. Theoretic Conclusions . . .118 
 
 VIII. Bibliography 131 
 
 IX. Explanation of Figures . . . 141 
 
 Introduction. 
 
 Some four years ago, at the suggestion of my instructor, Dr. E. L, 
 Mark, I began the investigation of the compound eyes in Crustaceans. 
 In order to familiarize myself with the subject, I determined to study 
 at first in detail the structure of the eyes in a single species, and for 
 this purpose I t ;d my attention to our common lobster, Homarus 
 americanus. My results were published in a paper entitled “ The His- 
 tology and Development of the Eye in the Lobster.” Since the publica- 
 tion of that paper, I have had the opportunity of examining the eyes in 
 a number of other Crustaceans, and my observations and conclusions 
 concerning these eyes are contained in the following pages. 
 
 The material which I have used in the present study was in part sup- 
 plied to me through the kindness of several friends, and in part collected 
 by myself. Of that which I obtained myself, some was gathered in 
 the immediate vicinity of Cambridge, but much of it came either from 
 Wood’s Holl, Mass., or from Newport, It. I. The material which I 
 obtained at Newport was collected at the Newport Marine Laboratory 
 during the summer of 1890, and consisted of specimens of Idotea, 
 Evadne, and Pontella ; that which I got at "Wood’s Holl was collected 
 at the United States Fish Commission Station during a brief period 
 
 1 Contributions from the Zoological Laboratory of the Museum of Comparative 
 Zoology, under the direction of E. L. Mark, No. XXV. 
 
 VOL. xxi. — no. 2. 
 
46 
 
 BULLETIN OF THE 
 
 which I spent there in the summer of 1889, and included much of 
 the material which I used in studying the eyes of Decapods. For the 
 opportunities of collecting, both at Newport and Wood’s Holl, I am 
 indebted to Dr. Alexander Agassiz. I also desire to express my thanks 
 to Prof. M. McDonald, the United States Commissioner of Fish and 
 Fisheries, for many courtesies shown me while at the government 
 station at Wood’s Holl. 
 
 Essentially the same methods as those which I used in investigating 
 the eyes in the lobster were employed in studying the eyes in other 
 Crustaceans. As these methods have been described at some length in 
 my paper on the lobster’s eye (Parker, ’90 a , pp. 3, 4), further mention 
 of them in this connection is unnecessary. 
 
 Before proceeding to an account of the eyes in Crustaceans, a few 
 statements should be made concerning the use of terms. In the fol- 
 lowing anatomical descriptions, I have very generally adhered to the 
 older and more established terms. It must be admitted that some of 
 these, on account of their derivation, are not entirely satisfactory, but 
 because of their general acceptance I have chosen to retain them rather 
 than to attempt to replace them by new ones. 
 
 The term retinula, the use of which varies with different writers, was 
 introduced by Grenacher (’77, p. 17), who employed it to designate the 
 rhabdome and the group of cells by which this structure is surrounded. 
 Subsequently, Patten (’86, p. 544) used the same term as a name for 
 a single cell of the group to which Grenacher gave the name retinula. 
 In my paper on the eyes of the lobster I followed Patten’s usage, but 
 in the present paper I have decided to employ the term as originally 
 defined by Grenacher, and to designate the individual cells in the 
 retinula as retinular cells , — a translation of the term already used for 
 this purpose in many German publications. 
 
 The greater part of the present paper is taken up with descriptions 
 of the eyes in different Crustaceans. The amount of detail thus col- 
 lected is considerable, and might appear at first sight to include many 
 unimportant particulars; but the number of observations recorded is 
 justifiable, I believe, on the ground that the majority of them bear 
 more or less directly upon the solution of the principal question dealt 
 with in the paper. 
 
 The following statements will make clear the character of this ques- 
 tion. It is now well recognized that the retina in compound eyes is 
 composed of a number of similar units or ommatidia, and that each 
 ommatidium consists of a cluster of cells regularly arranged around a 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 47 
 
 central axis. With very few exceptions, the different ommatidia in the 
 retina of any given Crustacean agree with one another in the number 
 and arrangement of their cells ; in other words, in a given retina any 
 ommatidium is the structural duplicate of any other. This uniformity 
 suggests the idea of a structural type, and already a number of such 
 types have been described. Some of these find representatives appar- 
 ently only in the ommatidia of a single species, but more frequently the 
 type characterizes a genus, family, or even a sub-order. Types differ 
 from one another, either in the number of their cells or in the arrange- 
 ment of these cells. Of these differences, the one which involves a 
 variation in the number of cells is the more fundamental. This dif- 
 ference, however, has probably arisen by the gradual modification of 
 an ancestral type, and, granting this, it follows that the ommatidia of 
 one type are genetically connected with those of other types. This 
 leads directly to the statement of the principal question, namely, What 
 are the means by which ommatidial types are modified, and what is the 
 significance of the changes through which these types pass ? 
 
 This question, although easily stated, is not so easily answered ; the 
 facts presented in the following pages cannot be said to settle it, and 
 yet they seem to me to increase materially the possibilities of its 
 solution. 
 
 A partial answer to at least the first portion of the question has al- 
 ready been suggested (Parker, ’90 a , pp. 56-58) ; it can be briefly stated 
 as follows. There is reason for believing that those ommatidia which are 
 composed of a small number of cells more closely resemble the ancestral 
 type than those composed of many cells. Granting this statement, one 
 would naturally expect that the more complex ommatidia had been de- 
 rived from the simpler ones by an increase in the number of their ele- 
 ments. Perhaps the most natural method by which this increase could 
 be accomplished would he by the further division of the cells already 
 forming the ommatidium. Consequently, cell division in this sense 
 seemed to me to afford a sufficient means for the modification of om- 
 matidial types. In the present paper it is in part my purpose to show 
 precisely to what extent cell division can be said to have modified om- 
 matidia, and to determine whether any other factors have been involved 
 in this process. 
 
 The Retina. 
 
 The retina in those Crustaceans in which its development has been 
 studied originates as a thickening in the superficial ectoderm. At least 
 
48 
 
 BULLETIN OF THE 
 
 three types of retinal structure can be distinguished, depending upon 
 the ultimate form which this thickening assumes. 
 
 The first type which will be described is in several particulars 
 the simplest, and probably represents a primitive form from which 
 the other two are derived. This type is characteristic of the eyes 
 in Decapods, Schizopods, Stomatopods, Isopods, the Nebalise, and the 
 Branchiopodidse, and is represented by a simple thickening in the super- 
 ficial ectoderm. 
 
 Branchiopodidce. — In the eye of adult specimens of Branchipus the 
 retina is a lenticular thickening occupying the inner concavity of the 
 distal end of the optic stalk. Near its edges the retina is directly con- 
 tinuous with the adjoining hypodermis. Its proximal face is bounded 
 by a basement membrane which is also continuous with the corre- 
 sponding membrane of the hypodermis, and its distal face is closely 
 applied to the inner surface of the superficial cuticula. Thus the retina 
 in the adult has in every respect the appearance of a simple thickening 
 in the hypodermis. 
 
 The way in which the retina originates in Branchipus confirms the 
 opinion that this organ has the simple structure suggested in the fore- 
 going paragraph. The development of the retina in this genus has been 
 studied by Claus (’86, p. 309), whose account can be summarized as 
 follows. In that part of the head from which the optic stalks eventu- 
 ally arise, the ectoderm becomes considerably thickened ; this thickening 
 is subsequently divided into a superficial and a deep portion ; the latter 
 sinks into the head and becomes a part of the central nervous system ; 
 the former retains its external position and is converted into the retina. 
 In Branchipus, therefore, the retina originates as a simple ectodermic 
 thickening which retains its superficial position throughout the life of 
 the individual. This method of origin, and the position permanently 
 retained by the retina, are the two principal characteristics of the first 
 retinal type. 
 
 Isopoda. — In adult specimens of Idotea irrorata, as sections perpen- 
 dicular to the external surface of the eye. show (Plate Y. Fig. 49), the 
 retina bears the same relation to the hypodermis as it does in Branchi- 
 pus. Similar structural relations occur also in the eyes of Idotea ro- 
 busta and of young specimens of Serolis Schythei. 
 
 The development of the retina in Isopods has been observed by Dohrn 
 and Bullar. As early as 1867, Dohrn (’67, p. 256) described the eye in 
 Asellus as originating in connection with a thickening in the lateral 
 wall of the head, presumably in the ectoderm of that region. The de- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 49 
 
 tails of the development of this organ were not followed on account of 
 the continual increase of pigment. Bullar (79, pp. 513, 514) in a paper 
 on parasitic Isopods described the development of the retina in Cymothoa. 
 His account is substantially as follows. In the course of the develop- 
 ment of the cerebral ganglion, when this structure is separated from the 
 superficial ectoderm, the latter remains on the exterior of the embryo as 
 a layer of considerable thickness. From this superficial layer is devel- 
 oped the retina, i. e. all parts of the eye which in the adult lie between 
 the basement membrane and the corneal cuticula. 
 
 I have studied a few stages in the development of the eyes in Idotea 
 robusta. The retina in this species originates as a simple thickening in 
 the superficial ectoderm, in essentially the same manner as Bullar has 
 observed in Cymothoa. 
 
 The retina in Isopods, both in respect to its method of development 
 and its general structure in the adult, is unquestionably a representative 
 of what I have called the first type of retinal structure. 
 
 Nebalice. — In Nebalia, as the figures given by Claus (’88, Taf. X. 
 Figs. 8 and 17) show, the retina and adjoining hypodermis are directly 
 continuous, and the former presents all the characteristics of a simple 
 thickening in the hypodermis. 
 
 Stomatopoda. — In an adult specimen of Gonodactylus which I ex- 
 amined, the relation between retina and hypodermis was the same as in 
 Nebalia. 
 
 Nothing is known, I believe, of the development of the retina in either 
 the Nebalise or the Stomatopods. The structure of the eyes in the adults ' 
 of both groups, however, shows very conclusively that their retinas belong 
 to the same structural type as those of Branchipus. 
 
 Schizopoda . — In describing the development of Mysis chamelio, Nus- 
 baum (’87, pp. 171-185) states that the retina arises from a thickening 
 in the superficial ectoderm, and adds that its formation, so far as his 
 observations extended, was not complicated by an involution. 
 
 In Mysis stenolepis, a Schizopod whose eyes I have studied, the 
 retina and hypodermis in the adult are directly continuous, as in Bran- 
 chipus. This relation is what would be expected from the method of 
 development described by Nusbaum. 
 
 Decapoda. — Oarriere (’85, p. 169), in his account of the eyes in Asta- 
 cus, showed very clearly that in the adult the retina and hypodermis 
 formed a continuous layer. This relation was subsequently observed by 
 me in Homarus (Parker, ’90 a , p. 5), and I have since seen the same con- 
 dition in Gelasimus, Cardisoma, Cancer, Hippa, Palinurus, Pagurus, 
 VOL. xxi. — no 2. 4 
 
50 
 
 BULLETIN OF THE 
 
 Cambarus, Crangon, and Palaemonetes. There is, therefore, considera- 
 ble ground for the support of Carriere's generalization, that the relation 
 of the retina to the hypodermis as shown in Astacus is characteristic 
 of all Decapods. 
 
 The development of the retina has been more fully studied in Deca- 
 pods, perhaps, than in any other group of Crustaceans. Nevertheless, 
 the accounts given by various writers are by no means in agreement, but 
 differ in several important particulars. In a former paper (Parker, ’90% 
 pp. 31-43), I devoted considerable space to the discussion of these 
 accounts, and I shall therefore not reopen the subject here. Suffice 
 it to say, that since the publication of the paper referred to nothing has 
 transpired to alter my belief that the retina in Decapods originates as 
 a simple thickening in the superficial ectoderm. 
 
 In a recent preliminary communication by Lebedinski (’90) on the 
 development of a marine crab, Eriphya, a brief description of the origin 
 of the eye is given. This description, however, is so very much con- 
 densed that it is not easily understood, and since the author himself 
 confesses that, on account of the complexity of the subject, a descrip- 
 tion without figures must be almost unintelligible, it would be unwise 
 to hazard a presentation of his views. I shall therefore pass over this 
 paper without further comment. 
 
 The evidence advanced in the course of the preceding paragraphs 
 leaves no doubt in my mind that the retinas in the Branchipodidae, the 
 Nebaliae, the Isopods, Stomatopods, Schizopods, and Decapods, belong to 
 the same structural type, and that this type is represented by a thick- 
 ening in the external ectoderm (hypodermis), which retains permanently 
 its superficial position. 
 
 The second retinal type is more complicated than the first, and 
 differs from it in that the retina does not retain its position at the 
 surface of the body, but becomes buried beneath a fold of integument. 
 Our knowledge of this type is largely due to the researches of Grobben 
 (’79). The type is represented in the eyes of the Apusidae, the Estheridae, 
 and the Cladocera. 
 
 Estheridce . — In adult specimens of Limnadia Agassizii the two lat- 
 eral eyes are rather closely approximated, and occupy a position in 
 the ventral anterior portion of the animal’s body (Plate IV. Fig. 33). 
 The relation of the eye to the surface of the body can be seen most 
 satisfactorily in sagittal sections. In such a section (Fig. 35) the eye 
 has the appearance of a stalked structure which projects anteriorly into 
 a cavity, the optic pocket ( brs . oc.) ; this pocket communicates with the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 51 
 
 exterior by means of a small opening (po. brs.), the optic pore. The 
 free surface of the stalked portion of the eye is covered with a delicate 
 cuticula, which, after being reflected from the base of the stalk over the 
 inner surface of the wall of the pocket, becomes continuous at the pore 
 of the pocket with the superficial cuticula. The retina (Fig. 35, r.) 
 occupies the greater portion of the optic stalk. Its distal face is bounded 
 by the delicate cuticula already mentioned, and its proximal face is lim- 
 ited by a basement membrane (mb. ba.). This membrane becomes indis- 
 tinct as the base of the stalk is approached, but the retina itself is 
 apparently continuous in this region with the layer of cells which rests 
 on the cuticular wall of the optic pocket, and which finally unites at the 
 pore of the pocket with the superficial hypodermis. Thus the retina 
 may be said to be continuous with the hypodermis. 
 
 The structure of the eyes in Limnadia Agassizii is such that they 
 can be described as stalked eyes which have been surrounded by a fold 
 of the integument, so as to become enclosed within a space, the optic 
 pocket, which communicates with the exterior only by means of the 
 optic pore. 
 
 An eye of essentially this structure has been described by Grobben 
 (’79, p. 255) in Limnadia Hermanni, Limnetis brachyurus, and Estheria 
 ticinensis, and in the last genus enough of the development of the eye 
 was observed to indicate that the optic pocket was formed by the growth 
 of a fold of integument over the optic stalk. 
 
 Apusidce. — In Apus, according to Grobben (’79, p. 256), the plan of 
 the eye is essentially similar to that in the Estheridae. The eyes pro- 
 ject into an open pocket, the cavity of which permanently communi- 
 cates with the exterior. Judging from the figure given by Claus (’86, 
 Taf. VII. Fig. 11, compare p. 366), the right and left retinas in Apus 
 are not so close to one another as in the Estheridse (compare Plate IV. 
 Fig. 34). 
 
 Cladocera. — The structure and development of the retina in the 
 Cladocera has been carefully studied by Grobben. My own observa- 
 tions on this group have been limited to a single genus, Evadne, and 
 as this genus is not very favorable for the determination of the general 
 relations of the retina I must rely almost entirely upon Grobben’s 
 descriptions. 
 
 In the development of Moina, according to Grobben (79, p. 253), the 
 retinal thickening is covered by a fold of the integument in such a 
 manner that an open optic pocket is produced, as in Limnadia. By the 
 closure of what corresponds to the optic pore, this pocket eventually 
 
52 
 
 BULLETIN OF THE 
 
 loses its connection with the exterior, and becomes reduced to a closed 
 sac on the distal face of the retina. With the closure of the sac, the 
 continuity of the retina with the superficial hypodermis becomes in- 
 terrupted. 
 
 In other Cladocera, especially the genera Sida and Daphnia, Grobben 
 has found evidence to believe that the eyes are of essentially the same 
 structure as in Moina. In a majority of the Cladocera the two com- 
 pound eyes coalesce even more completely than in Limnadia. 
 
 In the development of Moina, as the preceding description indicates, 
 the eye passes through a phase which closely resembles the permanent 
 condition in Limnadia. The eye in the latter may therefore be inter- 
 preted as representing a stage in the phylogeny of the eye in Moina. 
 
 In accordance with the facts presented in the foregoing account, the 
 second retinal type can be described as one in which the retina does not 
 retain its primitive external position, but sinks below the surface of the 
 animal and becomes covered by a fold of the integument.. The optic 
 pocket thus formed may remain permanently open, as in the Apusidse 
 and Estheridse, or may become closed and partially obliterated, as in 
 the Cladocera. The right and left retinas either remain separated, as in 
 the Apusidse, or become closely approximated, as in the Estheridee, or 
 fused, as in the Cladocera. 
 
 The minor modifications which this retinal type presents are not with- 
 out importance. Bearing in mind the general statement that the com- 
 pound eyes in Crustaceans are separate, paired, superficial structures, it 
 is evident that the eyes in the Apusidse, in which the retinas are sepa- 
 rate and the optic pocket permanently open, depart only slightly from 
 the primitive condition. In the Estheridee, in which the two retinas 
 are closely approximated, the eye is farther removed from the original 
 type ; but not so far as in the Cladocera, in which not only the two 
 retinas are fused, but the optic pocket is closed and partially obliterated, 
 thus entirely disconnecting the retina from the hypodermis. The three 
 groups — the Apusidse, the Estheridee, and the Cladocera — may con- 
 sequently be taken to represent a series in the differentiation of the 
 second retinal type. That this series is a natural one, and that it cul- 
 minates in the Cladocera, is shown from the fact that in the develop- 
 ment of Moina, and perhaps many other Cladocera, the eyes pass 
 through stages which reproduce the essential features of the perma- 
 nent condition in the Apusidse and Estheridse. 
 
 In the third retinal type, as in the more differentiated form of 
 the second, the retina is completely separated from the hypodermis. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 53 
 
 The method by which the separation is here accomplished is not by 
 the closure of an involution, as in the second type, but by a process 
 the nature of which will be described in the following pages. The 
 third type is represented by the eyes in Amphipods, and possibly in 
 Copepods. 
 
 Amphipoda. — The peculiar relation which the retina bears to the 
 hypodermis in Amphipods can be easily seen in Gammarus. In this 
 genus, as Carriere (’85, pp. 156-160) has clearly demonstrated, the 
 retina lies immediately below the hypodermis, and is separated from the 
 latter by a well defined structure, the corneo-conal membrane (Fig. 1, 
 mb. crn’con.). This membrane, although visible with perfect clearness, 
 is nevertheless extremely delicate, and has the appearance of a single 
 lamella. I believe, however, that its structure is more complex, and 
 that it is composed of two very intimately united membranes, one of 
 which is produced by the retina, the other by the corneal hypodermis. 
 This belief is based upon the fact that at the edge of the retina the 
 apparently single membrane separates into what may be considered its 
 two constituents. One of these becomes the basement membrane of 
 the general hypodermis, and the other, which I have called the cap- 
 sular membrane, passes over the edge and proximal face of the retina, 
 and is finally reflected over the optic nerve (Fig. 1, mb. n. opt.). In 
 addition to the capsular membrane, the eye in Gammarus possesses 
 still another membrane (Fig. 1, mb. ba.). This is a delicate lamella, 
 which is approximately parallel to the deep face of the eye at a level 
 between the rhabdomes and retinular nuclei (compare Fig. 2), and which 
 consequently divides the space within the capsular membrane into two 
 chambers, a larger distal and a smaller proximal one. At its periphery 
 this intercepting membrane unites with the capsular membrane. 
 
 The corneo-conal and capsular membranes in Gammarus show no evi- 
 dence of being perforated, but together constitute a closed capsule, which 
 separates the retina from all adjoining tissues except the optic nerve. 
 Both membranes are composed apparently of a homogeneous substance, 
 in which I have never been able to distinguish any trace of cells. It 
 is therefore probable that these membranes are cuticular. 
 
 The intercepting membrane, unlike either the capsular or the corneo- 
 conal membrane, is pierced by a great number of holes, through which 
 the proximal ends of the retinular cells project. This membrane, there- 
 fore, has the form of a mesh work. According to Carriere (’85, p. 158) 
 it is composed of numerous connective-tissue cells, but this statement 
 is not confirmed by my own observations. In depigmented sections of 
 
54 
 
 BULLETIN OF THE 
 
 the retina the intercepting membrane had the appearance of a delicate 
 lamella, in which I was unable to find any trace of cells. Not unfre- 
 quently the nuclei of certain accessory pigment cells (Fig. 2, nl. h'drm.) 
 appear to touch the membrane, and even at times to lie with their long 
 axes parallel to it, but in no case could these nuclei be said to be in the 
 membrane. In sections of the retina from which the natural pigment 
 had not been removed, it was often difficult to decide whether a given 
 nucleus was in the membrane or only next to it. Possibly appearances 
 such as these have led Carriere to believe that the membrane was cel- 
 lular. My own opinion is, that the intercepting membrane, like the 
 other two membranes, is a cuticula, and does not contain cells. 
 
 From the foregoing account, it will be seen that in an adult Gammarus 
 the retina lies immediately under an undifferentiated corneal hypoder- 
 mis, and is enclosed, excepting where the optic nerve emerges from it, 
 by a non-perforated cuticular capsule. The space within this capsule 
 is divided by a perforated cuticular membrane into a large distal and a 
 small proximal chamber. 
 
 In Hyperia, judging from the figure given by Carriere (’85, p. 161 ? 
 Fig. 123), the retina has essentially the same structure as in Gammarus. 
 The intercepting membrane is in a position proximal to the rhabdomes 
 and distal to the retinular nuclei. The layer of pigment cells, which 
 Carriere (’85, p. 161, Fig. 124) apparently considers the intercellular 
 membrane itself, in my opinion marks only approximately the position 
 of that membrane. Probably in Hyperia, as in Gammarus, these cells 
 rest on the distal face of the intercepting membrane. 
 
 In Phronima each side of the head is occupied by two eyes, instead of 
 one, contrary to the condition in the more typical Amphipods. Of the 
 two eyes, one is dorsal, the other lateral. This difference in position 
 affords a convenient means of distinguishing them. The lateral eye pre- 
 sents all the essential structural features of the single eye in Gammarus 
 (compare Carriere, ’85, Figs. 125 and 121). The dorsal eye, although 
 differing considerably in shape from the lateral one, is nevertheless con- 
 structed upon the same morphological plan. Its most important pecu- 
 liarity is the shape of its intercepting membrane and the adjoining 
 structures. In the dorsal eye the intercepting membrane, instead of 
 lying in a plane nearly parallel with the external surface of the retina, 
 as in the lateral eye, is cone-shaped. The axis of this cone corresponds 
 to the axis of the eye ; its apex is near the brain, and its base faces the 
 external surface of the eye (compare Claus, ’79, Taf. III. Fig. 20, and 
 Taf. VII. Fig. 58). The ommatidia are arranged approximately parallel 
 
 i 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 55 
 
 to its principal axis ; distally, they terminate in the region of its base ; 
 proximally, they end either at its apex or on its lateral walls near the 
 apex. The rhabdomes lie within the cavity of the cone, i. e. they are 
 distal to the intercepting membrane, as in other Amphipods. The retin- 
 ular nuclei cover the apical portion of the external surface of the cone, 
 i. e. they are proximal to this membrane. These nuclei are covered ex- 
 ternally by a second cone-shaped membrane, which separates them from 
 the surrounding tissue. This membrane occupies the position of the cap- 
 sular membrane of other Amphipods, and is unquestionably homologous 
 with it. 
 
 The fact that both the lateral and dorsal eyes in Phronima are con- 
 structed upon the same plan as the single eye in Gammarus, supports 
 the view that these two eyes have arisen by the division of a primitively 
 single retina into two parts, and the subsequent independent differentia- 
 tion of each part. 
 
 As the preceding account shows, in all Amphipods whose eyes have 
 been studied carefully, the retinas conform to one structural type well 
 exemplified by Gammarus. In this type the retina is characterized by 
 two peculiarities : first, it is not continuous with the hypodermis, but 
 lies immediately below that layer ; and secondly, it possesses what appear 
 to be two basement membranes, the capsular and the intercepting mem- 
 branes. The significance of these peculiarities will be discussed in the 
 following paragraphs. 
 
 The separation of the retina from the hypodermis is characteristic of 
 only the more mature conditions of the eye in Amphipods ; for as Pereyas- 
 lawzewa (’88, p. 202) has shown in Gammarus, and Rossiiskaya (’89, 
 p. 577, and ’90, p. 89) has demonstrated in Orchestia and Sunamphitoe, 
 the retina originates as a thickening in the superficial ectoderm, in the 
 same manner as in the majority of Crustaceans. So far as I am aware, 
 however, no one has observed the detachment of the retina from the 
 hypodermis, a process which must take place before the adult condition 
 is reached. In the figure of the developing eye in Gammarus given by 
 Pereyaslawzewa (’88, Plate VI. Fig. 120), the distal portion of the retinal 
 thickening contains almost nothing but developing cones. In sections of 
 my own from a corresponding region in a young specimen of Gammarus, 
 the distal portion of the retina contains not only developing cones, but 
 also isolated nuclei, which occasionally lie between the cones, but more 
 frequently occur in positions distal to them. These nuclei are as numer- 
 ous in the centre of the distal face of the retina as on its edges, and at 
 this stage can always be easily distinguished from the nuclei of the cone 
 
56 
 
 BULLETIN OF THE 
 
 cells. I believe they represent the nuclei of the corneal hypodermis. 
 The retina proper is probably separated from this hypodermis by delami- 
 nation ; at least, the corneo-conal membrane is formed at a stage slightly 
 older than that last mentioned, and, judging from the appearances at 
 this stage, its formation is not accompanied by any folding of the hypo- 
 dermis or retina, but is the result of a differentiation in place. Unfor- 
 tunately, none of the specimens which I studied showed any steps in the 
 formation of the corneo-conal membrane, and I am therefore uncertain as 
 to the exact method of its growth. 
 
 Of the two membranes in the basal portion of the retina of Gammarus, 
 presumably only one corresponds to the basement membrane of other 
 Crustaceans. The position occupied by the two membranes, as wmll as 
 their structure, serves to indicate which is the true basement membrane. 
 At first sight one might suppose that the capsular membrane, at least 
 in its proximal portion, corresponds to the basement membrane, but this 
 interpretation is not probable, for the reason that the capsular mem- 
 brane is not pierced by the fibres of the optic nerve, a characteristic of 
 the true basement membrane of the eye. I therefore believe that the 
 intercepting membrane, since it is perforated by these fibres, is the homo- 
 logue of the basement membraffe, and that that portion of the capsular 
 membrane which might be regarded as a basement membrane is in 
 reality merely the cuticular sheath of the optic nerve. 
 
 So far as I can foresee, the only objection to be urged against this 
 interpretation of the intercepting membrane is found in its relation to 
 the retinular nuclei. These nuclei in the eyes of almost all other Crusta- 
 ceans lie on the distal side of the basement membrane. Granting that 
 the intercepting membrane is the basement membrane, one must admit 
 that in Amphipods they lie on the 'proximal side of this membrane. 
 This admission might at first sight appear to offer an obstacle to the 
 homology which I have suggested ; but it can be made with consistency, 
 I believe, provided one can show that the position of the retinular nuclei 
 is not necessarily fixed. That such is the case is evident from the fol- 
 lowing facts. In Decapods the retinular nuclei usually occupy a position 
 in their cells distal to the rhabdome. In Porcellio, as Grenacher (’79, 
 Taf. IX. Fig. 96) has shown, they have a more proximal position, lying 
 in the same transverse plane as the rhabdome itself. In Serolis they 
 are midway between the rhabdome and the basement membrane. These 
 conditions show 7 , I believe, that the retinular nuclei may occupy very 
 different positions in their cells, and that the step from the condition 
 shown in Decapods to that shown in Serolis is not greater than that 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 57 
 
 from Serolis to tlie Amphipods. It seems to me, therefore, that the 
 objection suggested at the beginning of this paragraph is almost without 
 weight. This conclusion, moreover, is supported by the fact that in 
 Idotea (Plate V. Fig. 49) the retinular nuclei lie proximal to the base- 
 ment membrane, whereas in the majority of other Isopods they are 
 distal to that membrane. 
 
 From the preceding discussion, I conclude that the retina in Amphi- 
 pods originates as a simple thickening in the superficial ectoderm, and 
 that this thickening subsequently becomes separated, probably by a pro- 
 cess of delamination, into a deeper portion, the retina proper, and a 
 more superficial portion, the corneal hypodermis. The latter alone re- 
 tains its original connection with the adjacent hypodermis. Of the two 
 membranes present in the basal portion of the eye in Amphipods, that 
 which I have called the intercepting membrane is homologous with the 
 basement membrane of the retina in other Crustaceans, and that which 
 has been designated as the capsular membrane is in large part the 
 cuticular sheath of the optic nerve. 
 
 Copepoda . — The retinas in the Branchiura and Eucopepoda, the two 
 divisions of the Copepods, present such different structural conditions 
 that for purposes of description it is better to consider them separately. 
 
 Branchiura . — In adult specimens of Argulus, the retina is completely 
 separated from all surrounding tissue, excepting the optic nerve, by an 
 intervening blood space (Plate II. Fig. 11, cod.'). This peculiar condi- 
 tion was first clearly described by Ley dig (’50, p. 331), although as early 
 as 1806 Jurine (’06, p. 447) remarked that the eye in this genus was 
 contained in a transparent membranous sac, which apparently contained 
 a fluid, and Muller (*31, p. 97) some twenty-five years later described the 
 retina as separated from the “cornea” by an intervening space filled 
 with fluid. It remained, however, for Leydig to determine the extent 
 of this space, and to demonstrate that the fluid which it contained was 
 blood. The more essential features of Leydig’s description have since 
 been confirmed by Claus (’75, pp. 254-256). 
 
 The development of the eye in Argulus has not been studied with 
 sufficient fulness to allow one to determine the relation of its retina to 
 the hypodermis. But from the strong resemblance which the eye in the 
 adult bears to that in Amphipods, it is probable that the course of 
 development in the two cases is not unlike. Probably the retina in 
 Argulus originates as a thickening in the superficial ectoderm, and subse- 
 quently not only suffers delamination, as in the Amphipods, but becomes 
 actually withdrawn from the superficial layer (corneal hypodermis). 
 
58 
 
 BULLETIN OF THE 
 
 If this course of development really takes place, the various structures 
 in the eye of an adult Argulus can be easily homologized with those 
 in Amphipods. Thus the corneal hypodermic and corneal cuticula of 
 Amphipods would probably be represented by the hypodermis and cu- 
 ticula dorsal to the eye in Argulus (Fig. 11). The basement mem- 
 brane of this hypodermis would correspond to the corneal component 
 of the corneo-conal membrane of Amphipods, and the conal constituent 
 would be represented by what is called the preconal membrane in Argu- 
 lus (Fig. 11, mb. pr’con.). Proximally, the preconal membrane becomes 
 continuous with the sheath of the optic nerve (Fig. 11, mb. n. opt.), 
 the equivalent of the capsular membrane of Amphipods. The basement 
 membrane of the retina in Argulus, as in Amphipods, is the membrane 
 pierced by the fibres of the optic nerve (Fig. 11, mb. ha.). 
 
 Grobben (’79, p. 258) has suggested that possibly the eye in Argulus 
 is of the same type of structure as in Phyllopods, but I do not share 
 in this opinion for the following reasons. In Estheria, the delicate 
 cuticula which covers the optic stalk is morphologically a portion of 
 the outer surface of the body, and, as I hope to show subsequently, is 
 subtended by a true corneal hypodermis. There is no corneal hypo- 
 dermis beneath the preconal membrane of Argulus. Moreover, there 
 is nothing in the eye of Argulus to correspond to the optic pocket of 
 the Estheridse, or to the optic sac of the Cladocera, except the circum- 
 retinal blood space, and it seems to me very improbable that this space 
 was once a cavity in communication with the exterior, and afterwards 
 became converted into a blood space. I therefore believe that the 
 plan of the eye in Argulus is not similar to that in the Phyllopods, 
 but rather that it represents a modification of the type presented by 
 the Amphipods. The satisfactory determination of this question can 
 be settled, however, only by embryological evidence. 
 
 Eucopepoda . — In adult specimens of those true Copepods which 
 possess rudiments of the lateral eyes, — the Pontellidae and Corycaeidae, 
 — the retina is apparently separated from the hypodermis. In the 
 Corycaeidae it usually lies at some considerable distance from the hypo- 
 dermis, and in Pontella the two structures, although near one another, 
 are nevertheless not continuous. 
 
 The development of the lateral eyes in the Corycaeidae and Pontel- 
 lidae has not been studied, and consequently it cannot be stated with 
 certainty whether the retinas in these Crustaceans originate from the 
 hypodermis or not. In the metanauplius larva of Cetochilus, a Copepod 
 which as an adult has no lateral eyes, Grobben (’80, p. 262) has de- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 59 
 
 scribed a pair of thickenings, which extend from the superficial ectoderm 
 of the antero-lateral part of the head to the brain. These thickenings 
 are present only in the early stages of development, and represent the 
 unsevered connection between the brain and the superficial ectoderm. 
 They closely resemble the developing lateral eyes of Branchipus, and 
 Grobben has therefore very justly considered them rudiments of the 
 lateral eyes. If the rudiments of the lateral eyes in Cetochilus de- 
 velop from the superficial ectoderm, it is probable that the lateral eyes 
 in other Copepods have a similar origin. 
 
 To which of the three retinal types already described the eyes in 
 Copepods belong is not easily decided. The absence of any indication 
 of an optic pocket, either in the development of what Grobben con- 
 siders the rudiments of the lateral eyes in Cetochilus, or in the fully 
 formed eyes in other genera, seems to me to preclude the possibility of 
 these eyes belonging to what I have described as the second type. 
 The separation of the retina from the hypodermis prevents them from 
 being classed with the first type, and, especially in the case of the 
 Branchiura, brings them into close relation with the third type. It is 
 my opinion, that, if the lateral eyes in Copepods are not representatives 
 of a fourth type, essentially different from the three already described, 
 they must be considered members of the third retinal type. 
 
 Certain species of Cumacese, Ostracods, and Cirripeds possess optic 
 organs which probably represent the compound eyes of other Crusta- 
 ceans ; but so far as I am aware, the relation of these structures to the 
 hypodermis is unknown. It is therefore impossible to state whether 
 those eyes represent other retinal types, or belong to one of the three 
 already described. 
 
 According to the preceding account, three retinal types can be dis- 
 tinguished in the compound eyes of Crustaceans. In the first of these 
 the retina is a simple thickening in the superficial ectoderm (hypo- 
 dermis). This type is characteristic of the eyes in Isopods, the Bran- 
 chiopodidae, the Nebalise, Stomatopods, Schizopods, and Decapods. In 
 the Isopods, the eyes are sessile ; in the other groups of the first type, 
 they are borne on the distal ends of movable optic stalks. 
 
 In the second type, although the retina, as in the first type, originates 
 as a thickening in the superficial ectoderm, it ultimately becomes en- 
 closed within an optic pocket. This may remain permanently open, as 
 in the Apusidse and Estheridae, or it may become closed, as in the 
 Cladocera. In the Apusidae, so far as I am aware, the eyes a,re not 
 
60 
 
 BULLETIN OF THE 
 
 capable of motion, and in the Estheridse they are, if at all, only slightly 
 movable. In the Cladocera, where the second type probably reaches its 
 greatest differentiation, the retina is remarkable for the freedom of its 
 motion. 
 
 In the third type the retina originates from thickened hypodermis, 
 which subsequently separates into two layers, the corneal hypodermis 
 and the retina proper (a layer of cones and retinulae). This separation 
 is accomplished either by the formation of a corneo-conal membrane, as 
 in Amphipods, or by what I believe to be an actual withdrawal of the 
 retina proper from contact with the hypodermis, as in Copepods. Only 
 in the representatives of the extreme modification of this type, the Cope- 
 pods, are the eyes movable. 
 
 The course of development taken by each of the three types very 
 clearly indicates their mutual relations. Evidently the first type is a 
 primitive one, and since the first steps in the development of the second 
 and third reproduce the permanent condition of the first, these two may 
 therefore be considered derivatives from the first. It is interesting to 
 observe that in the simpler condition of each type the retina is fixed, 
 whereas in the more differentiated form it has become movable. The 
 sinking of the retina into the deeper parts of the body, as represented in 
 the second and third types, may have been induced by the protection 
 thus obtained for the eye. After the three types were differentiated, 
 each one seems to have been modified in a special way to give rise to a 
 movable retina. 
 
 Arrangement of the Ommatidia. 
 
 The ommatidia in the retinas of some Crustaceans are so few’ in num- 
 ber that they can scarcely be said to be grouped according to any system. 
 Where they are numerous, however, they are arranged upon one or the 
 other of two plans. These may be designated the hexagonal and tetrago- 
 nal plans of arrangement. In the hexagonal plan the imaginary outline 
 of the transverse section of an ommatidium is a hexagon, and each 
 ommatidium, excepting those on the edge of the retina, is surrounded by 
 six others. In the tetragonal arrangement the ideal transverse section 
 of an ommatidium is a square. Each of the four sides of this square 
 is occupied by one of the four faces of an adjoining ommatidium. 
 
 The arrangement of the ommatidia can usually be determined by 
 a careful inspection of the external surface of the eye ; this determina- 
 tion is considerably facilitated by the presence of a facetted cuticula. 
 Sometimes the form of a single facet is sufficient to indicate the plan of 
 
MUSEUM OF COMPAEATIVE ZOOLOGY. 
 
 61 
 
 arrangement. Thus, hexagonal facets have never been observed except 
 in connection with the hexagonal plan of arrangement. Circular facets 
 are likewise known to occur only with this method of grouping. Square 
 facets, on the other hand, may accompany either the hexagonal or te- 
 tragonal arrangement of deeper parts. 
 
 The hexagonal arrangement is apparently characteristic of the om- 
 matidia in all Crustaceans , 1 except the Decapods. In the Decapods, as 
 will be shown presently, the ommatidia are arranged either upon the hex- 
 agonal or the tetragonal plan. Before proceeding, however, to a descrip- 
 tion of the arrangement of the ommatidia in Decapods, it would be well 
 perhaps to call attention to the rather peculiar grouping of these struc- 
 tures in Gonodactylus, a Stomatopod. 
 
 For a clear understanding of the arrangement of the ommatidia in 
 this Crustacean, it is necessary to have some previous knowledge of the 
 shape of its optic stalk. In Gonodactylus the stalks are elongated cyl- 
 inders, the distal ends of which are rounded. In alcoholic specimens 
 the stalks in an undisturbed position rest with their longitudinal axes 
 approximately parallel with the chief axis of the animal, and with their 
 distal ends directed forward. The retina occupies the free end of the 
 stalk. Dorsally it extends over the distal half, ventrally over only the 
 distal third of the stalk. 
 
 The ommatidia in Gonodactylus are of two kinds, large and small, 
 which are always easily distinguishable from each other, although they 
 differ in no essential respect except size. The large ommatidia are defi- 
 nitely arranged in six rows, which extend as well defined bands from 
 the dorsal posterior edge of the retina anteriorly over its rounded distal 
 end, and posteriorly over its ventral surface to its ventral posterior edge. 
 This band thus occupies both dorsally and ventrally the median portion 
 
 1 Judging from the figures as well as the statements made by the authors 
 quoted, the hexagonal arrangement is characteristic of the ommatidia in the fol- 
 lowing Crustaceans (exclusive of the Decapods) : Branchlpus (Burmeister, ’35, 
 p. 531, Spangenberg, ’75, p. 30), Nebalia (Claus, '89, Taf. X. Fig. 10), Gam morns 
 (Sars, 67, p. 62), Orchestia (Frey und Leuckart, ’47, p. 204), Plironima (Claus, ’79, 
 Taf. VI. Fig. 48), Cymothoa (Muller, '29, Tab III. Figs. 5, 6, Bullar, ’79, p 514), 
 Lygidium (Lereboullet, ’43, p. 107, Planche 4, Fig. 2 b ), Serolis (Owen, '43, p. 174), 
 Arcturus (Beddard, 90, Plate XXXI. Fig. 4), Anceus (Hesse, ’58, pp. 100 and 103, 
 Dohrn, 70, Taf. VIII. Figs. 33, 34), Squill a (Miln e-Ed wards, ’34, p. 117, Will, '40, 
 p. 7, Frey und Leuckart, ? 47, p. 204, Leydig, '55, p. 411), and Mysis (Sars, ’67, 
 Planche III. Fig. 7, Grenadier, 79, Taf. X. Fig. 112). I have observed the hexag- 
 onal arrangement in the following genera: Apus. Branchipus, Estheria, Evadne, 
 Argulus, Gammarus , Caprel/a, Talorchestia, Idotea, Serolis , Porcellio, Sphceroma , 
 Mysis , and Gonodactylus. 
 
62 
 
 BULLETIN OF THE 
 
 of the retina, and separates the remaining retinal surface into two parts, 
 one on either side of the stalk. In alcoholic specimens this median band 
 is readily visible with the aid of a hand lens, and a little closer scrutiny 
 shows that it is composed of six lines. These lines, of course, correspond 
 to the six rows of ommatidia previously mentioned. The smaller om- 
 matidia, on either side of the median band, are also arranged in lines 
 parallel to those in the band ; but, on account of their smaller size, the 
 lines formed by them are not visible with an ordinary lens. 
 
 The smaller ommatidia in Goniodactylus are arranged upon the tvpi- 
 cal hexagonal plan (see the left half of Fig. 93, Plate VIII.). The 
 larger ones have a somewhat similar grouping, although the fact that 
 they are in six longitudinal rows rather obscures their hexagonal ar- 
 rangement. (See the right half of Figure 93, in which three rows, and 
 a part of a fourth, of large ommatidia are shown.) The hexagonal 
 arrangement is not disturbed, as might be expected, on the line which 
 separates the larger from the smaller ommatidia, but both kinds form 
 parts in a common system. That this is true can be seen from Figure 
 93, where it will be observed that the centres of any two small ommatidia 
 lying in the same vertical line are as far apart as the centres of the cor- 
 responding larger ommatidia. Moreover, as I have demonstrated by 
 actually counting the ommatidia of long parallel series, a vertical band 
 which contains twenty-five large ommatidia has the same length as one 
 composed of a corresponding number of small ones. The apparent differ- 
 ence in numbers at first sight presented by lines of the two kinds of 
 ommatidia is principally due to the fact that the larger ommatidia are 
 arranged in distinct rows, whereas the smaller ommatidia are so grouped 
 that the individuals in one row are slightly interpolated between those 
 of the two adjoining rows (compare Fig. 93). 
 
 In Decapods the ommatidia are arranged either upon the hexagonal 
 or tetragonal plan. In the Brachyura, 1 as well as in three families of the 
 Macrura, the Hippidse, Paguridse, and Thalassinidse, 2 the arrangement 
 
 1 The presence of hexagonal facets has been recorded in the following genera 
 of Brachyura: Portunns (Will, '40, p. 7) ; Ilia (Will, ’40, p 7, Leydig, ’55, p. 411) ; 
 Cancer; Maja; Carpilius (Frey und Leuckart, '47, p. 204): Herbstia , Dorippe, and 
 Lambrtis (Leydig, ’55, pp. 407, 410, and 411, respectively). This form of facet is 
 present only when the ommatidia are hexagonally arranged. Leydig (’55, p. 411) 
 states that the outline of each facet in Dromia Rumphii is square, but, as his 
 description clearly indicates, the facets are arranged upon the hexagonal plan. 
 As my own observations show, the ommatidia in Cardisoma Gvanhumi, Latr., 
 Cancer irroratus, Say, and Gelasimus pugdator , Latr., are hexagonally grouped. 
 
 2 The outline of the corneal facets is stated to be hexagonal in the following 
 genera: Pagurus (Swammerdam, ’52, p. 88, Cavolini, ’92, p. 130, Milne-Edwards, 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 63 
 
 of the ommatidia is invariably hexagonal. In the remaining macrurous 
 Decapods 1 the ommatidia are grouped on the tetragonal plan. This last 
 statement, however, is not without exceptions, for in Typton, and at 
 times also in Galathea, 2 the hexagonal arrangement appears to prevail. 
 An explanation of these exceptions will be offered in a subsequent 
 paragraph . 
 
 Before attempting this explanation, however, it will be well to gain a 
 precise idea of the relation of the hexagonal and tetragonal methods of 
 arrangement. At first sight, it might appear that these two methods 
 had no definite relations, and were simply characteristic of different 
 Decapods. Such, however, is not the case ; for, as the development of 
 the lobster show’s, the ommatidia in a single animal can be arranged at 
 first according to one plan, and afterward according to the other. In 
 the lobster the hexagonal arrangement characterizes the earlier stages 
 of development, and is replaced only subsequently by the tetragonal 
 grouping. A similar change also occurs in the spiny lobster. Thus, 
 in Phyllosoma, the larva of either Palinurus or Scyllarus, the hexagonal 
 facets observed by Milne-Edwards (’34, p. 115) afford unquestionable 
 evidence of the hexagonal arrangement at this stage. In the adult con- 
 dition, however, both of Palinurus and of Scyllarus, according to my own 
 observations, the ommatidia are tetragonally grouped. In the common 
 lobster and the spiny lobster, then, the hexagonal arrangement of the 
 early stages is replaced by the tetragonal one in the adult. These ob- 
 
 *34, p. 117, Will, ’40, p. 7, Frey und Leuckart, *47, p. 204, Chatin, 78, p. 8); 
 Calltancissa; and Gebbia (Milne-Edwards, ’34, p. 117). In Pagurus longicarpus, 
 Say, and Hippa talpoida, Say, I have observed a hexagonal arrangement of the 
 ommatidia. 
 
 1 Judging from the figures given by various authors, the ommatidia of the fol- 
 lowing genera are characterized by the tetragonal arrangement: Galathea (Will, 
 ’40, Fig. III. c.) ; Astacus (Muller, ’26, Tab. VII. Fig. 13, Leydig, ’57, p. 252, 
 Fig. 134, Reichenbaeh, ’86, Taf. XIV. Fig. 226, Huxley, ’57, p. 353) ; Homarus 
 (Newton, 73, Plate XVI. Fig. 3, Parker, '90 a , p. 8) ; Palcemon (Grenacher, 79, 
 Taf. XI. Fig. 118 A, Patten, ’86, Plate 31, Fig. 115); Penczus (Patten, ’86, Plate 
 31, Fig. 75). As my present observations have shown, the tetragonal arrangement 
 is characteristic of the ommatidia in Palinurus Argus , Gray, Cambarus Bartonii , 
 and Palcemonetes vulgaris , Say. 
 
 2 According to Chatin (78, p 13) the outline of the facet in Typton is hexagonal. 
 Presumably the arrangement of the ommatidia in this genus is upon the hexagonal 
 plan. In Galathea , according to the figures given by Patten (’86, Plate 31, Fig. 
 116), the ommatidia are hexagonally arranged, although it must be borne in mind 
 that Will's ('40, Fig. III. c.) figure of the facets in Galathea strigosa affords unmis- 
 takable evidence of a tetragonal arrangement. 
 
64 
 
 BULLETIN OF THE 
 
 servations appear to me to afford considerable evidence in favor of the 
 view that the hexagonal arrangement is phylogenetically more primitive 
 than the tetragonal. 
 
 Granting this conclusion, a number of otherwise exceptional observa- 
 tions can be explained. Thus, as long ago as 1840, Will (’40, p. 7) 
 called attention to the fact that in Astacus, where the ommatidia are 
 normally arranged upon the tetragonal plan, facets near the edge of the 
 retina are often irregularly hexagonal. The edge of the retina is well 
 known to be the last part produced, and therefore it is probably the 
 part least differentiated. Admitting the hexagonal arrangement to be 
 a primitive one, it is only natural to expect that, if it persists at all, 
 it will persist in the less modified portion of the retina. Hexagonal 
 facets also occur on the periphery of the retina in Homarus, and are to 
 be explained, I believe, in the same way. 
 
 On the assumption that the hexagonal plan is primitive, the occur- 
 rence of a few genera with ommatidia hexagonally arranged, in a group 
 in which the tetragonal arrangement is the rule, can also be explained. 
 In Typton, for instance, the hexagonal plan obtains, although in almost all 
 Crustaceans closely related to it the tetragonal system prevails. This 
 condition may be explained, however, by the fact that the eyes in Typton 
 show evident signs of degeneracy, due in all probability to the parasitic 
 habits of the Crustacean. If the hexagonal arrangement represents an 
 early ontogenetic phase in the development of Decapods related to Typ- 
 ton, it would be natural to expect that in Typton itself, where the normal 
 development of the eyes is interrupted by parasitism, this arrangement 
 would persist permanently. 
 
 In Galathea, as I have already mentioned in a note on page '63, the 
 ommatidia according to Will are arranged tetragonally ; according to 
 Patten, hexagonally. At first sight these observations might appear 
 to be irreconcilable, but such is not necessarily the case. So far as I 
 have been able to ascertain, Patten does not mention the name of the 
 species which he studied. Possibly he may have examined some other 
 than G. strigosa, the one from which Will’s figures were drawn. In 
 such an event, a difference in the arrangement of the ommatidia may 
 have been characteristic of the two species, although, if both possessed 
 well developed eyes, this difference would be somewhat anomalous. If 
 this is not the true explanation, it is still possible that the specimens 
 studied by Patten were somewhat immature, in which case the hexagonal 
 arrangement might very naturally be present. From what has been said, 
 I think it must be evident that the apparent contradiction in Will’s and 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 65 
 
 Patten’s statements is not so serious as might at first be supposed, and 
 that, admitting the relations already mentioned between the two plans 
 of arrangement, the observations of these two writers can be explained 
 without supposing either of them to be wrong. 
 
 The probable method of rearrangement by which the hexagonal plan 
 is converted into the tetragonal has been suggested in a previous paper 
 (Parker, ’90 a , p. 50). It involves two changes: the conversion of the 
 hexagonal outline of the ommatidium, as seen in the corneal facet, into 
 a square one, and the slipping of the rows of omnmtidia one on the other, 
 so that the lines which bound the four sides of each facet finally form 
 parts of two series of lines which cross each other at right angles. 
 
 A condition somewhat intermediate between the hexagonal and tetrag- 
 onal arrangement is shown in the retina of Crangon (Plate X. Fig. 123). 
 In this genus the outlines of the ommatidia as seen in the facets are 
 square, although their arrangement suggests the hexagonal type. The 
 permanent grouping of the ommatidia in Crangon represents a stage 
 slightly in advauce of the condition seen in some young lobsters (com- 
 pare Parker, ’90 a , Plate IV. Fig. 55), and the particular features in 
 which this advance is shown are two. First, the distal retinular nuclei 
 in Crangon (Fig. 123) are grouped in pairs, more as they are in adult 
 lobsters, and not in circles of six, as in young ones (compare Parker, 
 ’90 a , Figs. 5 and 55). Secondly, the arrangement of the ommatidial 
 centres in reference to the hexagonal plan is more symmetrical in the 
 young lobster than in Crangon, where the rows of ommatidia have ap- 
 parently slipped somewhat upon one another so as to resemble more 
 nearly the condition in the adult lobster. 
 
 I have been unable to determine with certainty what occasions the 
 change from the hexagonal to the tetragonal arrangement. Apparently 
 it accompanies an excessive growth on the part of the individual omma- 
 tidia. In the lobster, for instance, the ommatidia rearrange themselves 
 between the times when the young animal is one inch and eight inches 
 long. During this period the ommatidia increase about ten times in 
 length and about five times in breadth. The increase is especially 
 noticeable at their distal ends, and particularly in the cone cells. In 
 young lobsters of one inch in length (Parker, ’90 a , Plate IV. Fig. 55), 
 the space between the cones of adjoining ommatidia. is considerable; in 
 adults, it is proportionally very much less (compare Parker, ’90 a , Plate I. 
 Fig. 5), and the cones are crowded against one another. Under these 
 conditions, the hexagonal arrangement apparently gives way to the te- 
 tragonal. So far as I am aware, the tetragonal arrangement occurs only 
 
 VOL xxi. — no. 2. 5 
 
66 
 
 BULLETIN OF THE 
 
 in connection with this crowding of the cones, a condition found for the 
 most part only in macrurous Decapods. 
 
 In accounting for the rearrangement of the ommatidia, the eyes in 
 the Stomatopod Gonodactylus afford some important evidence. As I 
 have previously mentioned, the ommatidia in this genus are of two sizes. 
 The larger ones have several of the peculiarities characterizing the tetrag- 
 onal arrangement : their facets are generally square ; they are arranged 
 in single lines, and these lines, so far as the relations of the individual 
 ommatidia are concerned, show evidences of having slipped upon one 
 another. The smaller ommatidia have hexagonal facets, and are clearly 
 arranged according to the hexagonal plan. The larger ommatidia are 
 rather closely packed ; the smaller ones are arranged with more open 
 space between them (compare Plate VIII. Fig. 93). In this genus, 
 then, as in the lobster, the tetragonal arrangement occurs in connec- 
 tion with the crowding of the ommatidia. 
 
 How an increase in size, accompanied by a crowding of the retinal 
 elements, can induce the change in arrangement which seems to follow 
 it, I am at a loss to explain. Nevertheless, the two phenomena ap- 
 pear to be in some way connected. 
 
 From the preceding discussion concerning the arrangement of the 
 ommatidia, the following conclusions can be drawn. The ommatidia, 
 when numerous enough, present one of two plans of arrangement, 
 the hexagonal or the tetragonal. The hexagonal plan is phylogeneti- 
 cally the older, and is characteristic of the eyes of all Crustaceans 
 except some families of the macrurous Decapods, especially the Gala- 
 theidse, Palinuridse, Astacidse, and Carididse. In these the hexagonal 
 arrangement is usually replaced by the tetragonal ; but in the adults of 
 some species, especially those in which the eyes are partially rudi- 
 mentary, the hexagonal arrangement persists. The change from the 
 hexagonal to the tetragonal arrangement is connected apparently with 
 an increase in size, and consequent crowding, of the ommatidia. 
 
 The Structure of the Ommatidia. 
 
 Each ommatidium, as I have previously mentioned, consists of a 
 cluster of cells more or less regularly arranged about a central axis. 
 The greatest number of kinds of cells which an ommatidium is known 
 to contain is five. These are the cells of the corneal hypodermis, the 
 cone cells, the proximal and distal retinular cells, and the accessory 
 cells. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 67 
 
 The cells of the corneal hypodermis are usually arranged in a very 
 thin layer, and constitute the most superficial tissue in the retina. 
 They either present no definite arrangement, as in Amphipods, or they 
 are regularly grouped in pairs, one pair for each ommatidium, as in 
 the majority of Crustaceans. On their external faces they produce the 
 corneal cuticula. This is unfacetted in those Crustaceans in which the 
 corneal cells are not regularly arranged and facetted when they are 
 grouped in pairs. 
 
 The cone cells in each ommatidium are united to form the cone, a 
 transparent body which extends from the corneal hypodermis proximally 
 through the ommatidium at least as far as the rhabdome. The cone 
 occupies the axis of the distal portion of the ommatidium. 
 
 The proximal retinular cells are usually limited to the proximal por- 
 tion of the ommatidium. They are definitely arranged around the 
 axial structure of that region, the rhabdome, and together with it form 
 a single body, the retinula. The optic nerve fibres terminate in the 
 proximal retinular cells. 
 
 The distal retinular cells are present in only the more differentiated 
 ommatidia. They are two in number, and invest the sides of the cone 
 distal to the plane at which this structure emerges from the retinula. 
 When distal cells are present, the remaining cells of the retinula will be 
 distinguished as proximal cells ; when the distal cells are wanting, the 
 other cells will be called simply retinular cells. 
 
 The accessory cells fill the space between the elements of an omma- 
 tidium, or between separate ommatidia. Their number is apparently 
 inconstant, and they present a variety of forms. They may or may 
 not contain pigment. Depending upon their source, two kinds can be 
 distinguished, ectodermic and mesodermic. 
 
 In describing the ommatidia, I shall consider them according to the 
 groups of Crustaceans in which they occur. Under each group the 
 elements comprising the ommatidium will be described in the order 
 in which they have just been mentioned. 
 
 My object in the following account is to determine, as far as possible, 
 what the different kinds of ommatidial types are, and to define these 
 types by a brief statement of the number and kinds of cells which char- 
 acterize them. 
 
 Compound eves are known to occur in some Ostracods, and in the 
 larvae of some Cirripeds, but their histological structure, I believe, has 
 never been studied. I am therefore compelled to dismiss these two 
 groups without further comment, and proceed with the description of 
 
68 
 
 BULLETIN OF THE 
 
 the ommatidia in other Crustaceans. The order in which the groups 
 will be considered is one which is intended to emphasize their relations 
 only in so far as the structure of their ommatidia is concerned. Natu- 
 rally, this order will vary somewhat from the one usually given in sys- 
 tematic treatises. I shall begin with the Amphipods. 
 
 Amphipoda. 
 
 Within recent years the more important types of eyes in the Amphi- 
 pods have been studied with such care that the structure of their om- 
 matidia is perhaps better known than that of any other large group of 
 Crustaceans. My own observations do little more than confirm the 
 accounts already published. 
 
 The species of Amphipods whose eyes I have examined are Gammarus 
 ornatus, M. Edw., Talorchestia longicornis, Say, and an undetermined 
 species of Caprella. Of these the specimens of Gammarus and Caprella 
 were collected at Nahant, Mass., where I also obtained several sets of 
 eggs representing stages in the development of the former. Examples 
 of Talorchestia were kindly supplied me from the collections in the 
 Museum. 
 
 The corneal hypodermis in Amphipods was first satisfactorily described 
 by Claus (79, p. 131) in his account of the eyes in Phronima. It is 
 represented in this genus by a layer of undifferentiated cells lying be- 
 tween the corneal cuticula and the membrane which limits the distal 
 ends of the cone cells. A corneal hypodermis similar to that in Phro- 
 nima has likewise been described by Mayer (’82, p. 122) in Caprella and 
 Protella, by Carri^re (’85, p. 156) in Gammarus, by Claus (’87, p. 15) in 
 the Platyscelidae, by Della Valle (’88, p. 94) in the Ampeliscidae, and 
 by Watase (’90, p. 295) in Talorchestia. I have also identified this struc- 
 ture in Gammarus, Caprella, and Talorchestia. 
 
 In Gammarus, as Carriere (’85, p. 156, Fig. 121) has clearly shown, 
 the corneal hypodermis at the edges of the retina is directly continuous 
 with the general hypodermis. According to my own observations this 
 condition is not only met with in Gammarus, but also in Caprella and 
 Talorchestia. 
 
 In Phronima, according to Claus’s figures (’79, Taf. VI. Figs. 48 and 
 49, Ma Z .), the arrangement of the cells in the corneal hypodermis 
 bears no definite relation to the subjacent cones ; the distal end of each 
 cone presents an area which is covered by about a dozen hypodermal 
 cells. In Gammarus I have observed (Plate I. Figs. 2 and 3) an essen- 
 tially similar distribution of the hypodermal cells ; as in Fhronima, the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 69 
 
 number of cells which cover the area of each cone is about twelve. A 
 corneal hypodermis of this same character also occurs in Talorchestia, 
 although in this instance the number of cells over a cone is only about 
 nine. « 
 
 According to Watase (’90, p. 295), in the species of Talorchestia 
 which he studied there were only two cells in the corneal hypodermis 
 opposite each cone, or, as he expresses it, under each facet. "W hen com- 
 pared with the results recorded in the preceding paragraph, this observa- 
 tion appears somewhat striking, and the more so since two, the number of 
 cells recorded, is the usual number found under each facet in other Crus- 
 taceans. If Watase’s observation be correct, the relation which would 
 thus be established between this Amphipod and other Crustaceans would 
 be an interesting one. The desirability of confirming Watase’s observation 
 must, therefore, be evident ; but unfortunately he has not given the name 
 of the species of Talorchestia which he. studied, and I have therefore 
 not been able to verify his statement. In the only species of this ge- 
 nus which I have examined, viz. T. longicornis, the arrangement of the 
 cells in the corneal hypodermis is very different from that described 
 by Watase. 
 
 The conclusions which I draw from the preceding account are, that 
 in the eyes of Amphipods a corneal hypodermis is present, and the cells 
 composing it are usually not arranged with regularity. 
 
 The peculiar bodies observed by Schmidt (’78, p. 5) in the membrane 
 between the corneal hypodermis and the retina proper in Phronima, and 
 considered by Claus (’79, Taf. YT. Figs. 48, 49, B. nu .) as nuclei, are 
 apparently not represented in other Amphipods. Their significance is 
 still a matter of doubt. 
 
 The corneal cuticula in Amphipods has been described by almost all 
 observers as unfacetted. 1 According to Della Valle (’88, p. 94), how- 
 ever, in some of the Ampeliscidae this cuticula is facetted, and Watase 
 (’90, p. 295) has also observed facets in Talorchestia. But with these 
 two exceptions the corneal cuticula of Amphipods has been described 
 
 1 An unfacetted corneal cuticula has been recorded in the following genera of 
 Amphipods : Amphithoe (Milne-Edwards, ’34, p. 116) ; Caprella (Frey und Leuck- 
 art, ’47 a , p. 103) ; Cyamns (Muller, ’29, p. 58, Frey und Leuckart, ’47, p. 205) ; Gam- 
 marus (Muller, ’29, p. 57, Frey und Leuckart, ’47, p. 205, Pagenstecher, ’61, p. 31, 
 Sars, ’67, p. 61, Leydig, ’78, p. 235, Grenacher, ’79, p. 109) ; Hyperia (Gegenbaur, 
 ’58, p. 82, Grenacher, ’79, p. Ill, Carriere, ’85, p. 160) ; Phronima (Pagenstecher, 
 *61, p. 31, Schmidt, ’78, p 5, Claus, ’79, p. 131) ; Talitrus (Grenacher, ’79, p. 109) ; 
 and the Platyscelidce (Claus, ’87, p. 15). I have observed an unfacetted corneal 
 cuticula in Gammarus, Caprella , and Talorchestia longicornis. 
 
70 
 
 BULLETIN OF THE 
 
 as smooth. The absence of facets from Amphjpods is naturally ac- 
 counted for by the absence of a definite arrangement among the cells 
 of the corneal hypodermic. 
 
 In the genus Tenais, the systematic position of whidh is probably 
 somewhere between the Amphipods and Isopods, the corneal cuticula is 
 stated by Muller (’64, p. 2) to be facetted, at least in the males. Ac- 
 cording to Blanc’s (’83, p. 635) more recent observations, however, it is 
 claimed to be unfacetted. 
 
 The cones in Amphipods have long been known to be segmented. 
 The number of segments of which each cone is composed has been dif- 
 ferently stated, however, by different observers. According to Clapa- 
 rede (’60, p. 211), the cones in Hyperia are each composed of four seg- 
 ments. This also is the number given by Sars (’67, p. 61) and by 
 Leydig (’79, p. 235) for Gammarus. Both Hyperia and Gammarus 
 have since been carefully studied, and these observations are now 
 known to be inaccurate. Claparede was perhaps influenced in his 
 statement by his belief that all cones were composed of four cells. 
 Sars was probably misled by the supposed fact that in Gammarus the 
 cone is surrounded by four bands of pigment, which sometimes give it 
 the appearance of being divided into four segments. 
 
 The actual number of segments in the cone of Amphipods is two. 
 This number was first recorded by Pagenstecher (’61, p. 31) for the 
 cones of Phronima. Pagenstecher believed, however, that the cones 
 in this Crustacean increased in numbers by division, and that they 
 showed no indication of being composed of two segments except when 
 they were undergoing this process. I need scarcely add that subse- 
 quent investigations have not confirmed Pagenstecher’s belief. Cones 
 composed of two segments have been observed in some six or seven 
 genera of Amphipods. 1 
 
 The retinula in Amphipods is stated by different observers to consist 
 of either four or five cells. Five have been seen by Grenacher (’74, 
 p. 653) and Carriere (85, p. 160) in Hyperia; by Grenacher (’79, 
 p. 112), Claus (’79, Taf. VIII. Fig. 65), and Carriere (’85, p. 164) in 
 Phronima; and by Mayer (’82, p. 122) in Caprella. 
 
 In Gammarus, Sars (’67, p. 61) observed that the cone had four 
 
 1 In Caprella (Mayer, ’82, p. 122), in Gammarus (Grenacher, ’79, p. 110, Car- 
 riere, ’85, p. 156), in Hyperia (Grenadier, 74, p. 652), in Oxycephalus (Claus, *71, 
 p. 151), in Phronima (Schmidt, ’78, p. 5, Grenacher, ’79, p. 112, Claus, ’79, p. 130), 
 in Talorchestia (Watase, ’90, p. 296), and in the Platyscelidce (Claus, ’87, p. 15). 
 In Gammarus ornatus, Talorchestia longicornis, and Caprella , each cone is composed 
 of two cells. 
 
MUSEUM OF COMPAEATIVE ZOOLOGY. 
 
 71 
 
 longitudinal bands of pigment on it. Grenacher (’79, p. 110) took 
 this as an indication that there were at least four retin ular cells in 
 the ommatidium of this genus, but he was unable to satisfy himself as to 
 whether there were a greater number or not. Carriere (’85, pp. 156, 
 157) easily identified the four cells first seen by Sars, and in favor- 
 able cases observed what he thought might be indications of a fifth cell. 
 In Gammarus ornatus, as the present observations show, the retinula 
 is certainly always composed of five cells, one of which, as Carriere 
 observed, is usually much smaller than the other four (compare cl. rtn 
 Figs. 4-7). 
 
 In Talorchestia, according to Watase (’90, p. 296), the retinula is 
 composed of only four cells. I have studied T. longicornis with the 
 purpose of determining the number of retinular cells, and I find that, 
 although there are four large retinular cells, there is also one small one, 
 which is even more reduced than in Gammarus. Hence I conclude that 
 the total number of retinular cells in an ommatidium of Talorchestia 
 is five, not four. 
 
 Claus’s statement (’71, p. 151), that in Oxycephalus the retinula is 
 usually composed of four cells, is probably inaccurate, as Grenacher 
 (’79, p. 114) suggests; and the same is perhaps true of Della Valle’s 
 (’88, p. 94) observation, that in the Ampeliscidse the retinulse contain 
 only four cells each. It is therefore probable that the retinula in all 
 Amphipods is composed of five cells, although possibly in some excep- 
 tional cases the number may be four. 
 
 The retinular cells in Gammarus envelop the sides of the cone, as 
 Carriere suspected, and extend distally as far as the corneal hypodermis 
 (Plate I. Fig. 2). In Hyperia and Phronima, according to the descrip- 
 tion and figures given by Carriere (’85, p. 161, and Fig. 128, p. 165), 
 these cells appear to be limited to the proximal part of the retina. 
 
 The rhabdome in Amphipods, first described by Pagenstecher (’61, 
 p. 30) as the cylindrical element in the eye of Phronima, presents a 
 very simple structure. In Hyperia, according to Grenacher (’77, p. 31), 
 it is a simple rod-like body, composed of five rhabdomeres, one for each 
 retinular cell. In Phronima, as Claus (’79, p. 128) has shown, the 
 rhabdome is a tubular structure with five sides. Each side of the tube, 
 as can be seen in the figure given by Carriere (’85, p. 165, Fig. 128), 
 corresponds to a rhabdomere. In Gammarus locusta, Grenacher (’77, 
 p. Ill) has shown that, in transverse section, the distal end of the 
 rhabdome is cross-shaped. In G. pulex, according to Carriere (’85, 
 p. 157), the distal end of the rhabdome in section shows four rays, the 
 
72 
 
 BULLETIN OF THE 
 
 proximal five. In Carriere’s opinion, these rays indicate the five rhab- 
 domeres. In Gamniarus ornatus, the species -which I have studied, the 
 rhabdome (Plate I. Fig. 6, rhb.) is cross-shaped in transverse section 
 throughout its length. Each rhabdomere has the form of an elon- 
 gated plate, which is folded on its longest axis, so that its halves are 
 at right angles to each other. In the rhabdome, the four rhabdomeres 
 lie so that their folded edges occupy the axis of the ommatidium. 
 Each of the four large retinular cells rests in the furrow produced 
 by the folding of a rhabdomere (compare Fig. 6). The fifth retinular 
 cell always lies at the end of one arm of the cross-shaped rhabdome. 
 The two rhabdomeric constituents of that arm usually separate slightly, 
 so as to allow the small retinular cell to slip in between them. Possi- 
 bly this cell produces a small rhabdomere, as the corresponding cell in 
 G. pulex does ; but if such is the case, the rhabdomere must be a very 
 small one, for I have not been able to discover it. A rhabdome of 
 essentially this structure occurs in Talorchestia. 
 
 As the preceding account shows, the rhabdome in Amphipods always 
 presents some indication of the number of rhabdomeres of which it is 
 composed. This number is usually five, although it is possible that in 
 Gammarus it may be only four. 
 
 In addition to the cells which have thus far been described as entering 
 into the composition of the retina in Amphipods, certain other cells may 
 be present. These may be embraced under the one head of accessory 
 
 'pigment cells. 
 
 In Gammarus, as Carriere (’85, p. 159) has shown, the space between 
 the ommatidia is filled with rather large cells, the nuclei of which are 
 usually- visible with ease (Fig. 2, nl. k’drm.). These cells extend from 
 the basement membrane very nearly, if not quite, to the corneal hypo- 
 dermis. In the fresh condition they contain a whitish opaque pigment. 
 On account of their having no definite arrangement, it is difficult to esti- 
 mate their number, but there are probably two or three for each omma- 
 tidium. Cells similar in position to these have been described by Watase 
 (’90, p. 29G) in Talorchestia. 
 
 In Hyperia there are apparently three kinds of accessory pigment 
 cells. One kind occurs in the region of the basement membrane (Car- 
 riere, ’85, p. 161, Fig. 124, m.) ; another kind surrounds the proximal por- 
 tion of the cones (Carriere, ’85, p. 161) ; a third kind is applied to the 
 retinulae, and, according to Carriere, exactly equals in number the cells 
 of the retinula itself. Possibly the cells which Grenacher (’79, p. 112) 
 described as lying at the distal end of the retinula in Hyperia belong 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 73 
 
 to this third kind, although, as must be remembered, Grenacher states 
 that there are only two such cells for each ommatidium. 
 
 These three kinds of accessory pigment cells, with the possible excep- 
 tion of those which surround the retinula, occur in the lateral eyes of 
 Phronima (Carriere, ’85, p. 164). 
 
 Almost nothing is known about the source of the accessory pigment 
 cells in Amphipods. Those in Gammarus have no resemblance to the 
 loose mesodermic tissue which lies in the neighborhood of the eye, and 
 they are probably derived from the original ectodermic thickening which 
 gave rise to the retina. Although some of the accessory pigment cells 
 in Hyperia and Phronima have been called connective-tissue cells (Claus, 
 ’79, p. 125, Carriere, ’85, p. 160), a name which might be taken to im- 
 ply that they have come from a mesodermic source, nothing is really 
 known about them which would be inconsistent with an ectodermic 
 origin. 
 
 From the foregoing account of the ommatidia in Amphipods the follow- 
 ing summary can be made : cells of the corneal hypodermis not definitely 
 arranged, from about nine to twelve, — possibly two to each ommatidium ; 
 cone cells, two; retinular cells, five, — possibly in some cases four; ac- 
 cessory pigment cells (ectodermic 1 ?) present. Of these last there may be 
 only one kind, as in Gammarus and Talorchestia, or there may be three 
 kinds, as in Hyperia. 
 
 Phyllopoda . 
 
 The ommatidia in the eyes of Phyllopods present at least two struc- 
 tural types, one of which obtains in the Branchiopodidse and Apusidse, 
 
 the other in the Estheridm and Cladocera. On account of the greater 
 
 © 
 
 convenience, the eyes in the Apusidse and Branchiopodidse will be con- 
 sidered first, then the eyes in the Estheridm, and finally those in the 
 Cladocera. 
 
 Branchiopodidce and Apusidce. — The ommatidia in these two families, 
 and especially in the Branchiopodidse, have been carefully studied by a 
 number of competent investigators ; their structure is consequently 
 well known. 
 
 The material which I used in studying these eyes consisted of speci- 
 mens of Branchipus, probably B. vernalis, Verrill, which I had collected 
 in the neighborhood of Philadelphia, and which had been preserved 
 for some time in strong alcohol. Through the kindness of Dr. W. A. 
 Setchell, I was also able to examine a specimen of Apus lucasanus, 
 Packard. 
 
74 
 
 BULLETIN OF THE 
 
 A corneal hypodermis has been described by Claus (’86, pp. 321, 322) 
 in Branchipus and Apus. In Branchipus torticornis, according to Claus, 
 the nuclei of the hypodermal cells are arranged around the distal end of 
 each cone in circles of six ; each nucleus participates in three circles, so 
 that there are in reality only twice as many hypodermal cells as there 
 are ommatidia. The corneal hypodermis in the eye of Branchipus ver- 
 nalis (Plate TV. Fig. 30, nl. Kdrm.) is similar to that described by Claus 
 in B. torticornis. According to Patten (’86, p. 645), a corneal hypoder- 
 mis is present in Branchipus Grubii, but the cells, instead of being 
 regularly placed, as in either Branchipus torticornis or B. vernalis, are 
 stated to be indefinitely arranged. 
 
 The corneal cuticula in Apus is described as unfacetted by Miiller 
 (’29, p. 56), Burmeister (’35, p. 533), Zaddach (’41, p. 46), and Frey 
 und Leuckart (’47, p. 205). In Branchipus stagnalis the cuticula is 
 smooth according to Spangenberg (’75, p. 30), marked by concavo- 
 convex facets according to Grenacher (’79, p. 114), and smooth exter- 
 nally but facetted internally according to Leydig (’51, p. 295). This 
 difference of opinion is probably due to the fact that in this species the 
 facets are so poorly developed that their form can be determined only 
 with difficulty. In Branchipus vernalis, although the corneal cuticula 
 is facetted, the facet is not thickened in its centre, but has the form 
 of a simple concavo-convex elevation, as described by Grenacher in 
 B. stagnalis. In Branchipus paludosus according to Burmeister ('35, 
 p. 531), in B. torticornis according to Claus (’86, p. 320), and in 
 B. Grubii according to Patten (’86, p. 645), the corneal cuticula is 
 unfacetted. 
 
 The cone in Branchipus, as Spangenberg (’75, p. 30) first demon- 
 strated, is composed of four segments. This observation has since been 
 confirmed by Grenacher (’79, p. 115), Claus (’86, p. 320), and Patten 
 (’86, p. 645). In Branchipus vernalis (Fig. 31, con.) the cone, according 
 to my observation, consists of four segments. The cellular nature of 
 each segment was first clearly stated by Grenacher. Each cone in 
 Apus, according to both Grenacher (’79, p. 115) and Claus (’86, p. 321), 
 is composed of four cells. 
 
 The retinula in both Apus and Branchipus consists of five cells. This 
 number has been seen in both genera by Grenacher (’74, p. 653) and by 
 Claus (’86, p. 319). Spangenberg, however, (’75, p. 31) counted four 
 nuclei in the retinula of Branchipus. Since these unquestionably rep- 
 resent the nuclei of the retinular cells, and since these cells are usually 
 five in number, Spangenberg’s enumeration is probably inaccurate. Pos- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 75 
 
 sibly he was influenced when counting the nuclei by his belief that the 
 number four was characteristic of many structures in the ommatidium. 
 In Branchipus vernalis (Plate IV. Fig. 32, cl. rtnJ) the retinula contains 
 five cells. 
 
 The rhabdome in Apus is short ; in Branchipus (Fig. 30, rhb.) it is 
 relatively long. In transverse section (Fig. 32, rhb.) it is circular, or at 
 times squarish, but never pentagonal, as might be expected from the 
 fact that it is surrounded by Jive retinular cells. 
 
 The retina in B. vernalis contains no other cells than the three kinds 
 already described. According to Claus (’86, p. 319), blood corpuscles 
 may make their way into the base of the retina of B. torticornis. 
 
 From the preceding account, the number of cells in the ommatidia of 
 the Branchiopodidse and Apusidse can be stated as follows : cells of the 
 corneal hypodermis, usually two, possibly variable in number in some 
 species; cone cells, four; retinular cells, five. In Branchipus torticornis 
 the interommatidial space may contain blood corpuscles. 
 
 Estheridce. — The species which I studied as a representative of this 
 family was Limnadia Agassizii, Packard. This species can usually be 
 obtained in great abundance during summer in small fresh-water pools 
 in the neighborhood of Wood’s Holl, Mass., where my material was 
 kindly collected for me by Mr. W. M. Woodworth. 
 
 The external surface of the retina in Limnadia, as I have mentioned 
 in my account of the general structure of the eye in this genus, is cov- 
 ered with an extremely delicate corneal cuticula. This cuticula does 
 not show the least trace of facets. 
 
 Immediately below the corneal cuticula are numbers of small nuclei 
 (Plate IV. Fig. 37, nl. cm.). These, from their position, are probably 
 to be regarded as the nuclei of the corneal hypodermis. They are not 
 regularly arranged, and, although they sometimes lie between the cu- 
 ticula and the distal end of a cone, they more frequently occur next 
 to the cuticula in the spaces between the cones. 
 
 As a rule, each cone in Limnadia is composed of five cells (Plate IV. 
 Figs. 37 and 38). In this respect it resembles the cones in Estheria 
 californica and E. tetracera described by Lenz (77, p. 30). In Lim- 
 nadia Agassizii, however, cones composed of four cells are not infre- 
 quently met with (compare Figs. 37 and 38). Grube’s (’65, p. 208) 
 observation that the cone in Estheria is composed of two segments is 
 probably erroneous, but Claus’s (72, p. 360) statement that in Limnadia 
 the cone consists of four segments may be accurate, contrary to the 
 opinion of Lenz. 
 
76 
 
 BULLETIN OF THE 
 
 The retinular cells in Limnadia cover the greater part of the sides of 
 the cones, and completely hide the rhabdome (Plate IV. Fig. 36). Their 
 number can be determined in transverse sections in the region of the 
 rhabdome. In such sections each rhabdome is surrounded by five retin- 
 ular cells (Fig. 39, cl. rtnJ). Occasionally nuclei can be distinguished 
 in the pigment about the base of the cone. These are probably the 
 nuclei of the retinular cells. 
 
 Besides the elements thus far enumerated, the retina in the Estheridse 
 is not known to contain other kinds of cells. The cells in the omma- 
 tidia of this family are, therefore, as follows : cells of the corneal hypo- 
 dermis, not regularly arranged ; cone cells, usually five, sometimes four ; 
 retinular cells, five. 
 
 Cladocera. — The extreme minuteness of the ommatidia in the eyes of 
 the Cladocera renders their study especially difficult. In an undeter- 
 mined species of Evadne which I have studied, the ommatidia are 
 comparatively large, and in this respect are especially favorable for in- 
 vestigation. In the particular specimens which I used, however, I was 
 entirely unsuccessful in all attempts to differentiate the nuclei. Al- 
 though I tried a number of dyes and reagents, I was never able to make 
 these structures visible. In consequence of this, there are several impor- 
 tant questions concerning the eyes in the Cladocera which I have not 
 been able to answer. 
 
 It is reasonable to believe that a corneal hypodermis much like that 
 in Limnadia is present in Evadne, but, probably on account of my inabil- 
 ity to stain the nuclei, I have seen no traces of it. 
 
 The cones in Evadne are very clearly composed of five segments (Plate 
 IV. Figs. 41, 42). At their distal ends the cone cells are expanded so 
 that their peripheral membranes (Fig. 41, mb. pi’ph.) are in contact with 
 one another. At thn level, however, the substance of the cone proper is 
 collected about the axis of the ommatidium. Proximally the peripheral 
 membranes of each cone contract, and under these circumstances the 
 cavity of each cone cell is apparently filled completely with the differen- 
 tiated material of the cone itself (Fig. 42). 
 
 A cone composed of five segments has been observed in a considerable 
 number of Cladocera. Thus it is known to occur in Bythotrephys 
 (Leydig, ’60, p. 245, Claus, 77, p. 144), Daphnia (Spangenberg, 76, 
 p. 522, Grenacher, 79, p. 117), Polyphemus, Evadne (Claus, 77, p. 144), 
 Podon (Grenacher, 79, p. 117), and Leptodora (Carriere, ’84, p. 678). 
 Weismann’s assertion (74, p. 364) that the cone in Leptodora is com- 
 posed of four segments is disproved by Carriere’s later observations, and 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 77 
 
 Claus’s statement ( ? 76, p. 372) that the same number of segments oc- 
 curs in the cone of Sida is probably erroneous. There is, therefore, 
 reason to believe that the cones in the Cladocera are always composed 
 of five segments. 
 
 The composition of the retinula in Cladocera, so far as I am aware, has 
 never been fully worked out. In Evadne, on account of the relatively 
 large size of the ommatidia, the number of cells in the retinula can be 
 determined. At the proximal end of the cone, this structure is sur- 
 rounded by four distinct masses (Fig. 43). The regularity with which 
 these masses occur leaves no doubt as to their number. Each one prob- 
 ably represents a retinular cell. In transverse sections made through 
 the rhabdome (Plate IV. Fig. 45), this structure is surrounded by Jive 
 bodies, each one of which I take to be a retinular cell. It is therefore 
 probable that the retinula of Evadne is composed of five cells, four of 
 which approach nearer the surface of the eye than the fifth. 
 
 In Evadne I have seen no evidence of the existence of other cells than 
 those belonging to the cone and retinula. According to Carriere (’84, 
 p. 678), the interommatidial space in Leptodora contains a number of 
 cells which envelop the cones more or less completely. These are proba- 
 bly to be regarded as accessory pigment cells. 
 
 From the foregoing account the following general statement can be 
 made for the ommatidia in the Cladocera : corneal liypodermis, not 
 observed ; cone cells, five ; retinular cells, five (in Evadne ) ; accessory 
 pigment cells present (in Leptodora). 
 
 Copepoda. 
 
 I have studied the lateral eyeS in Pontella and Argulus, as representa- 
 tives of the Copepods. As is well known, the eyes in these two genera 
 differ greatly in structure, and I shall therefore describe them separately, 
 beginning with the eyes, in Pontella. 
 
 Eucopepoda. — The species of Pontella which I studied was extremely 
 abundant at Newport in August, 1890. This animal was so transparent 
 when living, that the general structure of its eyes could be ascertained 
 by a simple microscopic inspection of it. In addition to its median eye, 
 which occupies a ventral position, it possesses a pair of lateral eyes 
 (compare Claus, ’63, Taf. III. Fig. 5) situated one on either side of the 
 sagittal plane at the antero-dorsal angle of the head. 
 
 Each lateral eye in Pontella, as Claus (’63, p. 47) has already stated, 
 is provided with a spherical lens (Plate IT. Fig. 18, Ins.), which is usu- 
 ally firmly attached to the superficial cuticula. Immediately behind 
 
78 
 
 BULLETIN OF THE 
 
 this lens, and in fact covering much of its proximal face, is a rather 
 irregular mass of cells, the retina. In the living animal the cells of the 
 retina contain a great quantity of black or reddish black pigment. This 
 coloring matter, however, is so readily soluble in alcohol, that in speci- 
 mens preserved in that fluid all traces of it disappear. The optic nerve 
 (n. opt., Fig. 18), an imperfectly defined bundle of fibres, emerges from 
 the retina near its posterior dorsal edge, and passes directly backward to 
 the brain. 
 
 The lenses of the two lateral eyes in Pontella are so near each other 
 that their median faces are almost in contact (compare Plate III. Fig. 
 29). The retinas of the two eyes, as Claus (’63, p. 47) has observed, 
 are united with one another on their median faces, and so intimately 
 that they are apparently incapable of independent motion. 
 
 The two retinas together may be rotated on their lenses through an 
 angle of about forty-five degrees. The plane of this rotation corresponds 
 to the sagittal plane of the body, and the rotation is accomplished by 
 two pairs of muscles, one for each retina (compare Claus, ’63, Taf. III. 
 Fig. G). One pair of these muscles is shown in Figure 18. They occupy 
 a plane approximately parallel to the sagittal plane of the body, and 
 the effects of their contractions must be apparent from their positions. 
 When both muscles are relaxed, the retina occupies a position substantially 
 as shown in Figure 18. By the contraction of the posterior muscle, the 
 retina may be drawn upward and backward over the surface of the lens, 
 till its axis, instead of pointing dorsally, is directed forward and upward 
 at an angle of about forty-five degrees with its original position. The 
 retina is not usually held for any great length of time in this position, but 
 is soon returned by the contraction of the anterior muscle to its normal 
 place. The backward motion of the retina is accomplished with such 
 rapidity that the animal has the appearance of winking. The forward 
 motion is rather slower. 
 
 Each lens in Pontella is composed of concentric laminae (Plate III. 
 Fig. 29, Ins.). A considerable portion of its distal surface is intimately 
 connected with the superficial cuticula (Plate II. Fig. 18), although a line 
 of demarcation between lens and cuticula can always be distinguished. 
 
 When the anterior half of the body of Pontella is boiled in a strong 
 aqueous solution of potassic hydrate, and afterwards subjected to the 
 action of concentrated nitric acid, all the soft parts are dissolved, and 
 only the very resistant chitinons structures remain. In specimens 
 treated in this way, the lenses retain their firm connection with the 
 superficial cuticula, and differ in appearance from those in the living ani- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 79 
 
 mals only in that their concentric lamellae are somewhat more distinct. 
 The fact that the lens is composed of concentric layers indicates that 
 it is secreted, and the resistance which it offers to reagents is weighty 
 evidence in favor of its chitinons nature. In my opinion, therefore, the 
 lens in Pontella is a chitinons secretion. 
 
 The development of the lens in Pontella is rather peculiar. Appar- 
 ently a new lens is formed with each moulting of the general cuticula ; 
 at least, in a rather large proportion of the number of individuals exam- 
 ined, the lenses were abnormally small, having a diameter of one third 
 or even one fourth of that shown in Figure 18. Moreover, in all such in- 
 dividuals the superficial cuticula was correspondingly thin and delicate, 
 and when the animal was subjected to boiling potash, the segments of 
 its body and appendages separated with a readiness never observed in 
 specimens with large lenses. There can be no doubt, I believe, that the 
 small lenses are always accompanied by thin cuticula, a relation which 
 is to be explained by the immature condition of both structures. 
 
 The smaller lenses differ from the larger ones in only one important 
 particular besides that of size. They are not in contact with the super- 
 ficial cuticula. This relation can be determined better in optical sec- 
 tions than in actual ones, for in the latter the position of the lens is 
 usually somewhat changed by the resistance which it offers to the knife. 
 The centre of the small lens occupies a position relatively the same as 
 that of the large lens, the space between the surface of the small lens 
 and the external cuticula being filled with a cellular mass. This mass, 
 as seen in optical sections, apparently envelops the lens on all sides, 
 and is undoubtedly composed of the cells which secrete that structure. 
 As the lens increases in size, the cells are probably excluded from the 
 region between it and the cuticula, and as they retreat cement the lens 
 to the cuticula. Upon the completion of the lens, the cells which have 
 shared in producing it probably withdraw slightly from it to form the 
 hypodermal thickenings which occur beneath the adjoining cuticula 
 (Plate II. Fig. 18, and Plate III. Fig. 29, h'drm.). These thickenings 
 are rich in nuclei, and often have delicate strands of protoplasm stretch- 
 ' ing to the surface of the lens (Fig. 18). 
 
 I believe that these facts justify the opinion that the lenses in the 
 lateral eyes of Pontella are composed of chitin, that they are produced 
 unconnected with the superficial cuticula, and that they are secondarily 
 cemented to it. Like the cuticula itself, they are products of the hy- 
 podermis, a new lens being generated in all probability with each new 
 formation of cuticula. 
 
80 
 
 BULLETIN OF THE 
 
 Lenses similar in position to those in Pontella have been identified 
 in the lateral eyes of several other genera of Copepods. Gegenbaur 
 ('*58, p. 71) described such lenses in Sapphirina, and Leuckart (’59, 
 p. 250) observed similar ones in the lateral eyes of Corycseus and 
 Copilia. In all these genera the lenses, although biconvex, are not 
 spherical, as in Pontella. Gegenbaur (’58, p. 71), following Leydig’s 
 generalization, believed that in Sapphirina the lenses were thickenings 
 in. the cuticular covering of the body, and Claus (’59, p. 271) considered 
 them morphologically equivalent to a single corneal facet. Leuckart 
 (’59, p. 250), without definitely committing himself as to the nature 
 of the lens, states that in Copilia and Corycaeus the lens is implanted 
 in the superficial cuticula, and further describes it in Corycseus as com- 
 posed of two parts, an outer and an inner. According to Grenacher 
 (’79, p. 67), both parts can be identified in the lens of Copilia; the 
 outer part is a portion of the superficial cuticula ; the inner part, both 
 in its optical properties and its behavior toward reagents, is unlike the 
 cuticula. The inner part, however, contains no traces of cells, but is 
 composed of a homogeneous substance, probably secreted. This view of 
 the duplicity of the lens contrasts with the older idea of its origin as a 
 thickening in the superficial cuticula. 
 
 It is possible that the lenses in the Pontellidae and Corycseidae are not 
 homologous structures, but on account of their similarity I am inclined 
 to consider them as such. Since in Pontella both parts are derived 
 from the cuticula, 1 believe that a similar origin will be demonstrated 
 for these parts in the Corycseidae. The differences which Grenacher 
 has pointed out between the two parts of the lens in Copilia do not 
 necessarily oppose this view. It is possible that the cuticular secretion 
 which forms the proximal part of the lens may originate separately 
 from the other cuticula, as in fact it does in Pontella ; and it may also 
 be true, although this is not supported by the condition in Pontella, 
 that the two parts, although both secretions of the hypodermis, may 
 differ enough in their substance to account for all the peculiarities ob- 
 served by Grenacher. 
 
 The retina and lens in Pontella are not separated by an intervening 
 space as in the Corycseidse, but are in immediate contact. The retina 
 is composed of a mass of cells, the number and arrangement of which 
 can be seen in the figures on Plate III. These figures represent a 
 series of consecutive sections cut in planes transverse to the axis of 
 the eye, i. e. parallel to the horizontal plane of the animal (compare 
 Fig. 18, Plate II.). The series is complete in that it represents all 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 81 
 
 the sections which pass through the retina. The most ventral section 
 is shown in Figure 20, the most dorsal in Figure 29. 
 
 Immediately below the lens the central part of the retina is occupied 
 by a roundish granular mass (Fig. 18, con.), which in the living animal 
 is the only part without pigment. In transverse sections this mass is 
 seen to consist of two bodies (cl. con. 1, and cl. con. 2, Fig. 25), which 
 extend as far as to the lens (compare Figs. 25-27). Each body con- 
 tains a nucleus (nl. con., Figs. 25 and 27) and consequently represents 
 a cell. From the position which the mass occupies, and from the fact 
 that it contains no pigment, it represents, I believe, a cone, and the two 
 cells of which it is composed are its two segments. 
 
 Claus (’63, p. 47) states that in Pontella each retina is provided with 
 six or more small crystalline cones, but my own observations do not 
 confirm this statement. The body which, on account of its position, I 
 have described as the cone in Pontella, is probably homologous with 
 what Dana (’50, p. 133) first described as the inner lens in Corycseus, 
 and with what subsequent investigators have called the crystalline cones 
 in Sapphirina (Gegenbaur, ’58, p. 71) and Copilia (Leuckart, ’59, p. 252). 
 Nothing, I believe, is known of the cellular composition of the cone in 
 these genera. 
 
 The arrangement of the elements in that portion of the retina which 
 surrounds the cone in Pontella is not easily made out. The most con- 
 spicuous structures in this region are rod-like bodies, which probably 
 represent rhabdomeres. Eight of these, arranged in three groups, are 
 present in each retina. The largest group, composed of five rods, lies 
 directly beneath the cone. The rods of this group have been numbered 
 from one to five in the retina to the left in Figures 21, 22, and 23. 
 Posterior to this group, in the same retina, is the sixth rod, seen in 
 Figures 24, 25, and 26. Anterior to it are the seventh and eighth 
 rods, seen in Figures 26, 27, 28, and 29. 
 
 The outlines of the cells to which these rods belong cannot always be 
 distinguished ; that there is a cell for each rod is evident from the fact 
 that near each rod there is a large nucleus. The nucleus belonging to 
 the cell from which the eighth rod was produced is shown in Figure 28 
 (nl. rtnJ) ; those belonging to the cells from which the sixth and seventh 
 rods arose are indicated in Figure 26 (nl. rtn'.), and those belonging to 
 the cells from which the central group of five rods came are seen, four in 
 Figure 24 and one in Figure 25 (nl. rtnJ). 
 
 In addition to these nuclei, which judging from their positions and 
 number are unquestionably the nuclei of the cells to which the rhab- 
 vol. xxi. — xo. 2. 6 
 
82 
 
 BULLETIN OF THE 
 
 domeres belong, the retina contains a number of smaller nuclei (Fig. 21, 
 nl. tidrm.). These nuclei have been drawn in the figures of the various 
 sections in which they occur, and probably represent undifferentiated cells. 
 
 To what extent the retina of Pontella can be resolved into omma- 
 tidia may be seen from the foregoing account. Evidently the two 
 cone cells, the subjacent groups of five retinular cells, and probably 
 some of the undifferentiated cells, are the equivalent of one omma- 
 tidium. The sixth cell, with its rod, is probably the representative of 
 a second ommatidium, and the seventh and eighth cells are probably 
 representatives of one, or perhaps two, more. 
 
 If this interpretation be correct, the cells in the one complete omma- 
 tidium in Pontella would be as follows : corneal hypodermis, undifferen- 
 tiated ; cone cells, two ; retinular cells, five ; undifferentiated pigment 
 cells (ectodermic 1 ?) present. 
 
 Each retina in Sapphirina, according to Grenadier (’79, pp. 69, 70), 
 contains one group of three rhabdomeres. These are accompanied, 
 as in Pontella, by an equal number of large nuclei. The body desig- 
 nated at y, and perhaps some of those marked x, in Grenadier's figure of 
 Sapphirina (Fig. 43), may also represent isolated rhabdomeres. In Co- 
 pilia, Grenadier believes that the number of rhabdomeres in each retina 
 is three. Possibly in this genus, as in Sapphirina, the body marked x 
 by Grenadier (Taf. VI. Fig. 40) may represent an isolated rhabdomere. 
 Grenadier’s observations, when coupled with what I have seen in Pon- 
 tella, show that in Copepods the number of retinal elements is open to 
 considerable variation, and that what would correspond to the retinula 
 in Sapphirina, and perhaps in Copilia, consists of a cluster of only three 
 cells, instead of five, as in Pontella. 
 
 Branchiura. — The ommatidia in Argulus are rather small, and their 
 structure is consequently imperfectly known. The specimens of this 
 Crustacean which I studied were obtained from an aquarium in which 
 the common Killifish, Fundulus heteroclitus, had been kept. I have not 
 been able to determine the species to which these specimens belong. 
 
 The corneal hypodermis in Argulus is separated from the retina proper 
 by a space filled with blood (Plato II. Figs. 11, 12, ccel.). The cells in 
 this layer (Fig. 12, li'drm .), as in the corneal hypodermis of Amphipods, 
 are not arranged in groups, but are irregularly scattered. On their 
 distal faces they produce the corneal cuticula (Fig. 12, eta.), which, as 
 Muller (’31, p. 97) observed, is without facets. Proximally they are 
 separated from the blood space by the delicate corneal membrane (Fig. 
 12, mb. cm.). 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 83 
 
 The distal face of the retina proper in Argulus is bounded by a deli- 
 cate preconal membrane (Figs. 11—13, mb. pi'* con.) and its proximal face 
 is limited by the basement membrane (Figs. 11-13, mb. ba.). 
 
 The most conspicuous objects in the retina are the cones (Fig. 11, 
 con.), which lie with their distal ends usually somewhat below the 
 preconal membrane (Fig. 13). Each cone, as Claus (’75, p. 256) has 
 observed, is composed of four segments (Fig. 14). The segments corre- 
 spond to cells, and although the cone itself terminates proximally before 
 reaching the rhabdome, the cone cells form an axis free from pigment 
 and extending from the cone to the rhabdome (compare Fig. 12). In 
 depigmented sections the peripheral membranes of the cone cells (Fig. 
 13, mb. pi*ph.) can be distinguished as sharply marked lines which ex- 
 tend from the sides of the cone to the sides of the rhabdome. The 
 intercellular membranes of the cone cells in the region between the cone 
 and rhabdome are apparently marked by thickenings which appear in 
 both longitudinal and transverse sections (compare Figs. 13 and 15). 
 At the distal end of the rhabdome the four cone cells separate, and, after 
 passing partly around the rhabdome, become lost in the adjoining tissue 
 (Fig. 16, cl. con.). I have not been able to discover the nuclei of the 
 cone cells. 
 
 It is difficult to determine the number of cells in the retinula of Argu- 
 lus. Slightly below the proximal end of the rhabdome, the retinula is 
 divided into five distinct pigmented masses (Fig. 17, cl. rtnJ). Since the 
 rhabdome (Fig. 16, rhb.) is composed of five rhabdomeres, it is highly 
 probable that the retinula consists of five cells; but I have not been able 
 to determine with precision the outline and extent of these cells. 
 
 The nuclei which are visible in the retina of Argulus closely resemble 
 one another. They are limited for the most part to two regions (Fig. 
 13), one near the level of the cones, the other near the basement mem- 
 brane. Apparently there are no nuclei immediately below the preconal 
 membrane. Those which are near the cones (Figs. 13, 14, nl. h'drm .), 
 judging from their arrangement . and position, probably represent inter- 
 ommatidial pigment cells. Those near the basement membrane (Fig. 
 13, nl. rtnJ) may be the retinular nuclei, as their position seems to indi- 
 cate. For some distance proximal to the basement membrane, nuclei 
 (Fig. 13, nl. h'drm.') occur among the nerve fibres. Possibly they repre- 
 sent scattered cells in this region, but the strong resemblance which 
 they have to the nuclei on the distal side of the membrane induces me 
 to believe that they too are retinular nuclei, which, as in the Amphi- 
 pods, have migrated to a position below the basement membrane. 
 
84 
 
 BULLETIN OF THE 
 
 The cells in the ommatidium of Argulus are as follows : cells of the 
 corneal hypodermis, not arranged in definite groups ; cone cells, four ; 
 retinular cells, probably five ; accessory pigment cells probably present. 
 
 Isopoda. 
 
 The material which I used in studying the eyes in Isopods came from 
 several sources. I collected specimens of Asellus and Porcellio in the 
 neighborhood of Cambridge, and the two species of Idotea which I 
 studied were obtained at Newport. Specimens of Serolis Schythei, 
 Liitken, and of an undetermined species of Sphaeroma, were kindly fur- 
 nished me from the collections in the Museum. 
 
 The ommatidia in Isopods present two types of structure : one of 
 these is characteristic of the eyes in a majority of the members of this 
 group ; the other, so far as is known, is represented only in the genus Se- 
 rolis. These two types will be considered separately, and the one which 
 is common to the greater number of Isopods will be described first. 
 
 The corneal hypodermis in the more common of these two ommatidial 
 types was first identified by Grenacher. In Porcellio, according to this 
 author (’79, p. 107), the proximal surface of each facet is covered with 
 two comparatively thin cells. These are the cells of the corneal hypo- 
 dermis. Bellonci (’81 a , p. 98, Tav. II. Fig. 11 n.) figures similar cells 
 in the ommatidium of Sphaeroma, and Beddard (’90, p. 368) concludes 
 justly, I believe, that, of the four nuclei found near the distal end 
 of the cone in Arcturus, two represent cone cells and two cells in the 
 corneal hypodermis. In Idotea irrorata I have identified two cells in 
 the corneal hypodermis for each ommatidium. The nuclei of these cells 
 lie very near the nuclei of the cone cells (compare nl. con. and nl. cm. in 
 Figs. 50 and 51, Plate V.). In an ommatidium of Porcellio, Grenacher 
 (’79, pp. 107, 108) observed that the plane which separates the two 
 cone cells also separates the two cells in the corneal hpyodermis. In 
 Idotea, also, both kinds of cells are separated by a single plane. 
 
 The facetted condition of the corneal cuiicula of Isopods was observed 
 as early as 1816 by G. R. Treviranus (T6, p. 64), in wood-lice, and 
 subsequently in the same animals by Lereboullet (’43, p. 107, ’53, 
 p. 119). The shape of the facets in different Isopods has given rise to 
 some difference of opinion. According to Muller (’29, p. 42), in Cymo- 
 thoa each has the form of a biconvex lens. Leydig (’64% p. 40) states, 
 however, that in Oniscus the facets are concavo-convex with their hollow 
 faces innermost. In Asellus, according to the figure given by Sars 
 (’67, Planche VIII. Fig. 14), they are plano-convex with their flat faces 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 85 
 
 innermost. These differences, although at first sight somewhat con- 
 tradictory, are not matters of great importance, for it is probable that 
 each time an Isopod sheds its cuticula and a new one is formed, the 
 lens assumes, at successive stages of its growth, outlines which coincide 
 very closely with those recorded by the different observers. Thus, an 
 early stage would be represented by the concavo-convex lens described 
 by Leydig, an intermediate stage by the plano-convex lens figured by 
 Sars, and the final condition by the biconvex lens mentioned by Muller. 
 Either this is the explanation of the differences, or the observations of 
 Leydig and Sars are probably erroneous, for the results of the moie 
 recent investigations point to the conclusion that the facets in Isopods 
 have the form of a biconvex lens. Facets of this shape have been seen 
 by Grenacher (77, p. 29) in Porcellio, and by Bellonci (’81 a , p. 98) in 
 Sphaeroma. According to my own observations, they also occur in 
 Idotea, Asellus, Porcellio, and, as I shall show subsequently, in Serolis. 
 In the four genera mentioned the inner face of each facet is distinctly 
 convex ; this is also true of the outer face in Asellus and Porcellio. In 
 Serolis and Idotea (Plate Y. Fig. 50), however, the outer face is so 
 slightly curved that it is difficult to decide whether its curvature is that 
 of the general corneal cuticula or one peculiar to the facet itself. 
 
 That the cone in Isopods is composed of two segments was first ob- 
 served by Leydig (’64 a , p. 41, and ’64, Taf. VI. Fig. 8) in Oniscus. Ac- 
 cording to this author, each segment is spherical. Each ommatidium, 
 therefore, contains two spheres, and these, as Leydig’s figure shows, are 
 placed side by side immediately below the corneal facet. 
 
 It is now well known that in many Tsopods, especially in the wood- 
 lice, the cone itself is nearly spherical, and its two segments would con- 
 sequently be hemispheres, not spheres as figured by Leydig. How Ley- 
 dig’s statement of the spherical shape of the segments can be accounted 
 for, is not apparent. Since the two spheres described by him occupy 
 the same relative positions as the hemispherical segments of a normal 
 cone, there is not much question in my mind that they represent these 
 segments. Possibly their separation and spherical form may have been 
 due to the swelling action of some reagent which Leydig may have used 
 to make the tissue transparent. A cone composed of two segments has 
 been observed by Sars (’67, p. 110) in Asellus, by Leydig (78, p. 256) 
 in Ligidium, by Grenacher (77, p. 29) in Porcellio, by Bellonci (’81 a , 
 p. 98) in Sphseroma, by Sye (’87, p. 23) in Jsera, and by Beddard 
 (’90, p. 368) in Arcturus. In the three genera which I have examined, 
 Idotea, Asellus, and Sphseroma, each cone consists of two segments. 
 
86 
 
 BULLETIN OF THE 
 
 These observations naturally lead to the conclusion that in all Isopods 
 each cone is composed of two segments. To this general statement, 
 however, there are two noteworthy exceptions, one recorded by Sars, the 
 other by Beddard. Sars (’67, p. 110) has shown that, of the four om- 
 matidia in each eye of Asellus aquaticus, three have cones composed 
 each of two segments ; in the fourth, however, the cone is divided into 
 three parts. This observation has been confirmed by Carriere (’85, 
 p. 155). It is important to observe that in the figure given by Sars 
 (’67, Planche VIII. Fig. 12) the three parts of the cone are not of 
 equal size ; one is about as large as a single segment in the cones of 
 the other three ommatidia, whereas the remaining two are each about 
 half as large. In the eyes of the species of Asellus found about Cam- 
 bridge, the ommatidia are usually twice as numerous as in the European 
 species, A. aquaticus, and, so far as I could observe, the cones in the 
 American species were always composed of only two segments. In 
 Arcturus, according to the figures given by Beddard (’90, Plate XXXI. 
 Figs. 1 and 4), cones of three segments are occasionally met with. 
 
 The cellular composition of the retinula in Isopods was first made out 
 by Grenacher (’74, p. 653), who found that in Porcellio this structure 
 consisted of seven cells. Distally these cells surround the cone ; proxi- 
 mally they are continuous with the optic-nerve fibres. A retinula con- 
 sisting of seven cells has also been demonstrated by Buller ( ? 79, p. 513) 
 in Cymothoa, and by Beddard (’88, p. 443) in iEga and Ligia. As 
 Beddard (’88, Plate XXX. Fig. 13) has shown, the seven cells in the 
 retinula of A3ga pass through the basement membrane and become con- 
 tinuous with the nerve fibres. In Porcellio, as I have observed, the 
 fibrous ends of the seven retinular cells not only can be identified as nerve 
 fibres below the basement membrane, but each cell contains a well de- 
 veloped fibrillar axis (Plate V. Fig. 46, ax. n.), and I therefore conclude 
 that in Porcellio all seven cells are functional as nervous elements. 
 
 In Idotea robusta, transverse sections of the retinula in the region 
 where the rhabdome is thickest present the outlines of what seem to be 
 seven retinular cells (Plate V.,Fig. 48). In positions either distal or 
 proximal to this, however, only six cells appear. These six cells pass 
 through the basement membrane and taper into nerve fibres, and their 
 nuclei, unlike the corresponding nuclei in other Isopods, occur in that 
 part of the cell which is proximal to the basement membrane (Figs. 49 
 and 50, nl. rtn '.). The seventh body (Fig. 48, cl. rud.), in those sections 
 in which it occurs, has in all essential respects the same appearance as 
 any one of the adjoining six cells. It differs from these, however, in that 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 87 
 
 it is usually somewhat smaller, and I therefore conclude that it is a 
 rudimentary cell. It does not appear to contain a nucleus ; granting, 
 however, that it is a rudimentary retin ular cell, one would look for its 
 nucleus, not in the region about the rhabdome, but in the region of 
 the nuclei of the other retinular cells, i. e. proximal to the basement 
 membrane. Owing to the irregularity with which the fibrous ends of 
 the retinular cells are arranged in this region, I have not been able 
 to identify any nucleus with this rudimentary cell. Neither have I 
 found any fibrous projections reaching from the rudiment of the cell 
 toward the basement membrane such as might be expected provided 
 the nucleus and a part of the rudimentary cell persisted below the 
 membrane. Nevertheless, I believe, for the reasons already stated, 
 that the retinula in Idotea robusta is composed of seven cells, one of 
 which is extremely rudimentary. 
 
 In Idotea irrorata (Plate V. Figs. 53, 55) the retinula consists of only 
 six cells, all of which possess fibrillar axes, and are therefore probably func- 
 tional as nervous structures. In one retina of the several pairs of eyes 
 which I examined, there was a single ommatidium with seven functional 
 cells (Fig. 54). With this one exception, however, I have not been able to 
 find any trace of the seventh cell in Idotea irrorata. In Arcturus, accord- 
 ing to Beddard (’90, p. 368), the retinula is also composed of six cells. 
 
 In Sphteroma, Bellonci (’81, p. 98, Tav. II. Fig. 12) has figured and 
 described a retinula consisting of five cells. These cells alternate with 
 five other cells, which probably represent accessory pigment cells. If Bel- 
 lonci’s statement is correct, it must be admitted that the number of cells 
 in the retinulse of Isopods may be as few as five. My own observations, 
 however, do not confirm Bellonci’s account. In the species of Sph£eroma 
 which I have studied, there are seven cells in the retinula, four of which 
 are large and three small (Plate V. Fig. 58). All these cells pass through 
 the basement membrane ; all the large ones, and certainly some of the 
 small ones, are also connected with nerve fibres. 
 
 These observations indicate that in the Isopods the retinula is com- 
 posed of either six or seven cells. If Bellonci’s statements prove to be 
 correct, this structure may be composed in some cases of only five cells, 
 but my own observations are opposed to this view. 
 
 The rhabdome in Isopods presents tw r o types of structure, one of 
 which has been well described by Grenaoher (’77, p. 30) for Porcellio 
 scaber. In this species the rhabdome is composed of seven rhabdomeres, 
 each of which remains in connection with the retinular cell which pro- 
 duced it. In transverse section the rhabdome has the form of a seven- 
 
88 
 
 BULLETIN OF THE 
 
 pointed star, a ray corresponding to a rhabdomere. Each ray projects 
 into its retinular cell, not between two cells. My own observations on 
 Porcellio confirm Grenadier’s statements. A second representative of 
 this type of rhabdome has been described by Bellonci (’81, p. 98) for 
 Sphaeroma. Here, however, the rays, although they agree in number 
 with the retinular cells, project between the cells, not into them. 
 
 The second type of rhabdome is well represented in the eye of Arc- 
 turus furcatus. In this species, according to Beddard (’90, pp. 368, 
 369), the distal portion of the rhabdome, although surrounded by six 
 retinular cells, is bounded by four perpendicular sides. Each of the six 
 cells appears from its position to contribute to the formation of the 
 rhabdome, and yet in the greater part of this structure segments cor- 
 responding to rhabdomeres are not visible. In its proximal portion, 
 however, the rhabdome, according to Beddard, is divided into six rhab- 
 domeres, each of which is applied to its proper retinular cell. In Idotea 
 robusta the rhabdome (Plate Y. Fig. 48, rhb.) is nearly square in trans- 
 verse section. So far as I have been able to discover, it does not show 
 at its proximal end any indication of rhabdomeres. 
 
 Of these two types of rhabdome, the one in which the rhabdomeres 
 are evident is probably more primitive than the one in which their in- 
 dividuality is almost, if not completely lost. 
 
 The retinas of Isopods may contain, in addition to those already 
 mentioned, two other kinds of cells. Of these the one most frequently 
 met with fills the space between ommatidia. Cells of this kind have 
 been identified in Porcellio by Grenacher (’79, p. 107), and it is probable 
 that the pigment cells described by Bellonci (’81, p. 99) as intervening 
 between the retinular cells in Sphaeroma belong to this class. I have 
 observed interommatidial cells in Idotea ; here they contain few or no 
 pigment granules, but are easily recognized by means of their nuclei 
 (Plate Y. Fig. 54, nl. h'drrn.). 
 
 The source of these cells is not definitely known, but there appears to 
 be no evidence in favor of their having been derived from outside the 
 retina. Grenacher believed that those in Porcellio are undifferentiated 
 hypodermal cells ; this interpretation probably holds good for those in 
 Sphaeroma and Idotea. 
 
 The hyaline cells, the second kind of accessory cells, have been iden- 
 tified by Beddard (’87, p. 235, ’88, PI. XXX. Fig. 9, h.) in ASga and 
 Cirolana. Since these cells are best developed in the eyes of Serolis, a 
 full description of their structure will be deferred until the account of 
 the eyes in that genus is given. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 89 
 
 The cells which characterize the ommatidia in Isopods (except Serolis) 
 are as follows : cells of the corneal hypodermis, two ; cone cells, two ; 
 retinular cells, seven, six, or possibly five. Undifferentiated hypodermal 
 cells are sometimes present, and hyaline cells occur in a few genera. 
 
 The structural peculiarities of the ommatidia in Serolis were first de- 
 scribed by Beddard (’84, pp. 339-341) about seven years ago. Recently 
 Beddard’s observations have for the most part been confirmed by Watase 
 (’90), and it must now be admitted without question that the ommatidia 
 in Serolis differ in several important respects from those of many other 
 Isopods. 
 
 The material which t used in studying the eyes in this Crustacean 
 consisted of advanced embryos and matured individuals of Serolis 
 Schythei, Liitken. This material was collected in Patagonia by the 
 Hassler Expedition, and was preserved in strong alcohol. Fortunately, 
 it was in good histological condition, and sections prepared from it 
 showed very clearly the finer structure of the eyes. My observations, 
 as the following account will show, differ in no very important respects 
 from those of Beddard and Watase. 
 
 Although Patten’s generalization, that a corneal hypodermis was to be 
 found in the compound eyes of all Crustaceans, led Beddard (’88, p. 447) 
 to look for it in Serolis, he was not able to identify it. Watase (’90, 
 pp. 290 and 293) was more fortunate, and succeeded in finding under 
 each facet two cells in the corneal hypodermis. I have not been as 
 successful as Watase was in determining the exact number of hypo- 
 dermal cells in an ommatidium, but I have seen enough to convince me 
 that such cells are present. In sections approximately tangential to the 
 external face of the adult retina, one occasionally finds nuclei (Plate 
 YI. Fig. 60, nl. cm .) between the distal ends of the cone cells and the 
 corneal cuticula. These represent unquestionably the cells of the cor- 
 neal hypodermis, and are not to be confused with the nuclei of the cone 
 cells, which lie in a deeper plane. In making sections, the corneal 
 cuticula splintered so irregularly that the tissue immediately below it 
 was completely disarranged. It was therefore possible to get only ir- 
 regular fragments of the tissue in this region, such as Figure 60 shows, 
 and these fragments were always too small to admit of an accurate 
 determination of the number of hypodermal cells under a single facet. 
 I have also been equally unsuccessful in my attempts either to isolate 
 these cells or to study them in situ on the corneal cuticula. 
 
 The eyes in the aduu,, owing to the thickness of the cuticula, are 
 unfavorable for the study of the corneal hypodermis ; but in embryos of 
 
90 
 
 BULLETIN OF THE 
 
 even au advanced stage, the cuticula is so thin that the hypodermis can 
 be studied with comparative ease. Au ommatidium from the eye of an 
 advanced embryo is seen in Figure 65 ; the ommatidium is viewed from 
 the side. Distal to the cone ( con .) four nuclei can be seen ; one (nl. cm. 1) 
 is superficial in position, three are deep. The relation of these nuclei to 
 the ommatidium can be satisfactorily studied in sections transverse to 
 the axis of the ommatidium. A series of three such sections is seen 
 in Figures 66, 67, and 68. Of these, the most distal is that shown in 
 Figure 66. This includes only the most superficial layer of the retina, 
 and contains two nuclei (compare nl. cm. 1, in Figs. 65 and 66). These 
 nuclei, as their position clearly indicates, represent cells of the corneal 
 hypodermis. In the plane of the section which includes the three deeper 
 nuclei of Figure 65, four nuclei are in reality present (Fig. 67) ; two of 
 these (nl. con.) are large, and lie directly below the superficial ones in 
 the corneal hypodermis ; two are small (nl. cm. 2) and lie between the 
 ends of the deeper large nuclei. Of the deep nuclei, the two large ones 
 (nl. con.) rest one above each segment of the cone ; in fact, as a section 
 in a slightly deeper plane shows (Fig. 68, nl. con.), these nuclei coincide 
 so closely with the segments of the cone that they must be regarded as 
 the nuclei of the cone cells. 
 
 It is difficult to state what nuclei in the adult correspond to the 
 smaller of the four deep ones in the embryo. The number of these 
 nuclei (two) in the embryo equals the number of pigment cells which 
 Watase (’90, p. 294) has described as surrounding the cone ; but that 
 these nuclei do not belong to such cells is evident from the fact that in 
 the embryo, the nuclei of the pigment cells can be identified in a posi- 
 tion somewhat proximal to that in which the smaller of the four nuclei 
 occur (compare nl. dst. in Figs. 65 and 69.) Possibly the cells repre- 
 sented by these small nuclei in the embryo become in the adult the 
 small interommatidial pigment cells, or it may be that they retain 
 their relatively superficial positions, and, while occupying the space be- 
 tween the corneal facets, perhnps produce the cuticula of that region. 
 In the fragments of the adult retina, from immediately below the cor- 
 neal cuticula, small nuclei are not unfrequently met with in the spaces 
 between the ommatidia. These are possibly derived from the smaller 
 deep nuclei of the embr} T o. 
 
 It will thus be seen that my conclusions concerning the corneal hypo- 
 dermis agree in the main with those of Watase ; namely, that for each 
 ommatidium there are two cells in this layer. Besides these, however, 
 it is possible that the hypodermis may contain an equal number of other 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 91 
 
 cells, which occupy positions immediately under the cuticula and be- 
 tween the ommatidia. 
 
 The facets in the corneal cuticula of Serolis, when viewed from the 
 exterior, are irregularly circular in outline, often approaching a six-sided 
 form. As I have already observed, they are arranged on the plan of the 
 hexagonal type. The distal face of each facet is flat, or only slightly 
 convex ; the proximal face is decidedly convex. The curvatures of the 
 two faces and the thickness of the cuticula in the facet of S. Schythei 
 was about the same as that figured by Watase (’90, Plate XXIX. Fig. 1) 
 for the species which he studied. 
 
 The cone, as Beddard (’84, p. 340) first demonstrated, and as Watase 
 (’90, p. 290) afterwards confirmed, is composed of two nearly hemi- 
 spherical segments, which correspond to the two cone cells. The proto- 
 plasmic material of each cone cell covers the curved surface of the seg- 
 ment to which it belongs, and contains a nucleus in its distal portion. 
 These relations have been well shown by Watase (’90, Plate XXIX. 
 
 Fig- 1). 
 
 From the condition presented even in advanced embryos (Fig. 65) 
 it is evident that the part of the cone earliest formed, is the one which 
 is nearest the applied faces of the two cone cells, and that from this as 
 aeentre the cone has continued to increase outwards. Although at this 
 stage the outline of the cone itself is sharply marked (Fig. 65), the ex- 
 ternal limits of the cone cells are only approximately indicated by the 
 distribution of the pigment granules, which have begun to form in the 
 surrounding pigment cells. 
 
 In Serolis, as in Porcellio and Idotea, the cone cells and the cells of 
 the corneal hvpodermis are separated by the same perpendicular plane. 
 There are some complications in the structure of the cone cells which 
 can be discussed subsequently with greater clearness. 
 
 The retinula in Serolis, as Beddard (’84, p. 340) first observed, is 
 peculiar in that it is composed of only four cells. My own observations 
 add almost nothing that is new to the previous accounts of this structure. 
 The figure which Watase has drawn (’90, Plate XXIX. Fig. 1) of the 
 characteristic form of the retinular cell when viewed from the side and 
 its relation to its rhabdomere, reproduces very closely the structural 
 conditions which I have observed in S. Schythei. 
 
 The rhabdome in “Serolis has been carefully studied by Beddard (’88, 
 pp. 448-450). Owing to the complexity of its structure, one meets with 
 difficulties in attempting to interpret its parts in terms of the relatively 
 simple rhabdome of many Crustaceans. The peculiarities of this struc- 
 
92 
 
 BULLETIN OF THE 
 
 ture can be approached most satisfactorily perhaps from the side of its 
 adult anatomy. 
 
 In a transverse section of the distal end of the rhabdome, five struc- 
 tures can be observed (Fig. 61). Four of these (Fig. 61, rlib'm.) are 
 squarish pieces confluent on one side with a retinular cell, and in contact 
 with one another only at their angles The sides of these pieces which 
 are directed towards the axis of the ommatidium are convex, and to- 
 gether bound a central area which contains the fifth or axial structure 
 (cl. con.). Each of the squarish pieces also exhibits a line slightly concave 
 towards the axis of the ommatidium. This line, which might be taken 
 for the separation between the axial and peripheral structures, is in real- 
 ity entirely within the latter. That these are five separate structures 
 is indicated by the fact, that in transverse section, when for any reason 
 the elements have been broken apart, the separation almost always occurs 
 on the lines which 1 have described as the limits of the different pieces. 
 
 Evidently the squarish masses (rhb'm.) on the axial faces of the retinu- 
 lar cell correspond to the rhabdomeres of other Crustaceans, and like these 
 structures are produced by the cells to which they are attached. It is 
 more difficult to explain the axial element, for it shows no indication of 
 having been produced by the surrounding retinular cells, nor are there 
 other cells in the neighborhood to which its production could be 
 referred. 
 
 When the longitudinal extent of these structures is considered, the 
 difficulty of explaining the axial portion is increased. In S. Schythei 
 the rhabdomeres extend only a short distance distally and proximally, 
 but throughout the whole of that distance they are closely applied to 
 the axial face of the retinular cells. This condition has been well figured 
 by Watase (’90, Plate XXIX. Fig. 1), and supports the statement 
 already made that these bodies correspond to the rhabdomeres in other 
 Crustaceans. I have never observed a rhabdomere, such as that figured 
 by Beddard (’87, p. 234), in which the proximal half of the structure 
 is not in contact with the retinular cell. The axial part has a much 
 more considerable extent in a longitudinal direction than the rhab- 
 domeres. Apparently it is continued proximally into a fibrous bundle 
 which stretches towards the basement membrane, where according to 
 Beddard (’88, p. 449) it may terminate as a single fibre. 
 
 From what has just been stated it must be evident that the so called 
 rhabdome of Serolis consists of two sets of structures, one of which 
 includes the four rhabdomeres and the other the axial part with its prox- 
 imal fibrous prolongation. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 93 
 
 The development of these structures has been studied by Beddard 
 (’88, p. 450). In the youngest embryos which he examined, the axial 
 portion was already formed, and at that stage it was closely invested by 
 the four retinular cells and two other cells, the hyaline cells. Judging 
 from their positions, Beddard believes that both kinds of cells may con- 
 tribute to the formation of the axial structure, although the fact that 
 this body is squarish in transverse section leads him to conclude that the 
 four retinular cells play the more important part in its formation. Bed- 
 dard regards the axial body as the rhabdome of the immature eye. In his 
 opinion, the rhabdome in the adult is produced by subsequent secretions 
 from the retinular cells, and presents the form of the four rhabdomeres 
 already described. Although these rhabdomeres form the principal part 
 of the rhabdome in the adult eye, he believes that the rhabdome of the 
 earlier stages persists as the axial fibrous structure in the later stages, 
 and constitutes perhaps the greater part of its distal continuation 
 between the rhabdomeres. 
 
 Unless some such explanation of the origin of the axial part of the 
 rhabdome as that proposed by Beddard be accepted, it is difficult to 
 understand how the fibrous portion could arise as a secretion ; for in the 
 adult the proximal portion of it is touched by neither retinular nor 
 hyaline cells. 
 
 Granting for the moment the adequacy of Beddard’s explanation of 
 the origin of the axial part, we are still confronted by what appears to 
 me to be unparalleled in the structure of the eyes in Arthropods, namely, 
 an ommatidium which produces two distinct rhabdomes. This may not 
 be an impossibility, but if it occurs at all, it is certainly exceptional. 
 
 I believe, however, that th.e so called axial part of the rhabdome in 
 Serolis is capable of another interpretation, against which the objections 
 already suggested cannot be urged. That the axial portion terminates 
 proximally on the basement membrane has been fairly well established 
 by Beddard. The distal termination of it, however, has not been so 
 clearly made out. It is my belief that the axial structure is directly 
 continuous distally with the cone cells ; in other words, that this struc- 
 ture is to be regarded as a proximal extension of the cone cells, not as 
 a part of the rhabdome. The termination at the basement mem- 
 brane of this prolongation of the cone cells, as observed by Beddard, 
 is perfectly consistent with the interpretation which I have suggested, 
 and makes the condition in Serolis similar to that in Homarus, where 
 the fibrous ends of the cone cells also terminate on the basement mem- 
 brane. That the fibrous structure should be present in the embryo of 
 
94 
 
 BULLETIN OF THE 
 
 Serolis before the formation of the rhabdome proper is rather in favor 
 of my interpretation than opposed to it. The direct evidence that the 
 axial body is a proximal extension of the cone cells is not as conclusive 
 as could be desired. The condition which most favors this view is as 
 follows. In longitudinal and transverse sections of the ommatjdia, both 
 in adult and embryonic specimens, no line of separation has been observed 
 between the protoplasm at the deep end of the cone and the substance 
 which occupies the axial part of the ommatidium proximal to the cone 
 (compare Fig. 65). In attempting to determine the true relation, it is 
 important to keep clearly in mind the fact that the proximal end of the 
 cone, usually bounded by a sharply marked line, is not the proximal end 
 of the cone cells ; but, as Watase (’90, Plate XXIX. Fig. 1) has well shown, 
 the cone is surrounded proximally as well as laterally by the protoplasmic 
 material of its cells. It is this material, not that of the cone proper, 
 which forms the proximal elongation. 
 
 I had hoped that by isolating the elements of the retina I could ob- 
 tain more conclusive evidence of the connection of these parts, but my 
 efforts were of no avail. My ill success was due, I believe, not to any 
 want of connection between the structures treated, but to the fact that 
 the materia] at my disposal had been kept so long in strong alcohol that 
 it had become unfit to serve for isolation. This conclusion seems to me 
 to be confirmed by the fact that I was unable even to isolate satisfac- 
 torily the retinulse, structures which are usually separable with ease in 
 the fresh retinas of most Crustaceans. 
 
 If the view which I have set forth in the foregoing paragraphs con- 
 cerning the interpretation to be put upon the axial part of the so called 
 rhabdome of Serolis be correct, it follows that the true rhabdome of this 
 Crustacean must be considered as composed of four rhabdomeres, each 
 of which is applied to the axial face of its appropriate retinular cell, 
 and that these four rhabdomes are prevented from uniting with one 
 another by a proximal extension of the cone cells which occupies the 
 axis of the ommatidium from the cone to the basement membrane. 
 
 Beddard (’84 a , p. 21), in his account of the eye in S. Schythei, states 
 that the cone is “ enclosed in a sheath of deep black pigment cells,” and 
 Watase (’90, p. 294) has observed that in this genus there are two such 
 cells for each ommatidium. I believe that the number has been given 
 correctly, for although I have not satisfactorily isolated the cells, I feel 
 confident that I have identified their nuclei, and the number of these is 
 twice that of the ommatidia. 
 
 The nuclei of these pigment cells are most satisfactorily seen in ad- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 95 
 
 vanced embryos (compare nl. dst., in Figs. 65 and 69). In transverse 
 sections at this stage (Fig. 69) each cone is surrounded by a circle of 
 six nuclei. Each nucleus, however, participates in three adjoining cir- 
 cles, consequently there are only twice as many nuclei as ommatidia. 
 In the adult the nuclei of these pigment cells (Fig. 60, nl. dst.) occupy 
 the same relative positions as in the embryo ; in the latter, however, they 
 are usually somewhat hidden by the pigment which surrounds them. 
 
 In the embryo the nuclei of the pigment cells surrounding the cone 
 resemble very closely, except in point of size, the nuclei of the retinular 
 cells (compare nl. dst. and nl. px. in Fig. 65). In the nuclei of the 
 retinular cells there is usually one distinct nucleolus, sometimes two, but 
 as a rule no finer particles. This condition also obtains in the nuclei of 
 the pigment cells. Not only are the nuclei of these two kinds of cells 
 similar in the embryo, but they are also much alike in the adult (com- 
 pare nl. dst. in Fig. 60 with nl. rtnJ in Fig. 63). 
 
 Because of this resemblance, I believe that the pigment cells which 
 surround the cone can be fairly considered to be modified retinular cells, 
 which have lost their sensory function in precisely the same way as in the 
 case of the distal retinular cells in Decapods (see Parker, ’90 a , p. 57). If 
 this interpretation of the pigment cells be accepted, it follows that in 
 Serolis, as in Decapods, two kinds of retinular cells are present, proximal 
 and distal, and that the primitive ommatidimn from which that of Serolis 
 was derived probably contained six retinular cells functional as nervous 
 structures. It need scarcely be added, that this number is characteristic 
 for the ommatidia of many Isopods. 
 
 The retinula in the species* of Sphseroma which I studied presents 
 an appearance which suggests the differentiation of simple retinular cells 
 into proximal and distal cells. In Spheeroma there are seven retinular 
 cells (Plate V. Fig. 58) ; three of these are considerably reduced ; the 
 remaining four are large, and recall the four retinular cells of Serolis. 
 In transverse sections it can be shown that the four large cells in Sphse- 
 roma not only resemble in appearance the four proximal cells in Serolis, 
 but that they occupy the same relative positions in the ommatidium. 
 In Serolis the plane which separates the two cone cells of any given 
 cone, when extended, separates the four proximal retinular cells into two 
 groups of two cells each (compare Plate VI. Fig. 68 with Figs. 71 and 
 72). The plane of separation in the cone of Sphseroma divides the retin- 
 ula by passing through the single small retinular cell shown in the lower 
 part of Figure 58 (Plate V.) and between the two small cells on the oppo- 
 site side, thus separating the four large retinular cells into two groups, 
 as in Serolis. 
 
96 
 
 BULLETIN OF THE 
 
 The change which would convert an ommatidium like that in Sphse- 
 roma into one like that in Serolis is easily imagined. It would consist 
 in the complete abortion of one of the three small retinular cells, and the 
 conversion of the other two into the pigment cells surrounding the cone. 
 
 In addition to the elements which have already been described in the 
 ommatidium of Serolis, there are certain small pigment cells which oc- 
 cur for the most part in the region of the retinulae. Beddard (’84 a , 
 p. 21) describes these as long branching “connective-tissue cells,” a 
 name which might imply that they originated from the mesoderm, and 
 were therefore intrusive. Watase (’90, p. 293, Plate XXIX. Fig. 1) has 
 also described and figured these cells, but distinctly states his belief that 
 they are reduced ectodermic cells. In the adult I have observed in the 
 region of the cones, as well as near the retinulse, certain small nuclei 
 which are usually surrounded with more or less black pigment. These, 
 I believe, represent the cells described by Beddard and Watase. In the 
 embryo certain scattered nuclei ( nl . h’drm Figs. 65 and 70) occur in 
 the spaces between the ommatidia. It is probable that these nuclei are 
 ectodermic in origin, and I am at a loss to know what has become of 
 them in the adult, unless they form the pigment cells already men- 
 tioned. I am therefore inclined to believe, with Watase, that the small 
 additional pigment cells are reduced ectodermic cells. 
 
 The presence of the hyaline cells in the ommatidium of Serolis is, as 
 Beddard has pointed out, almost a unique feature. These cells, usually 
 two in each ommatidium, fill the space immediately below the rhabdome. 
 They are bladder-like (Fig. 62, cl. hyl.) and contain each a large gran- 
 ular nucleus. Although it is stated that there are usually two of these 
 cells in each ommatidium, I never found more than one to an ommatid- 
 ium in the several eyes of S. Schythei which I examined. This circum- 
 stance, however, is not surprising ; for, as Beddard (’84 a , p. 22) has 
 remarked, the number of these cells is subject to variation, there being 
 sometimes one, sometimes two, for each ommatidium. In S. Schythei 
 the single hyaline cell envelops more or less completely the distal part 
 of the fibrous portion of the cone cells, so that this part seems to pierce 
 the hyaline cell. A closer inspection, however, will usually show two 
 lines extending from the fibre to the periphery of the hyaline cell (com- 
 pare Fig. 62), and these lines indicate, I believe, the two walls of the 
 cell which have been infolded by the presence of the fibre during the 
 growth of the hyaline cell. 
 
 The source of the hyaline cells is not definitely known. Their nuclei 
 (Fig. 65, nl. hyl.), as Beddard (’88, p. 450) has observed, are present 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 97 
 
 in the retinas of embryos; and, although the cells may possibly be 
 intrusive, the evidence on the whole favors the view that they are 
 ectodermic in origin. 
 
 Several functions have been attributed to the hyaline cells. Their 
 close connection with what Beddard took to be the proximal extension 
 of the rhabdome led him (’88, p. 450) to suspect that they might be 
 rudimentary retinular cells, but, as he (p. 451) further remarks, the fact 
 that no nerve fibres are connected with them opposes this view. Their 
 transparency suggested to him (’84 a , p. 22) that they might form a part 
 of the dioptric apparatus ; but it is difficult to understand, consider- 
 ing their position, precisely what that function would be. I am inclined 
 to believe, with Watase (’90, p. 293), that they are chiefly concerned 
 with the support of the structures occupying the basal portion of the 
 retina. 
 
 In the retina of S. Schythei many of the open spaces between the 
 cones and the basement membrane contain free non-pigmented cells 
 (Fig. 61, cp. sng.). These have a distinct nucleus, finely granular pro- 
 toplasm, and a sharply marked outline. On account of the extreme va- 
 riations in form which the different cells present, it is probable that when 
 living they exhibited amoeboid motion. In appearance they correspond 
 exactly to the blood corpuscles of the body spaces, and as they occur not 
 only in the retina, but also in the rather large openings through the 
 basement membrane (compare Fig. 64), and in the space proximal to 
 this membrane, I am of opinion that they are blood corpuscles. 
 
 The peculiarities which have led me to consider the ommatidium 
 in Serolis separately from that of other Isopods, are two : the posses- 
 sion of one or more hyaline cells, and the presence of only four 
 retinular cells. The latter peculiarity, as I have already shown, is not 
 fully established : for in this genus, as in many other Isopods, the om- 
 matidium really contains six cells, although two of these, the distal ones, 
 are probably no longer functional as nervous structures. The other 
 peculiarity, the possession of hyaline cells, is not a very important char- 
 acteristic, for, as Beddard (’87, p. 235) has shown, these cells also occur 
 in iEga; and it is probable, moreover, that they must be regarded as 
 abnormally enlarged elements, specialized from among those cells which 
 in other Isopods fill the spaces between the ommatidia. What dis- 
 tinguishes the ommatidium in Serolis from that of other Isopods is, 
 therefore, not so much the possession of hyaline cells as the fact that 
 its retinular cells are differentiated into two sets, proximal and distal. 
 
 VOL. xxt — no. 2. 7 
 
98 
 
 BULLETIN OF THE 
 
 In accordance with the facts already presented, the number of cells 
 contained in the ommatidium of Serolis can be stated as follows : cells 
 of the corneal hypodermis, two, with possibly two others interomma- 
 tidial in position ; cone cells, two ; retinular cells, six, two distal and 
 four proximal ; hyaline cells, one or two ; a variable number of small 
 pigment cells of ectodermic (?) origin. 
 
 Leptostraca. 
 
 The histological structure of the ommatidia in the dSTebalise has been 
 investigated, so far as I am aware, only by Claus (’88, pp. 65-84). I 
 have had no material for the study of the eyes in these Crustaceans, 
 and I can therefore only present, in the form of a summary, the more 
 important results of Claus’s exhaustive study. 
 
 In Nebalia there is a corneal hypodermis (Claus, ’88, pp. 68 and 69), 
 the cells of which are grouped in pairs. As in many of the higher 
 Crustaceans, there is one pair of these cells for each ommatidium. The 
 corneal cuticula is facetted ; the outlines of the facets are circular, and ad- 
 joining facets are separated from one another by a small amount of inter- 
 vening cuticula (Claus, ’88, Taf. X. Fig. 10). The cones are composed 
 of four segments (Claus, ’88, p. 69). The structure of the retinula is 
 somewhat complex. The greater part of the rhabdome is surrounded 
 by seven retinular cells. Distal to these cells, however, are seven pig- 
 ment cells, which enclose the proximal prolongation of the cone cells and 
 the distal end of the rhabdome. Such a relation between pigment cells 
 and retinular cells is not of common occurrence among Crustaceans, and 
 it is possible that the bodies which Claus has taken for pigment cells are 
 really the distal ends of the retinular cells. Claus describes and figures 
 what he believes to be the nuclei of both kinds of cells, but I think 
 his figures fail to show that these nuclei are within the. limits of the 
 cells to which they are said to belong. It seems to me quite possible 
 that what he has described as two circles of seven cells each may 
 be merely one circle seen at two different levels, as the correspondence 
 in numbers suggests. This single circle would be of course composed 
 of retinular cells, the nuclei of which are probably the distal ones of 
 the two sets described by Claus. The proximal nuclei, which, accord- 
 ing to Claus, belong to the retinular cells, occupy positions not unfre- 
 quently taken by the nuclei of accessory pigment cells, and I am inclined 
 to think that such is their real nature. This interpretation would be 
 more in accordance with the conditions found in ommatidia which have 
 seven retinular cells than is the one given by Claus ; but as I have not 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 99 
 
 had the opportunity of studying the eyes in Nebalia, I can offer it 
 merely by way of suggestion. 
 
 Probably two kinds of accessory cells are present in Nebalia ; one of 
 these extends from the corneal cuticula to the basement membrane, the 
 Other, the presence of which is not so fully established, probably occurs 
 near the basement membrane. 
 
 Cumacece. 
 
 <4 
 
 Excepting what is contained in Burmester’s (’83, pp. 35-37) account 
 of the degenerate eyes in Diastylis (Cuma) Rathkii, nothing, I believe, 
 is known of the finer structure of the eyes in the Cumacese. The speci- 
 mens at my disposal for the study of these eyes proved upon examina- 
 tion to be blind. At least, the optic plate§ of all the individuals which 
 I examined, both when studied from the exterior and when examined 
 in sections, showed no evidence of eyes. My material consisted of 
 specimens of Diastylis quadrispinosa, G. 0. Sars, and of three other un- 
 determined species, two of which belonged to the genus Diastylis and 
 one to Eudorella. These were kindly sent me by Prof. S. I. Smith. 
 
 Schizopoda . 
 
 The species of Schizopod the eyes of which I have studied is Mysis 
 stenolepis, Smith. Specimens of this Crustacean were kindly collected 
 for me at Wood’s Holl, Mass., by Mr. C. B. Davenport. I am also 
 under obligations to Dr. H. V. Wilson, of the United States Fish Com- 
 mission, who at my request sent me specimens of this species freshly 
 preserved in Muller’s fluid. 
 
 In several of the previous accounts of the eye in Mysis the nuclei 
 of the corneal hypodermis, although recognized, have been described as 
 Semper’s nuclei, i. e. as nuclei of the cone cells. The differences between 
 the hypodermal nuclei and those of the cone cells can be easily seen in 
 Mysis stenolepis (Plate VII. Fig. 73). In this species the hypodermal 
 nuclei ( nl . cm.) lie in a plane somewhat nearer the external surface of 
 the eye than the nuclei of the cone cells ( nl . con.). In transverse sec- 
 tions at the proper levels, each ommatidium will be seen to contain two 
 elongated nuclei (Fig. 75, nl. cm.) belonging to the corneal hypodermis, 
 and two oval nuclei (Fig. 76, nl. con.) in the cone. The hypodermal 
 nuclei occupy such positions that the plane of separation between their 
 cells would be at right angles to that between the cone cells (compare 
 Figs. 75 and 76). The group of four nuclei, two belonging to the corneal 
 
100 
 
 BULLETIN OF THE 
 
 hypodermis, and two to the cone cells, correspond without much doubt 
 to the so called four Semper’s nuclei mentioned by Clapar&de (’60, 
 p. 194) in Mysis flexuosa, and described by Sars (’67, p. 33) in M. ocu- 
 lata. Nusbaum (’87, p. 179) also observed four similar nuclei in the 
 developing eye of Mysis chameleo, and Grenacher (’79, p. 118) described 
 the same number in Mysis vulgaris. In the last named species, accord- 
 ing to Grenacher, the four nuclei are grouped in two pairs, one of which 
 occupies a more distal plane in the ommatidium than the other. The 
 more superficial pair undoubtedly belongs to the corneal hypodermis, the 
 deeper pair to the cone cells. 
 
 It must be evident, then, that the nuclei of the cone cells and corneal 
 hypodermis have not always been carefully distinguished. In all cases 
 where they have been separated, the corneal hypodermis has been shown 
 to possess two nuclei for each ommatidium. 
 
 The corneal cuticula in Mysis, as Frey and Leuckart (’47 a , p. 113) 
 first pointed out, is facetted, and the outline of the facet is a circle. 
 In Mysis stenolepis the circumference of the facet is tangential to the 
 circumferences of six adjoining facets (Fig. 74). In Mysis vulgaris, 
 Grenacher (’79, p. 118) has shown that the facet is not lens-like, but is of 
 uniform thickness throughout. In M. stenolepis, however, the cuticula 
 is often slightly thicker at the middle of the facet than at its edges (Fig. 
 73, eta.). In this respect, therefore, different species probably vary. 
 
 The cones in Mysis vulgaris, according to Grenacher (’79, p. 118), are 
 composed of two segments. The same number is also present in the 
 cones of M. stenolepis (compare Figs. 76-78, con.). In longitudinal sec- 
 tions the cone (Fig. 73, con.) appears to consist of a uniformly and finely 
 granular substance enveloped in a delicate but distinct membrane. 
 Near the distal end of the cone the material which composes it becomes 
 more coarsely granular ; in this the nucleus of the cone cell is usually 
 lodged. Cones (Fig. 92) which have been isolated in macerating fluids 
 are plumper and apparently not so contracted as those which have been 
 subjected to the process of cutting. The nuclei also are rounder and fuller. 
 The cone proper (Fig. 92 con.) occupies a more central position in the 
 cone cells, and is surrounded by a finely granular material, which is es- 
 pecially abundant at the proximal end. The difference between the cone 
 proper and this granular material was not generally observable in sections 
 of the cones. In all of the many cones which I succeeded in isolating, 
 the proximal ends invariably had a broken appearance. Consequently, I 
 believe that I have never completely isolated a pair of cone cells. The 
 question of the proximal extent of the cone I shall recur to later. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 101 
 
 The retinular cells in Mysis are of two kinds, proximal and distal. 
 The proximal cells extend from the basement membrane distally to 
 the level at which the cone rapidly contracts. The pigment which they 
 contain is for the most part concentrated around the rhabdome, and 
 their nuclei occupy a distal position in the cell (Fig. 73, nl.px.). 
 
 In Mysis the number of cells comprising the ret inula is at least seven 
 (Figs. 85-87). Possibly, as I have elsewhere suggested (Parker, ’90 a , 
 p. 55), the total number of cells in this retinula, as in that of Homarus, 
 may be eight. 
 
 In order to determine this question, I have counted the number of 
 nuclei in several retinulae of Mysis. The enumeration of these can be 
 easily followed in Figures 79 to 82. These figures represent successive 
 transverse sections through four ommatidia, in the region occupied by 
 the proximal retinular nuclei. The axis of each ommatidium is marked 
 by the fibrous portion of the cone cells (cl. con.), and the same omma- 
 tidium is designated in different sections by the same Roman numeral. 
 The nuclei in ommatidium II. can be counted the most readily. In 
 Figure 79, which represents the most distal section of the series, the 
 cone in ommatidium II. is surrounded by a circle of six nuclei, which 
 have been numbered from 1 to 6. Each of these nuclei, however, par- 
 ticipates in three circles (compare nucleus 5), and hence only two of the 
 six can be referred to ommatidium II. Two similar circles occur, one 
 in the sections shown in Figure 80, and one in that shown in Figure 81. 
 As in the former instance, two nuclei in each circle belong to omma- 
 tidium II. In these three circles, then, there are in all six nuclei to be 
 allotted to ommatidium II. In addition to these nuclei, it will be no- 
 ticed that to the right of the cone in Figure 80 there is one more 
 nmcleus (No. 7), and still another in a similar position in Figure 82. 
 These two nuclei, when added to the six already summed up for om- 
 matidium II., make a total of eight nuclei for this ommatidium. 
 
 The same number of nuclei occurs in each of the other three omma- 
 tidia, but their arrangement is not quite so regular as in the one just 
 counted. From this I conclude that the number of nuclei in a retinula 
 of Mysis is eight. 
 
 The different nuclei in this retinula usually present a very uniform 
 appearance. The most proximal one differs somewhat from the others 
 . in being more elongated (compare Figs. 73 and 82). The seven distal 
 nuclei, on account of their general resemblance, belong, I believe, to the 
 seven functional retinular cells. The single proximal nucleus probably 
 represents an eighth rudimentary cell. The position of this nucleus, 
 
102 
 
 BULLETIN OF THE 
 
 proximal to the other retinular nuclei, is similar to that occupied by the 
 nucleus of the rudimentary retinular cell in Homarus (compare Parker, 
 ’90% pp. 20, 21). 
 
 The rhabdome in Mysis stenolepis lies in the proximal portion of the 
 retina. It is rather stout, blunt at its distal end, but sharper proxi- 
 rnally (Fig. 90). Its surface is marked with coarse corrugations. In 
 transverse section, its outline is a square ; this is subdivided by two 
 lines into four smaller squares, a condition already observed by Grena- 
 dier (79, p. 119) in M. flexuosa. The relation of the retinular cells 
 to these divisions of the rhabdome can be clearly seen in Figure 87. 
 
 According to Grenacher’s account (’79, p. 118), a rod-like structure 
 extends, in Mysis vulgaris and M. flexuosa, through the axis of the 
 ommatidium from the distal end of the rhabdome to the region of the 
 proximal retinular nuclei. Whether this rod be a proximal continuation 
 of the cone, or a distal extension of the rhabdome, Grenadier found it 
 difficult to decide. He is inclined, however, to the former opinion. 
 
 A similar structure occurs in the ommatidia of Mysis stenolepis. 
 Although I have made repeated attempts, I have never succeeded in 
 isolating the rod in connection with either the rhabdome or the cone 
 cells. In transverse sections, the distal end of it appears in a position 
 slightly proximal to the retinular nuclei (Figs. 73 and 83). The cone 
 cells extend proximally as a transparent axis to this region, and the 
 most distal indications of the rod are four fibres which lie on the 
 periphery of what I take to be the proximal end of the cone cells 
 (Fig. 83). Somewhat deeper than this, the four fibres thicken, and 
 finally fuse (Fig. 84), producing a body which in transverse section has 
 the outline of a four-pointed star. In a plane slightly more proximal, 
 the outline changes to a squarish one (Fig. 85), and this is retained 
 almost to the proximal end of the rod. Throughout its extent, this 
 problematic rod is closely surrounded by the seven proximal retinular 
 cells (Fig. 85). It is separated from the rhabdome by what appears to 
 be an open space (Fig. 90, at the level of the dotted line 86). In trans- 
 verse sections (Fig. 86), however, this space is seen to be divided by 
 delicate membranes into four compartments. 
 
 These facts, however, do not aid much in deciding the relationship 
 of the rod. The fact that it shows indications of being composed of 
 four parts suggests its connection with the rhabdome. The four parts • 
 of which it consists do not, however, correspond in position to the seg- 
 ments of the rhabdome, but fall between them. (Compare Figs. 83 and 
 87.) On the other hand, if.it were an extension of the cone, one would 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 103 
 
 expect it to be composed of two, instead of four parts. Its position, how- 
 ever, is one which is more frequently occupied in other Crustaceans by 
 a slender extension of the cone cells than by a process from the rhab- 
 dome, and, notwithstanding its division into four parts, I am inclined 
 to agree with Grenacher, and to regard it as belonging to the cone cells 
 rather than the rhabdome. 
 
 The distal retinular cells in Mysis surround the lateral faces of the 
 cones (Fig. 73, cl. dst .). Apparently they reach the cuticula . their 
 proximal ends are attenuated and become lost in the region of the 
 nuclei of the proximal cells. Their pigment is limited to their proximal 
 halves, and consists of a distal layer of brownish material, proximal to 
 which is a much more extensive deposit of blackish granules. Each cone 
 is surrounded by six of these cells, as can be seen from their outlines 
 (Fig. 78, cl. dst.), and still more satisfactorily from the arrangement of 
 their nuclei (Fig. 75, nl. dst.). Each cell, however, participates in three 
 circles ; consequently, there are only twice as many of these cells as 
 ommatidia. 
 
 The axis of each distal retinular cell is occupied by a transparent 
 rod, which in transverse section has the appearance of a light spot 
 (Fig. 77). In depigmented sections stained with Kleinenberg’s hema- 
 toxylin, these rods are deeply colored (Fig. 78). I shall recur to their 
 probable significance. 
 
 The pigment which is found in the region of the rhabdomes in Mysis 
 is of two kinds : blackish granules, and a fine flaky material, white by 
 reflected light, yellowish by transmitted light. The black granules are 
 for the most part contained in the retinular cells. The lighter pigment 
 is always associated with certain nuclei, two of which are shown in 
 Figure 90 {nl. ms’dr'm.). These nuclei are closely invested by the pig- 
 ment, and probably belong to the cells in which the pigment is con- 
 tained. 
 
 The source of the yellowish pigment cells is not easily determined. 
 Apparently they are not limited to the retina, but also occur in the 
 spaces below it. At least these spaces contain masses of pigment and 
 nuclei which in all essential respects are similar to those distal to the 
 membrane (compare the two nuclei, nl. ms'drm., Fig. 90). In one case 
 the nucleus of one of these cells was found apparently caught in its 
 passage through an opening in the basement membrane (Fig. 91). For 
 these reasons I believe that the yellowish pigment cells on the two sides 
 of the membrane have had the same origin. The question as to the 
 source of the yellowish pigment cells in the retina, therefore, appears 
 
104 
 
 BULLETIN OF THE 
 
 to me to involve that of the origin of the similar cells beneath the 
 retina. If I am right in this conclusion, all these cells must either have 
 arisen in the retina, many of them migrating in a proximal direction 
 out of it, or they must have had some extra-retinal origin, some of them 
 migrating into it. On account of the considerable numbers in which they 
 exist in the spaces below the retina, it seems to me much more probable 
 that they have had an extra-retinal origin than that they have come 
 from the retina itself. If this is their source, it is evident that those 
 which are in the retina are intrusive. The nucleus which has already 
 been mentioned as caught in an opening of the basement membrane 
 (Fig. 91) has more the appearance of a body which is making its way 
 into the retina than of one which is moving in the reverse direction, 
 and may therefore be regarded as confirming to some extent the view 
 of the extra-retinal origin of these cells. Their source, however, cannot 
 be stated with certainty. Their power of migration implies amoeboid 
 activity, and this might be taken as an indication of their mesodermic 
 origin. 
 
 The following cells characterize the ommatidium of Mysis : cells of 
 the corneal hypodermis, two : cone cells, two ; proximal retinular cells, 
 eight, one of which is rudimentary ; distal retinular cells, two ; accessory 
 pigment cells (mesodermic 1) present. 
 
 Stomatopoda. 
 
 The material which I have had for the study of the eyes in the Stoma- 
 topods consisted of two specimens of Gonodactylus chirarga, Latr. These 
 were kindly given me by Mr. W. S. Wadsworth, who had collected them 
 in the Bermudas. One of them had been killed in hot water and pre- 
 served in alcohol ; the other was both killed and preserved in strong 
 alcohol ; both were in excellent histological condition. 
 
 In Gonodactylus, as I have previously mentioned, there are two kinds 
 of ommatidia ; these differ in no important respect except size. 
 
 Longitudinal sections of both kinds are represented on Plate VIII. ; 
 the figure of the larger kind (Fig. 94) is taken from a depigmented sec- 
 tion, that of the smaller one (Fig. 95) from a section containing the 
 pigment in its natural condition. In the following description I shall 
 give an account of the structure of the larger ommatidia, alluding to 
 the condition of the smaller ones only when it differs in some important 
 respect from that of the others. 
 
 The corneal hypodermis is represented in the ommatidium of Go- 
 nodactylus by two cells, the nuclei (Figs. 94—96, nl. cm.) of which can 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 105 
 
 be recognized easily. Directly under the corneal cuticula each pair of 
 hypodermal cells is in contact with similar pairs belonging to adjoining 
 ommatidia, so that the layer here forms a continuous sheet. In a more 
 proximal plane the neighboring pairs of hypodermal cells are not in con- 
 tact (compare Fig. 93, a tangential section in which the extreme right- 
 hand edge represents the condition immediately below the cuticula, while 
 the parts to the left represent central portions successively more proxi- 
 mal in position). The only indication of a separation between the two hy- 
 podermal cells of each pair is seen in the distal projection of the cone 
 between the two hypodermal nuclei (compare Figs. 94 and 96, con.). 
 
 The corneal cuticula in Gonodactylus is facetted, but the proximal and 
 distal faces of the facets are apparently plane. Over the smaller om- 
 matidia the facets are hexagonal in outline, whereas over the larger ones 
 they are rectangular, and their arrangement is often indicative of the 
 tetragonal system. In Squilla mantis, according to Will (’40, p. 7), the 
 facets are hexagonal. 
 
 The cones in Gonodactylus are composed for the most part of a uni- 
 formly granular substance. Distally, they are pointed and probably 
 touch the corneal cuticula ; proximally, they terminate at the rounded 
 end of the rhabdome (Fig. 94). Each cone contains in its distal enlarge- 
 ment four nuclei (Fig. 97, nl. con .), two of which lie directly proximal 
 to the nuclei of the corneal hypodermis, while the remaining two alter- 
 nate with them (compare Figs. 96 and 97). The proximal part of the 
 cone is divided longitudinally into four segments (Fig. 98). Each seg- 
 ment, if extended distally, would include one of the four nuclei, and 
 corresponds to one of the four cells by which the cone was produced. 
 In Squilla mantis, according to Steinlin (’68, p. 17), the cone is also 
 composed of four segments. 
 
 The retinular cells of Gonodactylus are of two kinds, proximal and 
 distal. The proximal cells, constituting the retinula itself, surround the 
 rhabdome completely, and extend distally only a short distance beyond 
 it (Fig. 95). They contain only a small amount of pigment, which is 
 concentrated in two regions, at their distal ends and near the basement 
 membrane. The rhabdome is surrounded throughout its length by a 
 thin but rather dense layer of pigment. This layer is more extensive 
 in the smaller ommatidia (Fig. 102) than in the larger ones. The 
 nuclei of the proximal retinular cells (Figs. 94 and 95, nl. px.) are 
 located near their distal ends. 
 
 The number of cells in the retinula of Squilla, as described by Grena- 
 dier (’77, p. 33) and by Hickson (’85, p. 341, Fig. 2), is seven. In 
 
106 
 
 BULLETIN OF THE 
 
 Gonodactjdus (Fig. 101) the retinular cells are certainly as numerous 
 as in Squilla ; but seven obvious cells in the retinula, as I have already 
 shown in Mysis, may suggest the presence of eight in all, one of them 
 being rudimentary. This condition is in fact characteristic of Gonodac- 
 tylus also, as can be seen in the series of ommatidia shown in Fig. 100. 
 These six ommatidia represent consecutive individuals in one of the 
 bands of larger ommatidia previously mentioned. The band as a whole 
 is cut obliquely, and in such a way that the ommatidia from 1 to 6 are 
 cut successively in deeper or more proximal planes. In ommatidium 1 
 the rhabdome is surrounded by seven retinular cells, four of which are 
 upon the right side and three upon the left. In addition to these, a 
 large nucleus ( nl . px.) lies close to the rhabdome. Ommatidium 2 has 
 essentially the same structure as ommatidium 1. In ommatidium 3 the 
 nucleus corresponding to the one seen in ommatidium 1 and 2 is no longer 
 visible, but in its stead there is a small mass of granular protoplasm. 
 A similar mass is also seen in ommatidia 5 and 6. It is usually pres- 
 ent directly proximal to the nucleus figured in ommatidia 1 and 2, and 
 is, I believe, the protoplasmic body of the cell to which this nucleus 
 belongs. In ommatidium 4, the seven nuclei of the seven large (func- 
 tional) retinular cells can be seen. These nuclei appear very large in 
 transverse section compared with the cells in which they occur. It is 
 probable that the cell wall is distended by them, although, owing to the 
 indistinctness of the cell boundaries, I have not obtained positive evi- 
 dence of this. In ommatidium 6 the seven retinular cells are seen in 
 section at a plane proximal to that in which their nuclei lie. As in 
 ommatidium 1, three of them are upon one side of the rhabdome 
 and four upon the other. In a part of the ommatidium more proxi- 
 mal than that shown in number 6 (Fig. 100), the transverse section 
 of the retinula has the appearance seen in Figure 101. Here the 
 retinular cells have the same relation to the rhabdome that they do in 
 ommatidium 6 (Fig. 100), except in the case of the upper right-hand 
 cell of that figure. This cell enlarges in its more proximal portion, and 
 comes to occupy a position directly below the cell whose nucleus is shown 
 in ommatidium 1 (Fig. 100). The gradual disappearance of this distal 
 cell as one proceeds in a proximal direction from the plane of number 
 6, Figure 100, to that of Figure 101, and the gradual shifting in the 
 position of the cell which replaces it proximally, can be followed so 
 easily that there is not the least question as to the accuracy of the 
 relations described. It is evident, then, that in Gonodactylus, as in Mysis, 
 the retinula consists of eight cells, one of which is rudimentary. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 107 
 
 The rhabdome (Figs. 94 and 95, rhb.) in Gonodactylus is an elongated 
 rod-like structure of uniform thickness, which extends from the region 
 of the proximal retinular nuclei to the basement membrane. It shows 
 a distinctly toothed edge (Fig. 94), especially in specimens which have 
 been treated with potassic hydrate. In transverse section it is squarish. 
 Owing to its small size, the exact relation of the seven surrounding cells 
 to its four faces cannot be easily determined. The single unpaired cell 
 (Fig. 101) certainly lies opposite a face, hot an angle. In this respect 
 it agrees with the unpaired cell in Squilla as figured by Grenacher (79, 
 Taf. XI. Fig. 122). Probably in Gonodactylus the remaining six cells 
 are related to the sides of the rhabdome as the corresponding ones are 
 in Squilla (compare Grenacher’s Fig. 122). In Gonodactylus the retinu- 
 lar cells and rhabdome are in close contact with one another. The 
 separation of these elements as figured by Grenacher in Squilla is prob- 
 ably artificial, as Grenacher himself suggests. In Squilla, according to 
 both Steinlin (’68, p. 17) and Grenacher (79, p. 125), the rhabdome 
 in transverse sections is subdivided into four equal parts, somewhat as 
 in Mysis. I have not observed this condition in Gonodactylus. 
 
 The distal retinular cells in Gonodactylus occupy the usual position 
 near the cones. They contain very little pigment, and their number 
 can be determined only by that of their nuclei. These agree with the 
 nuclei of the proximal cells in the possession of a single well defined 
 nucleolus, which is most readily seen in depigmented sections (compare 
 nl. dst. and nl. px. in Fig. 94). The distal nuclei, especially in the 
 region of the larger ommatidia, are arranged in rows which alternate 
 with the rows of cones (Fig. 99, nl. dst.). Although the nuclei are not 
 very definitely arranged, they often show a tendency to be grouped in 
 pairs, and these pairs are so placed that in each row there is evidently 
 one for each adjacent ommatidium. Moreover, in equal lengths of ad- 
 joining rows of nuclei and cones, the nuclei are always double the num- 
 ber of cones. I am convinced by these facts that there are two distal 
 retinular cells for each ommatidium. 
 
 Besides the cells already described, certain others occur in the proxi- 
 mal part of the retina in Gonodactylus. These are represented by a 
 few small, elongated nuclei (Fig. 94, nl. ms’drm.), which are very similar 
 in appearance to certain nuclei occurring in the spaces below the base- 
 ment membrane. I therefore believe that in Gonodactylus, as in Mysis, 
 the proximal portion of the retina is occupied by intrusive cells, which 
 are probably mesodermic in origin. 
 
 The kinds of cells found in the ommatidium of Stomatopods are as 
 
108 
 
 BULLETIN OF THE 
 
 follows : cells of the corneal hypodermis, two ; cone cells, four ; proxi- 
 mal retiuular cells, eight, one of which is rudimentary ) distal retinular 
 cells, two ; accessory cells (mesodermic 1) present. 
 
 Decapoda. 
 
 I have studied the eyes of the following species of Decapods : Gelasi- 
 mus pugilator, Latr. ; Cardisoma Guanhumi, Latr. ; Cancer irroratus, 
 Say ; Hippa talpoida, Say ; Palinurus Argus, Latr. ; Pagurus longicarpus, 
 Say ; Homarus americanus, Edw. ; Cambarus Bartonii, Eabr ; Crangon 
 vulgaris, Fabr. ; and Palsemonetes vulgaris, Say. I collected much of 
 this material at the Station of the United States Fish Commission at 
 Wood’s Holl, Mass. The specimens of Cambarus were obtained in the 
 vicinity of Philadelphia. I am under obligations to Mr. Herbert M. 
 Eichards for specimens of Palsemonetes collected by him at Newport, 
 E. I. A number of eyes of two Crustaceans, Cardisoma and Palinurus, 
 were kindly obtained for me by Mr. Isaac Holden ; they were collected 
 on the coast of Florida by Mr. Ealph Munroe, to whom I am indebted 
 for the careful way in which they were preserved. 
 
 The corneal hypodermis in Decapods was first recognized by Patten 
 (’86, pp. 626 and 642), who observed it in Penaeus, Palsemon, Pagurus, 
 and Galathea. Since Patten’s announcement of the presence of this 
 layer in Decapods, it has been identified in a number of other genera : 
 in Crangon by Kingsley (’86, p. 863), in Alpheus by Herrick (’86, p. 43), 
 in Astacus by Carriere (’89, p. 225), in Cambarus and Callinectes by 
 Watase (’90, pp. 297 and 299), and in Homarus by myself (’90 a , p. 6). 
 More recently I have observed it also in Palsemonetes (Plate IX. Fig. 
 103, cl cm.), Crangon, Cambarus, Palinurus, Pagurus, Hippa, Cancer, 
 and Cardisoma. 
 
 In almost all Decapods in which the arrangement of the cells in the 
 corneal hypodermis has been observed, these elements have been 'found 
 to be grouped in pairs, and so distributed that each pair occupies the 
 distal end of an ommatidium (compare Figs. 103 and 106, Plate IX.). 
 This arrangement has been observed, either by others or by myself, in 
 the genera mentioned in the preceding paragraph, except Callinectes, 
 in which the exact arrangement of the cells has not been recorded. 
 Eeichenbach’s statement (’86, p. 91), that in Astacus there are four 
 hypodermal cells under each facet, is probably erroneous, as Carriere’s 
 observations show. 
 
 Although Patten was the first investigator who clearly demonstrated 
 the presence of the corneal hypodermis in Decapods, Grenacher, in 1879, 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 109 
 
 described, I believe, the nuclei of this layer, without however correctly 
 interpreting them. In his account of the ommatidium in Palsemon, 
 Grenacher (’79, p. 123) mentions two kinds of bodies in what he takes 
 to be the distal ends of the cone cells. Of these, the more distal ones 
 (Taf. XI. Fig. 117, n.) represent, in his opinion, the nuclei of the cone 
 cells; the more proximal (Fig. 117, Kk l .) he considers as differentiated 
 parts of the cone itself. The positions occupied by these bodies in 
 Palsemon, and by certain bodies which I have observed in Palsemonetes 
 (Plate IX. Fig. 103), are so similar that I believe the structures in the 
 two genera to be homologous. In Palaemonetes the distal bodies lie in the 
 cells of the corneal hypodermis (Fig. 103 cl. cm.), and are the nuclei of 
 these cells. They represent what Grenacher considered the nuclei of the 
 cone cells in Palaemon. The proximal bodies in Palaemonetes (Fig. 103, 
 nl. con.) are unquestionably the nuclei of the cone cells, yet they corre- 
 spond to what Grenacher considered the four pieces of the distal segment 
 of the cone. I therefore believe that what Grenacher has described as the 
 nuclei of the cone cells are really the nuclei of the corneal hypodermis, 
 and that what he considered distal segments of the cone are the nuclei 
 of the cone cells. 
 
 The corneal cuticula in Decapods, in correspondence with the differ- 
 entiated condition of the corneal hypodermis, is facetted. The outline 
 of the facets is either hexagonal or square. The particular genera in 
 which these different kinds of facets occur have already been mentioned 
 in dealing with the arrangement of the ommatidia in Decapods. The 
 faces of the facets in Decapods are usually very nearly plane, but in 
 Palsemon according to Grenacher (’79, p. 123), and in Palaemonetes 
 (Plate IX. Fig. 103, cm.) according to my own observations, the facets 
 are slightly biconvex. In Homarus, as Newton (73, p. 327) has ob- 
 served, and in Astacus according to Carriere (’85, p. 167), the distal 
 surface of the facet is plane, the proximal slightly convex. In even 
 the most extreme cases, however, the convexity of the facets in Decapods 
 is not sufficient to make them very effective as lenses. 
 
 The facets in Decapods are generally bisected by a fine straight line. 
 This line, as Patten has suggested, probably represents the plane of 
 separation between the two subjacent hypodermal cells. In the square 
 facets this line either divides the facet diagonally, as in Homarus 
 (Parker, ’90 a , Fig. 2), or transversely, as in Palaemonetes (Plate IX. 
 Fig. 105). In the hexagonal facets it either bisects opposite sides, as in 
 Cancer (Plate X. Fig. 126), or unites opposite angles, as occasionally in 
 * Galathea (Patten, ’86, p. 644, Plate 31, Fig. 114). Leydig’s (’57, p. 252, 
 
110 
 
 BULLETIN OF THE 
 
 Fig. 134) figure of Astacus, in which each facet is subdivided by two 
 diagonal lines into four areas, and Newton’s (’73, p. 327) statement 
 that the same condition occurs in Homarus, are probably incorrect. 
 
 The cones in Decapods are composed of four segments. This number 
 was first observed by Will (’40, p. 13) in Palsemon, and has since been 
 recorded in many other genera. So far as I am aware, there are no 
 Decapods in which the number of segments is not four. As Claparede 
 (’60, p. 194) first pointed out in Galathea and Pagurus, each segment 
 contains a nucleus and represents a single cell. Although the signifi- 
 cance of these nuclei was without doubt first fully appreciated by 
 Claparede, it is probable that they were previously seen by Leydig 
 (’55, Taf. XVII. Fig. 31) in the crayfish. 
 
 As a rule, the distal termination of the cone cells is on the proximal 
 side of the corneal hypodermis. In the lobster, however, and in Palae- 
 monetes (Plate IX. Fig. 104), the pointed ends of these cells pass 
 between the two cells of the corneal hypodermis, and probably come 
 in contact with the corneal cuticula near the middle of a facet. 
 
 It is difficult to determine with accuracy the proximal termination of 
 the cone cells. They can be easily traced to a region immediately distal 
 to the distal end of the rhabdome. In this region, as Schultze (’68, 
 Taf. I. Figs. 9 and 11) has clearly demonstrated in Astacus, the fibrous 
 ends of the four cone cells separate, and pass partially around the rhab- 
 dome. In Homarus, these fibres extend proximally, and finally ter- 
 minate at the basement membrane. A similar method of termination 
 also occurs in Palinurus. In the other genera which I have studied, the 
 fibres, although visible near the distal end of the rhabdome, are lost in 
 the adjacent tissue, and I do not know whether they terminate in this 
 tissue without special attachment, or whether they make their way as 
 excessively fine fibres to the basement membrane. The separation of 
 the fibrous ends of the cone cells, near the distal end of the rhabdome, 
 has been observed by Steinlin (’66, p. 93) in Palmmon, and by Schultze 
 (’67 and ’68) in several other Decapods. The statement made by many 
 of the older investigators, and recently reaffirmed by Patten, that the 
 cone and rhabdome are parts of one continuous structure, is without 
 doubt incorrect. 
 
 The resolution of the retinula into its cellular constituents was first 
 attempted in Decapods by Leydig (’55, p. 408), according to whom the 
 retinula of Herbstia contains four cellular bodies, the nuclei of which 
 can be distinguished in the distal part of the structure. A somewhat 
 similar condition was described by Newton (’73, p. 333) for Homarus ; 
 
MUSEUM OF COMPARATIVE ZOOLOGY. Ill 
 
 in this genus, as in Herbstia, it was maintained that there were only 
 four cells. Subsequent investigators have not confirmed this conclusion. 
 In transverse sections of the retinula of Palaemon, Grenacher (’77, p. 32) 
 has demonstrated that the rhabdome is surrounded by seven retinular 
 cells. He also (77, p. 33, and 79, p. 125) observed the same number 
 in the retinulae of Astacus and Portunus. Since the publication of 
 Grenadier’s observations, a retinula containing seven cells has been 
 seen in Astacus by Carriere (’85, p. 169), in Penaeus, Palaemon, Gala- 
 thea, and Pagurus by Patten (’86, pp. 630 and 643), and in Cambarus 
 by Watase ('*90, p. 299). 
 
 In Homarus, as I (’90 a , p. 21) have already shown, the retinula con- 
 tains, in addition to the seven functional retinular cells, an eighth rudi- 
 mentary one, which is little more than a nucleus. In order to ascertain 
 the presence or absence of this eighth cell in other Decapods, I have 
 been careful to record the number of retinular nuclei, as well as the 
 number of functional retinular cells. In some genera, such as Cardisoma 
 and Hippa, I have not been able, on account of the unfavorable condition 
 of the tissue, to make this determination ; but in Palaemonetes, Palinurus, 
 Cambarus, Crangon, and Cancer, I have succeeded in ascertaining the 
 number both of the functional cells and of the nuclei in the retinulae. 
 
 In Palaemonetes each rhabdome is surrounded by at least seven re- 
 tinular cells (Plate IX. Fig. 114, cl. px .). The nuclei of these cells 
 usually lie slightly distal to the rhabdome (Fig. 104, nl. px.). Their 
 arrangement is shown in Figures 110, 111, and 112, which represent a 
 series .of consecutive sections through the region occupied by the prox- 
 imal retinular nuclei of five ommatidia. The nuclei of the different 
 ommatidia are arranged upon the same plan, and the corresponding 
 nuclei in the different sets have been marked by the same number. In 
 several instances, nuclei have been cut in two, and their parts are found 
 in consecutive sections ; in such cases the separate portions have been 
 marked with the same number. As can be seen in these figures, 
 the number of nuclei in the distal portion of each retinula is seven. 
 But in addition to these, there is also another one, which occupies a 
 position near the rhabdome. This nucleus resembles the others in all 
 respects except that it is somewhat longer and narrower. It is drawn 
 in Figure 103 at the level marked 114, and in Figure 114 one can see 
 the regularity with which it occurs. This nucleus is the eighth in the 
 retinula of Palaemonetes, and since it differs somewhat in structure from 
 the other seven, and occupies a more proximal position, I believe it rep- 
 resents a rudimentary retinular cell. 
 
112 
 
 BULLETIN OF THE 
 
 la the distal portion of the retinula in Cambarus there are eight 
 nuclei. The arrangement of these, as seen in successive transverse 
 sections, is shown in Plate X. Figs. 118 to 122. In Figure 118, which 
 represents the most distal section of the series, there are four nuclei, 
 and these are so arranged that there is evidently one for each omma- 
 tidium. 1 In the next section (Fig. 119) there are seven nuclei, none' 
 of which were seen in Figure 118 ; the place for an eighth is indicated 
 by an open area, and the eighth nucleus itself is seen somewhat out of 
 place in Figure 120 (x). Four of the eight nuclei belonging in Figure 
 119 are arranged in a manner similar to those in the preceding sec- 
 tion, but are not to be confounded with them. The remaining four 
 are so placed that there are two for each ommatidium. Hence in this 
 plane there are, as a whole, three times as many nuclei as there are 
 ommatidia. In the next section (Fig. 120), omitting the nucleus 
 marked x , which has been recorded as belonging to the preceding 
 section, there are four nuclei, so arranged that there is one for each 
 ommatidium. In the following section (Fig. 121) the nuclei, omit- 
 ting the one marked x, which will be considered as belonging to the 
 next following section, are so arranged that there are two for each 
 ommatidium. In the last section (Fig. 122), the nuclei are not so 
 regularly grouped as in the previous section, but when taken with the 
 nucleus marked x in Figure 121, they constitute a group of four, the 
 arrangement in which is such that each nucleus is intermediate between 
 four groups of cone cells rather than between two , and therefore in the 
 plane of this section there is one nucleus for each ommatidium. . From 
 this enumeration it is evident that the total number of retinular nu- 
 clei is eight ; namely, one in the first section, three in the second, one 
 in the third, two in the fourth, and one in the fifth. The structure 
 
 1 The nuclei shown in Figures 118 to 122 are arranged upon either the plan 
 shown in Figure 118 or that in Figure 121 (omitting nucleus x). Imagine the 
 arrangement in Figure 118 extended over a large surface. The groups of four 
 cone cells could then be regarded as forming lines in the direction of the length 
 of the plate. These lines would alternate with lines of nuclei, and as the nuclei 
 in any line would alternate with the groups of cone cells in an adjoining line, the 
 number of nuclei must equal exactly the number of groups of cone cells ; i. e. in 
 this arrangement there is one nucleus for each ommatidium. In a similar way, 
 alternating vertical lines may be constructed from the arrangement in Figure 121. 
 One line would be composed entirely of nuclei situated one opposite each group 
 of cone cells; the other, of alternating nuclei and groups of cone cells. In the 
 former, as well as in the latter, there would be as many nuclei as groups of cone 
 cells. Hence, in this arrangement the nuclei are twice as numerous as the groups 
 of cone cells ; i. e. there are two nuclei for each ommatidium. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 113 
 
 of these nuclei affords no clue as to which one belongs to the rudi- 
 mentary cell. 
 
 In Palinurus (Plate X. Fig. 125, nl. px.), the eighth nucleus is regu- 
 larly present and easily seen. In Cancer (Fig. 129, nl. px. 8) it occu- 
 pies a position between the adjacent retinuhe. It can also be identified 
 in Crangon. 
 
 The retinulse in Decapods, according to all recent observers, contain 
 seven functional cells. In Homarus, Palinurus, Cambarus, Crangon, 
 Palsemonetes, and Cancer, the retinulse contain, in addition to the 
 seven nuclei of the functional cells, an eighth nucleus, which repre- 
 sents, I believe, a rudimentary cell. It is probable, therefore, that in 
 all Decapods each retinula really contains eight cells, one of which is 
 rudimentary. 
 
 The rhcibdome in Decapods presents a very uniform structure. It is 
 usually an elongated body, pointed both at its distal and its proximal end, 
 and completely covered, except at its distal tip, by the proximal retinular 
 cells. In those Decapods in which it is large enough to be conveniently 
 observed, its transverse section is squarish, and usually subdivided by 
 two straight lines into four smaller squares (Plate IX. Fig. 113). As 
 Grenacher (’77, pp. 31, 32) first demonstrated in Palaemon, the retinular 
 cells are rather peculiarly arranged around the rhabdome. One of its 
 four sides is flanked by one cell, the other, three by two cells each. This 
 arrangement can be seen in Palaemonetes (Fig. 113), and probably obtains 
 for all Decapods. 
 
 In Palinurus Argus (Plate X. Fig. 124) there appears to be no rhab- 
 dome, unless the translucent axial portion of each retinular cell can 
 be said to represent segments of it. The fibrous ends of the cone cells 
 (cl. con.) can be easily identified between the retinular cells, but the 
 centre of the retinula is filled with pigment, and shows not the least trace 
 of a rhabdome. This peculiarity of Palinurus was noticed as early as 
 1840 by Will (’40, p. 15), who described the ommatidium in this genus 
 as being without a transparent mass (= rhabdome). 
 
 Although the distal retinular cells in Decapods were collectively rec- 
 ognized by Muller (’26, pp. 355, 356) some sixty years ago as a definite 
 pigment band in the distal portion of the retina in the crayfish, they 
 were not identified as separate cells until quite recently. The first in- 
 vestigator to observe them was Carriere (’85, p. r 169), who described 
 them in Astacus as a pair of pigment cells flanking each cone. In Cam- 
 barus, Crangon, and Homarus, they also cover the sides of the cone, and 
 in the last named genus they are produced proximally into long fibres, 
 
 VOL. xxi. — no. 2 . 8 
 
114 
 
 BULLETIN OF THE 
 
 which perhaps pass through the basement membrane. In Palsemonetes 
 (Plate IX. Fig. 108, cl. dst.) and in Cancer (Plate X. Fig. 127, cl. dst.) 
 they are reduced to pigmented threads, which, starting from comparatively 
 large bases, twine around the lateral surfaces of the cones. 
 
 The arrangement and number of the distal retinular cells can be most 
 readily determined from their nuclei. In Cancer (Plate X. Fig. 128) 
 the cells are arranged in circles of six around each group of cone cells ; 
 each cell, however, participates in three circles, and consequently there 
 are in reality only twice as many cells as ommatidia. This arrangement 
 of the cells also occurs in Cardisoma, Hippa, and Pagurus. In Crangon 
 (Fig. 123), as I have previously remarked, the nuclei of the distal retinu- 
 lar cells are arranged in rows alternating with the rows of cones. There 
 are twice as many nuclei as cones; hence I conclude that here also 
 there are two distal cells for each ommatidium. In Homarus, Palinurus, 
 Gambarus, and Palsemonetes (Plate IX. Figs. 103 and 109, nl. dst.) the 
 nuclei are grouped distinctly in pairs, one pair for each ommatidium. 
 
 Each cone in Penseus, according to Patten (’86, p. 634), is surrounded 
 by two pairs of pigment cells, and Watase (’90, p. 299) states that in 
 Gambarus the dioptric part of the ommatidium is sheathed by four pig- 
 ment cells. In Cambarus Bartonii I have been able to find only two 
 such elements, the pair of distal retinular cells already described, and in 
 the other Crustaceans which I have studied I have observed nothing 
 which supports Patten’s statement concerning the four pigment cells in 
 Penseus. I am therefore inclined to doubt the accuracy of these two 
 observations. 
 
 The interommatidial space in the basal part of the retina in Palse- 
 monetes contains a light pigment similar to that described in the retina 
 of Mysis. Like this the pigment in Palsemonetes is white by reflected 
 light, and yellowish by transmitted light (compare Plate IX. Fig. 115). 
 It is apparently contained within cells (Fig. 103, cl. ms’drm.) whose out- 
 lines are very irregular, and whose nuclei (Fig. 104, nl. ms’drm.) are 
 small and somewhat variable in form. These cells occur on both sides of 
 the basement membrane. As in Mysis, they have probably migrated 
 into the retina, and are perhaps mesodermic in origin. They have been 
 Seen by Carriere (’85, p. 169) in Astacus, by Patten (’86, p. 636) in 
 Penseus, and by myself (’90 a , p. 25) in Homarus. I have also recently 
 observed them in Crangon, Cambarus, Cardisoma, Pagurus, and Pali- 
 nurus, as well as in Palsemonetes. 
 
 From what has preceded it is evident that the ommatidium in Deca- 
 pods contains the following elements : cells of the corneal hypodermis, 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 115 
 
 two ; cone cells, four ; proximal retinular cells, eight, one of which is 
 rudimentary ; dista\ retinular cells, two ; accessory cells, mesodermic (1) 
 in origin, often present. 
 
 Table of Ommatidial Formula:. 
 
 I have now concluded my account of the structure of the ommatidia 
 in Crustaceans, and for the purpose of presenting in a condensed form 
 its more important features I have devised the following table. This 
 consists of a series of ommatidial formulae constructed upon the plan 
 which I have described in the Introduction. The figures indicate the 
 numbers of particular kinds of cells present in the ommatidium of a 
 given group. The abbreviation pr. (present) marks the presence of any 
 kind of cell when the number of that kind is not constant for different 
 ommatidia in the same individual. 
 
 Table showing the Cellular Composition of the Ommatidian 
 Crustaceans. 
 
 Groups of Crustaceans. 
 
 Cells of 
 Corneal 
 Hypo- 
 dermis. 
 
 Cone 
 
 Cells. 
 
 Retinular Cells. 
 
 Accessory 
 
 Cells. 
 
 Undiffer- 
 
 entiated. 
 
 Differentiated. 
 
 Proxi- 
 
 mal. 
 
 Distal. 
 
 Amphipoda, 
 
 pr. 
 
 2 
 
 5 
 
 
 
 pr. (ect. ?) 
 
 Branchiopodidae and Apusidae, 
 
 2 
 
 4 
 
 5 
 
 
 
 0 
 
 Estheridae, 
 
 pr. 
 
 5(4) 
 
 5 
 
 
 
 0 
 
 Cladocera, 
 
 ? 
 
 5 
 
 5 
 
 
 
 pr. (ecu'?) 
 
 Copepoda : Pontella, 
 
 pr. 
 
 2 
 
 5 
 
 
 
 pr. (ect. ?) 
 
 Sapphirina, 
 
 
 i 
 
 3 
 
 
 
 2 
 
 Argulus, 
 
 pr. 
 
 4 
 
 5 
 
 
 
 2 
 
 Isopoda : Idotea, H 
 
 2 
 
 2 
 
 6 
 
 
 
 pr. (ect. ?) 
 
 Porcellio, 
 
 2 
 
 2 
 
 7 
 
 
 
 pr. (ect. ?) 
 
 Serolis, 
 
 2(+1) 
 
 2 
 
 
 4 
 
 2 
 
 pr. (ect. ?) 
 
 Nebaliae, 
 
 2 
 
 4 
 
 7 
 
 
 
 pr. (ect. ?) 
 
 Schizopoda, 
 
 2 
 
 2 
 
 
 7 + 1 
 
 2 
 
 pr. (mes. 1 ) 
 
 Stomatopoda, 
 
 2 
 
 4 
 
 
 7 + 1 
 
 2 
 
 pr. (mes. ? ) 
 
 Decapoda, 
 
 2 
 
 4 
 
 
 7 + 1 
 
 2 
 
 pr. (mes. ?) 
 
 A few features in the table require explanation. Among the number 
 of cells recorded for the Estheridae, the figure within the parenthesis 
 
116 
 
 BULLETIN OF THE 
 
 under the head of Cone Cells indicates the occasional occurrence of cones 
 containing only four cells, although the usual number is five. In the 
 line for Serolis, under the head of Corneal Hypodermis, the parenthesis 
 and included signs are intended to indicate the possibility of there being 
 more than two cells in the corneal hypodermis for each ommatidium. 
 In the Schizopods, Stomatopods, and Decapods, the number of prox- 
 imal retinular cells is expressed in the form of 7 + 1 instead of 8, be- 
 cause one of the cells is rudimentary. 
 
 The Innervation of the Retina. 
 
 The innervation of the retina in the compound eyes of Crustaceans is 
 chiefly interesting, because of its importance in relation to physiological 
 questions. As this paper deals with a morphological topic, it would be 
 obviously irrelevant to enter upon any extended discussion of this sub- 
 ject. Nevertheless, the innervation of the retina is not without some 
 bearing on the general question which I have set for myself, and I shall 
 therefore not pass it by, but put in as brief a form as possible what I have 
 observed concerning it. 
 
 In my account of the retina in the lobster, I described the optic- 
 nerve fibres as terminating in the proximal retinular cells. Near the 
 ganglion each fibre consists of a bundle of fibrils, simply enclosed 
 within a sheath, but as it approaches the retina it becomes coated with 
 pigment. The pigment increases in quantity and the fibre correspond- 
 ingly enlarges till it finally becomes continuous with the deeply pig- 
 mented retinular cell. The fibrillar axis can be distinguished in the 
 pigmented portion of the fibre as a transparent axial structure, and it 
 can also be traced distally through the pigment of each retinular cell 
 till it breaks up into its ultimate fibrillae, which are spread over the dis- 
 tal half of the rhabdome. This is the method of nerve termination in 
 the lobster, and points very conclusively to tlie rhabdome as the termi- 
 nal organ. 
 
 What I have seen of the termination of the nerve fibres in other 
 Crustaceans confirms the account which I have already given for the 
 lobster. In some species which I have studied, owing to the small size 
 of the retinal elements, 1 was unable to determine the cells with which 
 the nerve fibres connected. The termination of the fibres in the cells 
 of the retinula was observed, however, in the following genera : Bran- 
 chipus, Limnadia, Pontella, Gammarus, Talorchestia, Idotea, Porcellio, 
 Sphseroma, Serolis, Gonodactylus, Mysis, Palsemonetes, Crangon, Cam- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 117 
 
 barus, Palinurus, Pagurus, Cancer, and Cardisoma. In the majority of 
 these, a fibrillar axis could be distinguished. 1 In Cam barus, as in Homa- 
 rus, the nerve fibrillse spread over the distal portion of the rhabdome. 
 
 In Serolis an exceptionally interesting condition is presented. At the 
 level of the basement membrane each retinular cell contains a large fibril- 
 lar axis (Plate VI. Fig. 64, ax. n.). This becomes somewhat subdivided 
 in the more distal portion of the cell, and in the region of the retinular 
 nucleus it is represented by a cluster of several smaller axes (Fig. 63). 
 At the level of the hyaline cell, these however cannot be distinguished 
 (Fig. 62), but the scattered condition of the pigment granules in this 
 plane is probably to be accounted for by the presenoe of many separate 
 fibrils in the substance of the cell. In the region of the rhabdome an 
 immense number of fine lines can be seen extending from the retinular 
 cell into the substance of each rhabdomere (Fig. 61). These, I believe, 
 represent the fibrils of the nervous axis. They have been previously 
 observed in Serolis by Watase (’90, p. 291), and are so readily visible 
 that there can be no question as to their presence. Each fibril is per- 
 pendicular to the longitudinal axis of the ommatidium, and extends 
 through the rhabdomere to its axial surface. Before reaching this, 
 however, the fibril passes through what seems to be a delicate mem- 
 brane. When closely examined, this membrane often has the appearance 
 of a row of dots instead of a line, and in several cases I have been unable 
 to discover any traces of it. What its significance is, I am at a loss to 
 say. As I have previously observed, when the elements of the retinula 
 are separated the rhabdomere shows no tendency to break along this line. 
 Since the structure is pierced by the fibrils, and does not appear to be 
 a natural plane of rupture, and since sometimes it is apparently absent, 
 I believe it may be considered, from a morphological standpoint at least, 
 as a secondary and rather unimportant modification within the rhabdo- 
 mere itself. 
 
 If I am correct in maintaining that the nerve fibrils in Serolis terminate 
 in the rhabdomere, it is probable that they have a similar method of 
 ending in all other Crustaceans, and in such instances as Homarus, 
 where they have been traced only to the surface of the rhabdome, their 
 actual termination has probably not been seen. 
 
 1 A definite fibrillar axis was traced from below the basement membrane to the 
 region of the rhabdome in Gammarus (Plate I. Figs. 6-8), Porcellio (Plate V. Fig. 
 46), Idotea (Plate V. Figs. 53 and 55-57), Mysis (Plate VII. Figs. 87-89), Gono- 
 dactylus (Plate VIII. Figs. 101, 102), Palaemonetes (Plate IX. Figs. 116, 117), Cam- 
 barus, Pagurus, Cancer (Plate X. Figs. 130 and 131), and Cardisoma. 
 
118 
 
 BULLETIN OF THE 
 
 The termination of the fibrillae of the optic nerve in the rhabdome 
 supports Muller’s belief that the nerve fibres terminate in a region near 
 the proximal ends of the cones, and Grenacher’s more specific view that 
 they are connected with the retinular cells, and that the rhabdome is the 
 terminal organ. This method of termination is not consistent with the 
 opinion of Gottsche and Leydig, that the cone is the terminal organ, 
 nor with Patten’s rather similar belief that the ultimate nerve fibrillse 
 are distributed to the cone. I am therefore compelled to think that 
 these authors are mistaken in their conclusion. 
 
 Theoretic Conclusions. 
 
 In attempting to account for the variation in the number of cells in 
 different types of ommatidia, two courses naturally suggest themselves. 
 Either the different kinds of ommatidia vary in the number of cells 
 which they contain, because they have had separate origins, or they are 
 different because in some or all of them the ancestral ommatidium has 
 suffered modification. An examination of the table on page 115 shows 
 conclusively, I think, that in Crustaceans even the most extreme types 
 are so little removed from one another that it is much more probable 
 that the different kinds of ommatidia are genetically connected, than 
 that they have been produced independently. Granting this statement, 
 the question naturally arises, What are the means by which the primi- 
 tive ommatidium was modified h I believe that a close scrutiny of the 
 cellular structure of the ommatidia in living Crustaceans will disclose 
 some of the factors in this process. There are at least three of these to 
 be distinguished : the differentiation of cells, the suppression of cells, 
 and the increase in the number of cells by cell division. 
 
 By the differentiation of cells, I do not mean the process by which 
 hypodermal cells have become converted into retinular or cone cells, 
 "but that by vrhich an element already differentiated in the ommatidium 
 is secondarily modified to subserve another function. The only instance 
 of this kind with which I am acquainted occurs among the retinular cells. 
 In the majority of the simpler Crustaceans, the sides of the cones are 
 covered with pigment, which is almost always contained in the distal ends 
 of the retinular cells. In Serolis, among the Isopods, and apparently 
 in all the genera of Stomatopods, Schizopods, and Decapods, the cones 
 are surrounded by special pigment cells. These are always twice as 
 numerous as the ommatidia, and represent, I believe, retinular cells 
 which have become differentiated for the special purpose of sheathing 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 119 
 
 the cones. The way in which this differentiation may have occurred 
 has already been suggested in my paper on the lobster (’90 a , p. 57). 
 
 Although I have expressed the opinion that these cells are to be re- 
 garded as modified retinular cells, it might be maintained that they are 
 merely enlarged accessory pigment cells, such as occur in the inter- 
 ommatidial space of many Crustaceans. But I believe such an interpre- 
 tation of these cells would be erroneous, for the following reason. In 
 Serolis the nuclei of the pigment cells which surround the cone (Plate VI. 
 Fig. 65, nl. dst.) possess one, and sometimes two, well marked nucleoli, 
 but no fine chromatine granules. In this respect they closely resemble 
 the nuclei of the proximal retinular cells ( nl.px .), and differ consider- 
 ably from those of the accessory pigment cells (nl. h’drm.). The nu- 
 clei of the last named cells contain only fine granules. So far, then, 
 as their nuclei are concerned, the distal retinular cells bear a much 
 closer resemblance to the proximal cells than to the accessory pigment 
 cells. Each retinula in Serolis contains, moreover, only four cells, and 
 in this respect differs considerably from other Isopods, where the number 
 of retinular cells is either six or seven. On the supposition that the 
 pigment cells surrounding the cone in Serolis are accessory pigment 
 cells, one would be called upon to account for the exceptionally small 
 number of cells in the retinula of this genus ; whereas, if the cells 
 around the cone are regarded as modified retinular cells, they may be 
 taken to indicate for Serolis a primitive retinula composed of six cells, 
 a number characteristic of the retinulse in other Isopods. This inter- 
 pretation of the condition of the retinula in Serolis is borne out by 
 what is known of the retinula in Sphaeroma, where, it will be remem- 
 bered, a transition between the condition in Serolis and that in other 
 Isopods was distinctly indicated. 
 
 In the Stomatopods, Schizopods, and Decapods, if my observations 
 are correct, there are no ectodermic accessory pigment cells. Conse- 
 quently, a comparison between these cells and what I have called the 
 distal retinular cells cannot be drawn. In My sis (Plate VII. Fig. 73), 
 Gonodactylus (Plate VIII. Fig. 94), and Palaemonetes (Plate IX. Fig. 
 103), as well as in all other Decapods which I have examined, the resem- 
 blance between the nuclei of the retinular cells and those of the pigment 
 cells which surround the cone is as striking as in Serolis, and suggests 
 the origin of these cells from retinular cells rather than from any other 
 source. In Homarus, the pigment cells around the cone present a con- 
 dition of some interest in this connection. Each pigment cell is extended 
 proximally as a long fibre, which certainly reaches nearly to the base- 
 
120 
 
 BULLETIN OF THE 
 
 ment membrane, and probably passes through it in company with the 
 fibrous ends of the retinular cells (compare Parker, ’90 a , pp. 17-19). 
 Admitting that these cells are merely modified accessory pigment cells, 
 such a condition as this is quite unintelligible to me; but granting 
 them to be differentiated retinular cells, their fibrous extensions can 
 be easily explained as the rudiments of the fibrous portion of the cell 
 with which the nerve fibre was once connected. A somewhat similar 
 case occurs in Mysis, where the centre of each of the pigment cells 
 which surround the cone contains a small transparent axis. This axis 
 in every respect except that of connection with a nerve fibre corresponds 
 to the fibrillar axes described in the functional retinular cells of this 
 Crustacean (compare Plate YI I. Figs. 77, 78, and 87). Consequently, 
 the axis in the distal cells either represents a rudimentary nervous axis, 
 in which case the cell containing it must be regarded as a retinular cell, 
 or it is something for which I can suggest no explanation. 
 
 These facts lead me to conclude that the pigment cells which sur- 
 round the cone in Serolis, the Stomatopods, Schizopods, and Decapods, 
 are to be regarded as modified retinular cells, and I have therefore 
 described them under the name of distal retinular cells, in contrast to 
 proximal retinular cells, or those which retain their primitive position 
 around the rhabdome. In the differentiation of a group of simple 
 retinular cells into proximal and distal cells, the latter necessarily 
 change their function from that of terminal nervous organs to that of 
 screens chiefly concerned in excluding the light from the sides of the 
 cones. Wherever the distal retinular cells occur, they afford evidence, 
 I believe, that the structure of the ommatidium has undergone a modi- 
 fication from the primitive ommatidial condition. 
 
 The second method by which the structure of ommatidia may be 
 changed, namely, the suppression of cells, is perhaps the one whose 
 presence is most easily detected because of the frequent persistence of 
 the partially reduced cells. These rudimentary cells can be identified 
 most readily in the cases where they belong to groups in which the 
 number of elements is constant for different ommatidia. I know of no 
 evidence of suppression among the groups of cells in the corneal hypo- 
 dermis or the cones. Among the retinulte, however, it seems to be of 
 rather common occurrence. The first indication of this process is natu- 
 rally a diminution in the size of the cell to be suppressed. Such a step 
 is perhaps shown in the retinula of Gammarus (Plate I. Fig. 6), where 
 one of the five cells, although evidently functional, is nevertheless con- 
 siderably reduced. Without much doubt, the body described in the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 121 
 
 retinula of Idotea robusta represents, for reasons already stated, the 
 seventh cell present as a functional structure in Porcellio. In Idotea 
 irrorata the retinuhe, with very few exceptions (Plate Y. Fig. 54), contain 
 only six cells, showing no trace of the seventh cell. This condition, I 
 believe, is to be interpreted as one in which a cell has been completely 
 suppressed. In Stomatopods, Schizopods, and Decapods the retinulse 
 have been shown to contain, in addition to the nuclei of the seven func- 
 tional cells, an eighth nucleus, which may represent a rudimentary cell. 
 
 In alf of the cases thus far cited, it might be maintained that what I 
 have considered rudimentary cells are really cells newly acquired by the 
 ommatidia, and not old cells gradually undergoing suppression. The con- 
 dition in Idotea, however, where the body in question apparently contains 
 no nucleus, would be difficult to explain on this assumption, whereas, if 
 it be considered a cell undergoing reduction, its condition can be easily 
 accounted for. In Stomatopods, Schizopods, and Decapods, the con- 
 stancy in the number of cells and in the position of the eighth nucleus, 
 the small amount of protoplasm which surrounds it, and the striking 
 resemblance which it has to the other retinular nuclei, are facts difficult 
 to explain on the assumption that it represents a newly acquired cell, 
 but easily accounted for on the supposition that it is the remnant of a 
 partially suppressed cell. For these reasons, I believe that the instances 
 cited are valid cases of partial suppression, and that this must be regarded 
 as one of the actual means employed in the modification of ommatidia. 
 
 That ommatidia have been modified by an increase in the number 
 of their cells by cell division, is a proposition not easily established. 
 The difficulty of obtaining conclusive evidence on this point can be 
 made clear by an example. Let it be assumed that cones composed 
 of two cells are converted by the division of the cells into cones com- 
 posed of four cells. This step, even when first taken, would probably 
 be accomplished during the embryonic growth of an animal, and there- 
 fore before the cones themselves had begun to be differentiated. What 
 would actually happen would probably be this : the two cells, the 
 homologues of which in all previous animals had given rise to two cone 
 cells, would in this case each divide, thus producing a group of four 
 cells, which ultimately would form a cone of four segments. If we 
 could compare the adult animal in which such a process had occurred 
 for the first time with its immediate ancestors, the only important 
 difference that would be observed would be in the number of the 
 cells in each cone, and if the genetic relations of the two individu- 
 als were not known, it could not be stated with certainty whether in 
 
122 
 
 BULLETIN OF THE 
 
 one case we were dealing with an animal which had lost two cone cells 
 or in the other, with one which had gained two; in other words, it 
 would be impossible to determine which of the two conditions was the 
 primitive one. The importance of embryological evidence in determin- 
 ing this question must therefore be apparent. But evidence from even 
 this source might not be conclusive. Thus in the development of the 
 lobster I have traced in detail the steps by which the ommatidia are 
 formed, and although in this Crustacean the considerable number of 
 cells in each ommatidium would warrant one in expecting some evidence 
 of increase by division, the division of the cells in the retina is entirely 
 accomplished some time before these elements show any grouping into 
 ommatidia. Hence, the exact method of origin of the cells of the om- 
 matidium cannot at present be given. I have observed that the same 
 is also true in Gammarus ; cell division is completed before the cells are 
 grouped into ommatidia. Perhaps in the development of some other 
 Crustaceans evidence of the kind which I have sought may be obtained, 
 but in the few species which thus far have been studied the evidence 
 has not been produced. 
 
 Although the supposition that ommatidia may increase the number 
 of their cells by the division of those which they already possess is not 
 supported by any direct observations with which I am acquainted, there 
 are some facts recorded which are indirectly confirmatory of it. Thus, 
 in Phyllopods, an increase in the number of cone cells appears to accom- 
 pany a progressive differentiation of the retina itself. In this group, as 
 I have already pointed out, the simplest condition of the retina is found 
 in Branchipus and Apus. Prom the retina of Apus that of the Estheridse 
 can be easily derived, and the retina in the Estheridse represents a con- 
 dition from which the retina of the Cladocera may have arisen. That 
 this series of retinas, from Apus through the Estheridse to the Cladocera, 
 is a natural one is abundantly proved by the course taken in the develop- 
 ment of the eye in these groups. If we regard the condition of the 
 cones in these Crustaceans, we shall find that in the most primitive 
 retina, that of either Branchipus or Apus, they consist of four cells ; 
 that in the more complex retina of the Estheridse they are usually com- 
 posed of five cells, although cones of four cells are not unfrequent occur- 
 rences ; and finally, that in the Cladocera they are always composed of 
 five cells. Apparently in this series the development of the retina is 
 paralleled by a corresponding development in the cones, whereby one 
 composed of four cells is ultimately converted into one with five cells. 
 Since the resemblance between any two of the cells in a cone composed 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 123 
 
 of five elements is quite as close as that between the cells in cones con- 
 taining only four elements, I believe that the additional cell, which has 
 increased the number of segments from four to five, has been derived by 
 the division of one of the original four cone cells, and not from an extra- 
 ommatidial source. 
 
 Another instance of this kind occurs among the Isopods. The cones 
 in this group, it will be remembered, are usually each composed of two 
 segments. According to Beddard’s figures (’90, Plate XXXI. Figs. 1 
 and 4) in Arcturus, however, they occasionally consist of three segments, 
 and in Asellus aquaticus, according to Sars (’67, p. 110), although three 
 of the four cones in each eye are composed of only two segments each, 
 the fourth regularly contains three. The size of the segments in the 
 fourth cone differs ; tw T o are small, and together their bulk about equals 
 that of the third, and the last is approximately of the size of a segment 
 in one of the other cones. If we attempt to explain the condition of the 
 cone composed of three segments by supposing it to have been produced 
 by adding to the normal pair of cone cells a single cell from some source 
 external to the ommatidium, we are met with the difficulty, that what 
 is apparently the added cell — the larger one — resembles more closely 
 a segment in the other cones than do either of the tw T o remaining cells, 
 although the latter must on this assumption represent the original seg- 
 ments. If, however, we imagine the small segments to have arisen by 
 the division of a single larger one similar to the large one which remains 
 in the cone, the relation of the resulting segments both in size and num- 
 ber is a perfectly natural one. This explanation, therefore, seems to me 
 to be more probable than the former. For these reasons, I believe that 
 an increase in the number of cells in an ommatidium takes place by the 
 division of the cells already forming a part of that ommatidium, rather 
 than by the importation of new elements hitherto foreign to the om- 
 matidium. 
 
 The conclusion which I would draw from the preceding discussion is, 
 that there are at least three means of modifying the numerical formulae 
 of ommatidia, all of which involve only the cells primitively belonging 
 to the ommatidium, and therefore do not necessitate the introduction 
 of new cells from extra-ommatidial sources. They are cell differentia- 
 tion, cell suppression, and cell multiplication. 
 
 Having now determined the means by which the cellular structure of 
 the ommatidia in living Crustaceans is modified, we are prepared to ap- 
 proach the question of the structure of the primitive ommatidium. If 
 it could be shown that ommatidia w r ere modified only by increasing the 
 
124 
 
 BULLETIN OF THE 
 
 number of their elements, it would naturally follow that those com- 
 posed of the fewest cells would more nearly resemble the ancestral type 
 than those which consist of many cells. On the other hand, if the sup- 
 pression of cells were the only means employed in modifying structure, 
 the ommatidia containing the greatest number of elements would most 
 nearly approach the primitive type. Since, as I believe, both means 
 are employed in the Crustacea, the determination of the structure of 
 the ancestral ommatidium is evidently a difficult problem. Perhaps 
 the most satisfactory way of attempting its solution is to consider sep- 
 arately the different categories of cells which enter into the formation of 
 an ommatidium, and, after reviewing the conditions presented by each 
 in different Crustaceans, to determine, if possible, which of these condi- 
 tions is the most primitive. The conclusions thus arrived at concerning 
 each kind of cell will afford the necessary grounds for the construction 
 of an hypothetical formula of the ancestral ommatidium. Although it 
 is not necessary that this ommatidium should be represented in any liv- 
 ing Crustacean, for the ommatidia in all these may have suffered modifi- 
 cation, yet it is possible that a representative of it may still exist. 
 
 Turning now to the consideration of the different groups of cells, we 
 find that the corneal hypodermis presents two conditions ; one in which 
 its cells are not regularly arranged, and another in which they are 
 grouped in pairs, each pair lying at the distal end of an ommatidium. 
 The latter condition is characteristic of the Decapods, Schizopods, Sto- 
 matopods, Nebalise, Isopods, and some Branchiopods; the former, so far 
 as is known, occurs in the Amphipods, the Branchiura, and in some 
 Branchiopods (Limnadia and some species of Branchipus). In view of 
 the fact that the corneal hypodermis is a part of the retina which re- 
 tains the function of the general hypodermis but slightly modified, and 
 that in the latter the cells do not present a regular arrangement, it 
 is probable that a corneal hypodermis in which the cells are not regu- 
 larly arranged is of a more primitive character than one in which they 
 are definitely grouped. 
 
 The number of cells in the individual cones of Crustaceans varies 
 from two to five. Cones composed of two cells occur in Eucopepoda, 
 Amphipods, Isopods, and Schizopods ; cones of three cells are present 
 only exceptionally in Isopods ; cones of four cells are found in the 
 Decapods, Stomatopods, Nebalise, Branchiura, and some Branchiopods ; 
 cones of five cells characterize the Cladocera and some Branchiopods. 
 I have already given reasons for regarding the cones composed of three 
 cells as having been derived from those containing two, and cones com- 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 125 
 
 posed of five cells from those possessing four. Since there is no evidence 
 of degenerate cells in any of the cones composed of two segments, I am 
 .convinced that cones with four cells are derived from those with two cells, 
 and not the reverse. On these grounds, I conclude that the most primi- 
 tive form of cone in living Crustacea is that consisting of two cells. 
 
 The retinular cells in Crustaceans are subject to considerable varia- 
 tion. As I have previously shown, an ommatidium may contain one or 
 two kinds. When there is only one kind, all the cells are grouped 
 around the rhabdome, and are known simply as retinular cells. When 
 there are two kinds, one occupies a position around the rhabdome, and 
 the other around the cone ; the former I have called proximal retinular 
 cells, the latter distal retinular cells. Proximal and distal retinular 
 cells occur in Serolis, the Stomatopods, Schizopods, and Decapods 5 
 simple retinular cells apparently characterize the ommatidia of all other 
 Crustaceans. I have already presented reasons for considering the 
 distal retinular cells as modified simple retinular cells, which, in the 
 separation of the cone from the rhabdome by the elongation of the 
 ommatidium, have lost their connection with the nervous element, but 
 have retained their place next the dioptric one. A group of retinular 
 cells in which this differentiation has occurred is not so primitive in its 
 structure, therefore, as one in which all the retinular cells retain their 
 original position around the rhabdome, as in the groups of Crustacea 
 which possess simple retinular cells. 
 
 The number of simple retinular cells in Crustacean ommatidia varies 
 from five to seven. I11 Nebalia, and some Isopods, the retinula con- 
 tains seven cells ; in other Isopods it is composed of six cells, and in 
 the Branchiopods, the Cladocera, some Copepods, and Amphipods it 
 consists of five cells. It is difficult to state which of these numbers 
 represents the primitive condition. In the Isopods, as I have previ- 
 ously indicated (pp. 86 and 87), there is considerable evidence to 
 show that a retinula composed of six cells has been produced from one 
 containing seven by the suppression of one cell. Possibly in this way 
 the retinula with five cells was derived from that with six, but I know 
 of no observations which favor this supposition. 
 
 A small amount of indirect evidence on this question is to be ob- 
 tained from the other structural peculiarities of the ommatidia con- 
 taining retinulae with five, six, or seven cells. These retinulae occur 
 in connection with two kinds of rhabdomes, — one in wdiich the rhab- 
 domeric segments are easily distinguishable, and the other from which 
 they are apparently absent. Of these two kinds, the one in which the 
 
126 
 
 BULLETIN OF THE 
 
 segments persist is evidently more primitive than the one in which their 
 outlines are obliterated. 
 
 Probably in A : ebalia, in which the retinula is composed of seven cells, 
 and certainly in Idotea, where it consists of six, the rhabdome shows 
 no indication of being composed of rhabdomeres, but in Porcellio the 
 seven retinular cells surround a rhabdome composed of a corresponding 
 number of rhabdomeric segments. In Branchipus, the retinula consists 
 of five cells, but the rhabdome is apparently not composed of separable 
 rhabdomeres, whereas in Pontella, Argulus, Gammarus, Talorchestia, 
 Hyperia, and Phronima the five retinular cells are each represented by 
 a rhabdomere. The more frequent occurrence of a primitive condition 
 of rhabdome with the retinula having five cells than with that having 
 seven, favors indirectly the idea that the retinula with the smaller 
 number of cells is the more primitive of the two. The types of cones 
 associated with the two kinds of retinulse offer almost .no evidence on 
 the question in hand. Thus, a retinula of seven cells is associated with 
 a cone of four cells in Nebalia, and with one of two cells in Porcellio, 
 and a retinula of five cells is combined with a cone of four cells in 
 Branchipus and Argulus, and w 7 ith one of two cells in Amphipods. The 
 relation of the two kinds of retin ulse to the corneal hypodermis affords 
 some slight evidence in support of the opinion that the retinula of five 
 cells represents the more primitive type ; for although the differentiated 
 type of corneal hypodermis — the one in which the cells are regularly 
 arranged — may occur with either type of retinula, the undifferen- 
 tiated hypodermis — in which the cells are not regularly grouped — is 
 known to be associated only with retinulse containing five cells (some 
 Branchiopods, Argulus, and Amphipods). The evidence drawn from 
 these various sources is obviously very slight ; but such as it is, it indi- 
 cates that the retinula with five cells, rather than that with a greater num- 
 ber, represents the more primitive condition. This conclusion receives 
 some additional support from the fact that the retinula composed of 
 five cells characterizes the ommatidia in a number of not otherwise very 
 closely related Crustaceans (Pontella, Argulus, the Branchiopods, and 
 Amphipods), whereas the type possessing seven cells occurs only among 
 certain Isopods and in the Nebaliee. I believe, therefore, that all the 
 evidence at present deducible from the condition of the simpler retinula} 
 indicates that the one which contains five cells is more primitive than 
 that composed of six or seven cells. 
 
 In the present argument I have purposely omitted any mention of the 
 condition of the retinula in the Corycseidte, those Copepods in which the 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 127 
 
 lateral eyes present a highly modified condition. I have done this be- 
 cause I believe that the lateral eyes in many Copepods are degenerate, 
 and that therefore the evidence to be drawn from them cannot be as 
 trustworthy as that from other sources. That the lateral eyes in Cope- 
 pods are degenerate, is shown from the fact that in many members of 
 the group the eyes are entirely absent, and that in those in which they 
 do occur, their structure is subject to considerable variation. Thus in 
 Pontella the retina contains, besides one group of five retinular cells, three 
 isolated nervous cells, whereas in Sappliirina there is a group of three 
 retinular cells, and at least one isolated nervous cell. In Pontella, Sap- 
 phirina, Corycaeus, and Copilia each retina is provided with a single lens, 
 but iu Ireuseus, according to Claus (’63, Taf. II. Fig. 1), there are two 
 lenses in each eye. These variations, including the total disappearance 
 of the organ in some members of the group, lead me to believe that the 
 lateral eyes in the Copepods are degenerated, and therefore are organs 
 in which the suppression of cells may have reduced them to even a 
 simpler condition than that presented by the ancestral ommatidium. 
 
 The conclusion which I draw from the preceding argument is, that the 
 type from which the ommatidia in all living Crustaceans are probably 
 derived would exhibit the following structures : a corneal hypodermis ill 
 which the cells are not regularly arranged, and consequently an un- 
 facetted corneal cuticula ; a cone composed of two cells ; a retinula com- 
 posed of five retinular cells and having a rhabdome which consists of 
 five rhabdomeres. The retina of the primitive eye, a simple thickening 
 in the superficial ectoderm, would be composed of ommatidia of this 
 type arranged upon the hexagonal plan. None of the Crustaceans 
 with which I am acquainted possess an eye of exactly this structure. 
 The one in which this condition is most nearly represented is perhaps 
 Gammarus. In this animal all the requirements of the hypothetical 
 eye are fulfilled, except that the form of the retina as a whole is some- 
 what disturbed by the separation of the corneal hypodermis from the 
 layer of the cones and retinulae by a corneo-conal membrane, and by the 
 partially disguised condition of the basement membrane. 
 
 If my conclusions be correct concerning the structure of the primitive 
 ommatidium and the means by which it has been modified, it follows 
 that the principal types of ommatidia have been produced mainly by 
 increasing the number of cells in the primitive type, and that, of the 
 three means of modifying the structure of ommatidia, cell division has 
 been the most influential. 
 
 Although the hypothetical ommatidium which has been described in 
 
128 
 
 BULLETIN OF THE 
 
 the preceding paragraphs has been spoken of as ancestral, it is not to be 
 supposed that the condition which it presents must be regarded as necessa- 
 rily its simplest form. I feel tolerably confident, however, that the prim- 
 itive ommatidium must have been at least as simple as I have assumed 
 it to be. Possibly its retinula may have been composed of less than five 
 cells, as is that seen in some Copepods ; although, as I have previously 
 remarked, the condition of the lateral eyes in these Crustaceans is 
 probably influenced by degeneration, and therefore may not represent a 
 primitive stage. What might be regarded, however, as a more primitive 
 form of ommatidium than that which I have described, may be seen 
 in the eye of the Chsetopod Nais (Carriere, ’85, pp. 28, 29). In this 
 worm the eye lies in the hypodermis on the side of the head, and con- 
 sists of a few relatively large transparent cells, the proximal faces of 
 which are in part covered by pigment cells. It is probable that the 
 transparent cells are merely dioptric in function, and that the pigment 
 cells are nervous. The transparent cells may therefore be looked upon 
 as the forerunners of cone cells, and the pigment cells at their bases as 
 retinular cells not yet differentiated into a retinula. It is not difficult 
 to imagine the origin of an ommatidium from a single one of the trans- 
 parent cells and its accompanying pigment cells, and, by an increase in 
 the number of such groups, the production of a retina like that of the 
 compound eye of Arthropods. 
 
 This view of the origin of the ommatidia in Arthropods is irreconcila- 
 ble with that recently advanced by Watase (’90), according to whom 
 each ommatidium is to be regarded as a pit formed by an involution of 
 the hypodermis. The supposed cavity of this pit occupies nearly the 
 whole length of the axial portion of the ommatidium, and is filled bj' 
 the secretions of the cells constituting its wall. The secretion in the 
 deeper part of the pit forms the rhabdome ; that which is produced 
 nearer its mouth, the cone. During the formation of the pit, the hypo- 
 dermal cells are believed to retain such mutual relations that their mor- 
 phologically distal ends lie next its cavity ; hence the secretions produced 
 by these ends, the rhabdome and cone, are to be regarded as modifica- 
 tions of the chitinous cuticula of the outer surface of the body. 
 
 Ingenious as this theory is, I have not been able to convince myself of 
 its tenability. It may be urged against the assumption that the retinu- 
 lar cells occupy a proximal position and the cone cells a distal one on the 
 wall of a hypodermal pocket, that in Gammarus the retinular cells extend 
 from the distal to the proximal face of the retina, and that in Homarus 
 the cone cells have a corresponding extent ; these conditions show that 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 129 
 
 it is possible to interpret the cells in an ommatidium as elements in a 
 thickened epithelium, all of which originally extended from one face of 
 the layer to the other, and the grouping of which is not even now in- 
 terfered with by any process of involution. But granting that the ret- 
 inal cells are thus arranged, it must be admitted that the surface on 
 which the rhabdomeres are produced corresponds to the sides of the cells 
 rather than to their distal ends. This interpretation of the position of 
 the rhabdome is not, so far as I am aware, contrary to any well estab- 
 lished facts, and indeed it is rather more in accordance with the condi- 
 tion seen in the eyes of some Arthropods than that implied in Watase’s 
 theory. Thus, in the lateral eyes of scorpions the retinal cells are ar- 
 ranged as in an ordinary epithelium, and the lateral wall of each cell is 
 in part occupied by a rliabdomere. In this instance, then, it must be 
 admitted either that the rhabdomeres are produced on the sides of the 
 retinal cells, or that each cell has independently rotated upon itself, so 
 as to bring its morphologically distal end into a position corresponding 
 to the side of an ordinary epithelial cell. But there is neither direct 
 evidence to show that this rotation of single cells has occurred, nor, in 
 this case, can there be any motive assumed which might have induced 
 the rotation of single elements. I therefore believe that in the lateral 
 eyes of scorpions the rhabdomes are on the sides of the retinal cells in 
 the strictest morphological sense $ and if they can occur in this position 
 in the eyes of scorpions, I can see no reason why they might not occur 
 in similar positions on the retinal cells of compound eyes. Hence it 
 seems to me as reasonable to interpret the retina in compound eyes as a 
 layer of modified epithelium unaffected by involutions, as it is to con- 
 sider it a layer in which each ommatidium represents an infolding. 
 
 When, moreover, an attempt is made to show how a particular omma- 
 tidium has arisen by involution, some difficulties are encountered. Thus 
 in Gammarus, in which the ommatidium is of a primitive type, each om- 
 matidial pocket would involve seven cells, two of which, the cone cells, 
 must be imagined as forming the neck of the involution, while the re- 
 maining five, the retin ular cells, would constitute the deeper portion of 
 the pocket. The mechanical difficulty which would accompany the forma- 
 tion of an involution involving so small a number of cells must be obvi- 
 ous, and offers, I believe, an obstacle to the successful operation of the 
 process assumed in Watase’s theory. 
 
 The one instance in which Watase has described an actual involution 
 to form the eyes in Arthropods is the lateral eye of Limulus. These eyes 
 consist of a cluster of hypodermal pits, over each of which there is a cu- 
 
 vol. xxi — no 2, 9 
 
130 
 
 BULLETIN OF THE 
 
 ticular lens. Although there cannot be the least doubt that in this case 
 each pit is a hypodermal involution, the belief that each one is homolo- 
 gous with an ommatidium is by no means so well founded. In structure 
 the wall of the pit differs considerably from that of an ommatidium ; it 
 contains no cells which can be definitely denominated, either as cone 
 cells or as cells of the corneal hypodermis, and it does contain a large 
 ganglionic cell, which is only questionably homologous with any element 
 in an ommatidium. In most respects in which these pits differ from 
 ommatidia, they resemble simple eyes, and I therefore regard them as 
 such, rather than as representatives of an early condition in the forma- 
 tion of an ommatidium. 
 
 When to the objections raised in the preceding paragraphs the state- 
 ment is added, that in both Homarus and Gammarus — representatives 
 of the extremes of organization — the ommatidia are developed without 
 showing any trace of infolding, Watase’s theory of the formation of om- 
 matidia by means of involutions appears in a still less favorable light. 
 I therefore regard ommatidia, not as the result of involutions, but as 
 differentiated clusters of cells in a continuous unfolded epithelium. 
 
 I have not observed anything that would lead to the conclusion re- 
 cently expressed by Patten (’90), that an ommatidium is a hair-bearing 
 sense bud. I believe, on the contrary, that they have had a very differ- 
 ent origin. 
 
 In conclusion, I may add, that if my idea of the origin of ommatidia 
 be correct, it supports Grenacher’s opinion, that compound eyes are 
 not derived directly from aggregations of simple eyes, but from groups 
 of optic organs which were even more primitive in their structure than 
 simple eyes. Possibly such primitive organs were the antecedents of 
 both the compound and simple eyes of Arthropods, as Grenacher sug- 
 gests ; but possibly the two kinds of eyes may have had totally different 
 origins. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 131 
 
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 ’51. Ueber Artemia salina und Branchipus stagnalis. Zeitschr. f. wiss. 
 
 Zool, Bd. III. pp. 280-307, Taf. VIII. 1851. 
 
 ’55. Zum feineren Bau der Arthropoden. Arch. f. Anat, Physiol, u. wiss. 
 Med, Jalirg. 1855, pp. 376-4S0, Taf. XV.-XVIII. 1855. ‘ 
 
 \/ 57 . Lehrbuch der Histologie des Menschen und der Tliiere. Frankfurt 
 a. M., Meidinger Sohn & Co. 12 4 - 551 pp. 1857. 
 
 ’ 60 . Naturgeschichte der Daphniden (Crustacea Cladocera). Tubingen, 
 Laupp und Siebeck. 4 + 252 pp, 10 Taf. 1860. 
 
 ’ 64 . Tafeln zur Vergleichenden Anatomie. Tiibingen, Laupp & Siebeck. 
 
 Erstes Heft, 10 Tafeln nebst Erklarungen. 1864. 
 
 ’ 64 a . Das Auge der Gliederthiere. Tiibingen, Laupp & Siebeck. 50 pp. 
 1864. 
 
 ’ 78 . Ueber Amphipoden und Isopoden. Zeitschr. f. wiss. Zool, Bd XXX., 
 Supplement, pp. 225-274, Taf. IX.-XII. May, 1878. 
 
 Lubbock, J. 
 
 ’88. On the Senses, Insfincfs, and Intelligence of Animals, with Special 
 Reference to Insects. New York, D. Appleton & Co. 29 + 292 pp. 
 The International Scientific Series, Vol. LXIV. 1888. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 137 
 
 Mayer, P. 
 
 ‘82. Die Caprelliden des Golfes von Neapel und der angrenzenden Meeres- 
 Abschnitte. Fauna und Flora des Golfes von Neapel, VI. Monographic, 
 10 + 201 pp., 10 Taf. 1882. 
 
 Milne Edwards, H. 
 
 V' ’34. Histoire naturelle des Crustaces, comprenant l’Anatomie, la Physiologie 
 et la Classification de ces Animaux. Tome Premier. Paris, Rovet. 
 35 -|- 168 pp. 1831. 
 
 Muller, F. 
 
 ’64. Ueber den Bau der Scheerenasseln (Asellotes liereropodes M. Edw.). 
 Arch. f. Naturg, Jahrg. XXX. Bd. I. pp. 1-6. 1864. 
 
 Muller, J. 
 
 ’26. Zur vergleichenden Physiologie des Gesichtssinnes des Menschen und 
 der Thiere, nebst einem Versucli fiber die Bewegungen der Augen und 
 fiber den menschlichen Blick. Leipzig, C. Cnobloek. 32 + 462 pp., 8 Tab. 
 1826. 
 
 ’29. Fortgesetzte anatomische Untersuchungen fiber den Bau der Augen bei 
 . den Insekten und Crustaceen. Arch. f. Anat. u. Physiol. (Meckel), 
 Jahrg. 1829, pp. 38-64, Taf. III. Figs. 1-17- 1829. 
 
 ’29 a . Sur les yeux et la vision des Insectes, des Arachnides et des Crustaces. 
 Ann. Sci. Nat., Tom. XVII. pp. 225-253 and 365-386 ; Tom. XVIII. 
 pp. 73-106. 1829. 
 
 ’31. Ueber den Bau der Augen bei Argulus foliaceus mit Bemerkungen fiber 
 die Eintheilung der Crustaceen nach dem Bau der Augen. Zeitschr. f. 
 Physiol, Bd. IV. Heft 1, pp. 97-105, Taf. VI. Figs. 5, 6. 1831. 
 
 ’35. Anmerkung des Herausgebers. Arch. f. Anat., Physiol, u. wiss. Med, 
 Jahrg. 1835, pp. 613, 614. 1835. 
 
 '52. Anmerkung des Herausgebers. Arch. f. Anat, Physiol, u.wiss. Med., 
 Jahrg. 1852, p. 492. 1852. 
 
 Newton, E. T. 
 
 ’73. The Structure of the Eye of the Lobster. Quart. Jour. Micr. Sci., New 
 Ser, Vol. XIII. pp. 325-343, Pis. XVI, XVII. 1873. 
 
 Nusbaum, J. 
 
 ’87. L’embryologie de My sis chameleo (Thompson). Arch. Zool. exp. et 
 gen, Ser. 2, Tom. V. pp. 123-202, Pis. V.-XII. 1887. 
 
 Owen, R. 
 
 '43. Lectures on the Comparative Anatomy and Physiology of the Inverte- 
 brate Animals. London : Longman, Brown, Green, and Longmans. 392 
 pp. 1843. 
 
 Pagenstecher, H. A. 
 
 ’61. Phrouima sedentaria. Arch. f. Naturg, Jahrg. XXVII. Bd. I. pp. 
 15-41. 1861. 
 
 Parker, G H. 
 
 ’90. A Preliminary Account of the Development and Histology of the Eyes 
 in the Lobster. Proc. Amer. Acad. Arts and Sci, Vol. XXIV. pp. 24, 
 25. 1890. 
 
BULLETIN OF THE 
 
 138 
 
 ’ 90 a . The Histology and Development of the Eye in the Lobster. Bull. 
 Mus. Comp. Zool., Vol. XX. No. l,pp. 1-60, 4 Bis. May, 1890. 
 
 Parsons, G. 
 
 ’ 31 . An Account of the Discoveries of Muller and others in the Organs of 
 Vision of Insects and the Crustacea. Magazine of Natural History, 
 Edinburgh, Yol. IV. pp. 124-134, 220-234, and 363-372. 1831. 
 
 Patten, W. 
 
 ’86. Eyes of Molluscs and Arthropods. Mittheilungen Zool. Station zu Nea--- 
 pel, Bd. VI. Heft IV. pp. 542-756, Taf. 28-32. June, 1886. 
 
 ’ 90 . Is the Ommatidium a Hair-bearing Sense-Bud ? Anat. Anzeiger, 
 Jahrg. V. Nos. 13, 14, pp. 353-359. July, 1890. 
 
 Pereyaslawzewa, S. 
 
 '88. Le Developpement de Gammarus poecilurus, Bilik. (Etudes sur le 
 developpement des Amphipodes, Premiere Partie.) Bull. Soc. Imper. 
 Naturalistes de Moscou, Nouvelle Ser., Tom. II. No. 2, pp. 185-219, 
 Pis. Ill -VI, 1888. 
 
 Reichenbach, H. 
 
 ’86. Studien zur Entwicklungsgeschichte des Elusskrebses. Abhandl. Senck- 
 enb. Naturf. Gesellsch., Bd. XIV. Heft 1, pp. 1-137, Taf. I. -XIV, 
 1886. 
 
 Rossiiskaya, M. 
 
 ’ 89 . Le Developpement d’Orchestia littorea, Spence Bate. (Etudes sur le 
 developpement des Amphipodes, Deuxieme Partie ) Bull. Soc. Imper. 
 Naturalistes de Moscou, Nouvelle Ser., Tom. II. No. 4, pp. 561-581, 
 Pis. XVI., XVII. 1889. 
 
 Rossiiskaya-Koschewnikowa, M. 
 
 ’ 90 . Developpement de la Sunamphitoe valida, Czerniavski, et de l’Amphi- 
 toe picta, Bathke. (Etudes sur le developpement des Amphipodes, Qua- 
 trieme Partie.) Bull. Soc. Imper. Naturalistes de Moscou, Nouvelle Ser., 
 Tom. IV. No. 1, pp. 82-103, Pis. I , II. 1890. 
 
 Sars, G. O. 
 
 ’ 67 . Histoire naturelle des Crustaces d’eau douce de Norvege Les Mala- 
 costraces. Christiania, Chr. Jolinsen. 155 pp., 10 Pis. 1867. 
 
 Schaffer, J. C. 
 
 ’ 56 . Der krebsartige Kiefenfuss mit der kurzen und langen Scliwanzklappe. 
 Begensburg, E. A. Weiss. 142 pp. 7 Taf , 1756. 
 
 Schmidt, O 
 
 >78. Die Eorm der Krystallkegel im Arthropodenauge. Zeitschr. f. wiss. 
 Zool , Bd. XXX., Supplement, pp. 1-12, Taf. I 1878. 
 
 Schultze, M. 
 
 ’ 67 . Ueber die Endorgane des Sehnerven im Auge der Gliederthiere. Arch, 
 f. mikr. Anat., Bd. III. pp. 404-408. 1867. 
 
 ’68. Untersuchungen fiber die zusammengesetzten Augen der Ivrebse und 
 Insecten. Bonn, Cohen & Sohn. 32 pp., 2 Taf. 1868. 
 
MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 139 
 
 Schultze, M. ( continued ). 
 
 ’68 a . Bemerkuugen zu dem Aufsatze des Dr. W. Steinlin. Arch. f. inikr. 
 Anat., Bd. IV. pp. 22-25. 1808. 
 
 Spangenberg, F. 
 
 ’75. Zur Kenutniss von Brauchipus stagnalis. Zeit.sckr. f. wiss. Zook, Bd. 
 
 XXV., Supplement, pp. 1-64, Taf. I. -111. 1875. 
 
 ’76. Ueber Bau und Entwicklung der Dapliniden. Gottingen Nackrickten, 
 
 pp. 517-537. 1876. 
 
 Stebbing, T. R. R. 
 
 '88. Report on the Amphipoda collected by H. M. S. Challenger during the 
 Years 1873-1876. The Voyage of H. M. S. Challenger, Zoology, Vol. 
 XXIX. 1888. 
 
 Steinlin, W. 
 
 '66. Beitrage zur Anatomie der Retina. Bericht St. Gallischen naturwiss. 
 
 Geselsch., 1865-66, pp. 17-138, Taf. I.-III. 1866. 
 
 ’68. Ueber Zapfen und Stabchen der Retina. Arch. f. inikr. Anat., Bd. 
 IV. pp. 10-21, Taf. II. 1868. 
 
 Straus, H. E. 
 
 ’19. Memoire sur les Daphnia, de la classe des Crustacds, Premiere Partie. 
 Mem. Mus. Hist. Nat., Tom. V. pp. 380-425, PI. 29. 1819. 
 Swammerdam, J. 
 
 '52. Bibel der Natur. Edition Hermann Boerhave. Aus dem Iiolland- 
 ischen ribersetzt. Leipzig, J. F. Gleditsclien. pp. 12 + 410 + Regis- 
 ter, Tab. I.-LIII, 1752. 
 
 Sye, C. G. 
 
 ’87. Beitrage zur Anatomie und Histologic von Jaera marina. Inaugural- 
 Dissertation. Kiel, C Bockel. 37 pp-, 3 Taf. 1S37. 
 
 Treviranus, G. R. und L. C. 
 
 ’16. Vermischte Schriften anatomischen und physiologischen Inhalts. Er- 
 ster Band. Gottingen, J. F. Rower. 8 + 188 pp., 16 Taf. 1816. 
 Watase, S. 
 
 ^ ’89. On the Structure and Development of the Eyes of the Limulus. Johns 
 Hopkins Univ. Circulars, Vol. VIII. No. 70, pp. 34-37. March, 1889. 
 v ’90 On the Morphology of the Compound Eyes of Arthropods. Stulies 
 Biol. Lab. Johns Hopkins Univ., Vol. IV. No. 6, pp-. 2S7-334, Pis. 
 XXIX.-XXXV. Feb, 1890. 
 
 \r ’9o a . On the Migration of the Retinal Area, and its Relation to the Morphol- 
 ogy of the Simple Ocelli and the Compound Eyes of Arthropods. Johns 
 Hopkins Univ. Circulars, Vol. IX. No. 80, pp. 63-65. April, 1890. 
 
 - ’90 b . On the Morphology of the Compound Eyes of Arthropods. Quart. 
 
 Jour. Micr. Sci., Vol. XXXI. Pt. II. pp. 143-157, PI. XIX. June, 
 1890. 
 
 Weismann, A. 
 
 ’74. Ueber Bau und Lebenserscheinungen von Leptodora hvalina Lilljeborg. 
 Zeitschr. f. wiss. ZooL, Bd. XXIV. pp. 349-418, Taf. XXXIII.-XXXVIII. 
 1874. 
 
140 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
 
 Will, F. 
 
 ’40. Beitrage zur Anatomie der zusanimengesetzten Augen mit facettirter 
 Hornliaut. Leipzig, Leopold Yoss. 32 pp., 1 Taf.. 1S40. 
 
 Zaddach, E. G. 
 
 ’41. De Apodis cancriformis Scliaeff. Bonn®, Typis Caroli Georgii. 72 pp., 
 4 Tab. MLCCCXXXXI. 
 
 Zenker, W. 
 
 ’54. Monograpliie der Ostracoden. Arch. f. Naturg., Jahrg. XX. Bd. I. 
 pp. 1-87, Taf. I.- VI. 1854. 
 
EXPLANATION OF FIGURES. 
 
 All the drawings were made with the aid of an Abbe camera. Unless otherwise 
 stated, the specimens from which the drawings were made were stained in Czokor’s 
 alum-cochineal and mounted in benzol-balsam. The reagent used in depigmenting 
 sections was an aqueous solution of potassic hydrate £%. 
 
ABBREVIATIONS. 
 
 a. 
 
 Anterior. 
 
 mb. del. Intercellular membrane. 
 
 ax. n. 
 
 Axis of nerve fibrillse. 
 
 mb. n. opt. Membrane of optic nerve. 
 
 brs. oc. 
 
 Optic pocket. 
 
 mb. pi’ph. Peripheral membrane. 
 
 cl. con. 
 
 Cone cell. 
 
 mb. pr’con. Preconal membrane. 
 
 cl. cm. 
 
 Cell of corneal hypodermis. 
 
 mu. Muscle. 
 
 cl. dst. 
 
 Distal retinular cell. 
 
 n.fbr. Nerve fibre. 
 
 cl. hyl. 
 
 Hyaline cell. 
 
 nl. con. Nucleus of cone cell. 
 
 cl. ms’drm. 
 
 Mesodermic cell. 
 
 nl. cm. Nucleus of cell in corneal hy- 
 
 cl. px. 
 
 Proximal retinular cell. 
 
 podermis. 
 
 cl. rtn/ 
 
 Retinular cell. 
 
 nl. dst. Nucleus of distal retinular cell. 
 
 cl. rud. 
 
 Rudimentary retinular cell. 
 
 nl. h’drm. Nucleus of hypodermal cell. 
 
 cnch. 
 
 Shell. 
 
 nl. hyl. Nucleus of hyaline cell. 
 
 cod. 
 
 Body cavity. 
 
 nl. ms’drm. Nucleus of mesodermic cell. 
 
 con. 
 
 Cone. 
 
 nl. px. Nucleus of proximal retinular 
 
 cp. sng. 
 
 Blood corpuscle. 
 
 cell. 
 
 cm. 
 
 Corneal cuticula. 
 
 nl rtn.' Nucleus of retinular cell. 
 
 eta. 
 
 Cuticula. 
 
 n. opt. Optic nerve. 
 
 d. 
 
 Dorsal. 
 
 oc. Eye. 
 
 dsc. 
 
 Sucking disk. 
 
 omm.' Ommateum. 
 
 dx. 
 
 Right. 
 
 p. Posterior. 
 
 gn. opt. 
 
 Optic ganglion. 
 
 po. brs. Pore of optic pocket. 
 
 h’drm. 
 
 Hypodermis. 
 
 r. Retina. 
 
 hp. 
 
 Liver. 
 
 rhb. Rhabdome. 
 
 in. 
 
 Intestine. 
 
 rhb’m. Rhabdomere. 
 
 Ins. 
 
 Lens. 
 
 rtn.' Retinula. 
 
 mb. ba. 
 
 Basement membrane. 
 
 s. Left. 
 
 mb. cm. 
 
 Corneal membrane. 
 
 v. Ventral. 
 
 mb. cm’con. 
 
 Corneo-conal membrane. 
 
 va. sng. Blood-vessel. 
 
 Such other abbreviations as have been used are explained in the description of 
 the figures with which they occur. 
 

 
 
Parker. — Compound Eyes in Crustaceans. 
 
 PLATE I. 
 
 Gatnmarus. 
 
 Fig. 1. A section of the right eye in a plane transverse to the chief axis of the 
 body and through the central part of the retina. X 115. 
 
 “ 2. A section lengthwise of an ommatidium. The numbers at the left of 
 
 the figure correspond to the numbers of the six following figures 
 of transverse sections, and mark the levels at which the latter were 
 taken. X 475. 
 
 “ 3. A transverse section in the plane of the corneal hypodermis. X 475. 
 
 “ 4. A transverse section through the distal ends of the retinular cells and 
 
 cone. X 475. 
 
 “ 5. A transverse section through the proximal portion of the cone and 
 
 through the adjoining retinular cells. X 475. 
 
 6. A transverse section through the retinula in the region of the rhabdome. 
 
 X 475. 
 
 7. A transverse section through the retinular cells somewhat proximal to 
 
 the basement membrane. X 475. 
 
 “ 8. A transverse section through a single retinular cell in the region of its 
 
 nucleus. X 475. 
 
 “ 9. The proximal portion of a retinular cell viewed from the side. (Compare 
 
 Fig. 2.) Isolated in Muller’s fluid. Not stained. X 475. 
 
 “ 10. A. cone isolated in Muller’s fluid and viewed from the side. Not stained. 
 
 X 475. 
 
Parker - Crustacean Eye 
 
 Pl.I 
 
 GJ.R del. 
 
 Gammarus 
 
 B.Meisel jtth.B o s to n. 
 
the IIOTASY 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
Parker. — Compound Eyes in Crustaceans. 
 
 PLATE II. 
 
 Argulus. 
 
 (Figs. 11-17.) 
 
 Fig. 11. A section in a plane transverse to the chief axis of the body and through 
 the right eye. Depigmented. X 140. 
 
 “ 12. A longitudinal section of an ommatidiurn. X 475. 
 
 “ 13. A longitudinal section of an ommatidiurn which had been depigmented. 
 
 The numbers at the left of the figure correspond to the numbers 
 of the four following figures of transverse sections, and mark the 
 levels at which the latter were made. X 475. 
 
 “ 14. A transverse section through the distal end of a cone and the surround- 
 
 ing pigment cells. X 475. 
 
 “ 15. A transverse section through the proximal portion of a group of four 
 
 cone cells. The intercellular membranes of the cells present four 
 
 thickened regions. X 475. 
 
 “ 16. A transverse section through the rhabdome. Depigmented X 475. 
 
 “ 17. A transverse section through the retinula somewhat proximal to the 
 
 rhabdome. X 475. 
 
 Pontella. 
 
 Fig. 18. The left lateral eye seen from the left side. The section is an optical 
 one ; its plane is very nearly parallel to the sagittal plane of the 
 body. Depigmented in alcohol (see p. 78). X 275. 
 
 “ 19. A transverse section of the optic nerve from a region immediately poste- 
 
 rior to the retina. The sagittal plane divides the nerve into sym- 
 metrical halves ; the fibres in each half belong exclusively to the 
 lateral eye of the corresponding side. X 400 
 
Parker - Crustacean Eye 
 
 Pl.H 
 
 '^s0L... hUrm.. 
 mh.bri. 
 
 .nLhcLrm 
 
 mb.piph. 
 
 ; $%X..nlh<lrnv. 
 
 nltidrm. 
 
 P del. 
 
 mh.n.opt. 
 S. 
 
 mbpreon. 
 
 mb.bd. 
 
 n.opt 
 
 COPEPODA. 
 
 V 
 
 B.MeiseUttK.Bostan. 
 
THE LIBRARY 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
Parker. — Compound Eyes in Crustaceans. 
 
 PLATE III. 
 
 Pontella. 
 
 Figs. 20-29. A complete series of ten consecutive sections through the right and 
 left retinas in planes parallel to the horizontal plane of the animal. 
 The sections are viewed from their dorsal faces. Figure 20 represents 
 the most ventral section ; Figure 29, the most dorsal. The plane of 
 Figure 25 is approximately indicated by the incomplete dotted line 
 mu. con. in Figure 18 (Plate II.). In the sections on the present plate 
 the different bodies in the left retina have been designated by appropri- 
 ate letters and figures. The eight rhabdomeres have been indicated 
 simply by numbers ; the same number always refers to the same 
 rhabdomere. For the sake of distinction, the two cone cells have 
 been marked cl. con. 1 and cl. con. 2. Some of the nerve fibres 
 ( n.fbr . 7 and n.fbr. 8) have been numbered in reference to the par- 
 ticular rhabdomeres with which they are associated. X 400. 
 
Parker - Crustacean Eye 
 
 Pl.IE 
 
 nlhflrm 
 
 B.Meisel,ltth.Boslon. 
 
 PONTELLA 
 

 
 
 
 the library 
 
 OF THE 
 
 UNIVERSITY of ILLINOIS 
 

Parker — Compound Eyes in Crustaceans. 
 
 PLATE IV. 
 
 Branchipus. 
 
 (Figs. 30-32.) 
 
 Fig. 30. A longitudinal section of an ommatidium. X 400 
 “ 31. A transverse section through the distal end of four cones. X 400. 
 
 “ 82. A transverse section through the middle portion of a retinula. X 400. 
 
 Limnadia. 
 
 (Figs. 33-39 ) 
 
 A section through the anterior part of the body, including the eye, in a 
 plane transverse to the chief axis. X 25. 
 
 An enlarged portion of a section from the same series as that from which 
 Figure 33 was drawn, but in a position slightly anterior to the lat- 
 ter. X 115. 
 
 A section through the eye cut in the sagittal plane of the animal. De- 
 pigmented. X 90. 
 
 A lateral view of an ommatidium. The numbers at the left of the 
 figure correspond to the numbers of the three following figures 
 of transverse sections, and mark the levels at which the latter were 
 taken. X 475. 
 
 A transverse section through the corneal hypodermis and distal ends of 
 the cones. X 475. 
 
 A transverse section through four cones at the level where they are 
 thickest. X 475. 
 
 A transverse section through the central portion of four retinulae. X 475. 
 
 Evadne. * 
 
 (Figs. 40-45.) 
 
 An optical section through the eye and adjoining structures in a plane 
 approximately parallel to the sagittal plane of the body, but lying 
 somewhat to the right of it. X 140. 
 
 A transverse section through the distal ends of the cones. X 475. 
 
 A transverse section through the proximal end of a cone. X 475. 
 
 A transverse section through the distal ends of three groups of retinular 
 * cells. In each group the corresponding cells have been designated 
 by the same number. X 475. 
 
 A transverse section through the central part of four rhabdomes. X 475. 
 A transverse section through a retinula. Depigmented. Kleinenberg’s 
 alum-hsematoxylin. X 475. 
 
 Fig. 40. 
 
 “ 41. 
 
 " 42. 
 
 “ 43. 
 
 “ 44. 
 
 “ 45. 
 
 Fig. 33. 
 “ 34. 
 
 “ 35. 
 
 “ 36. 
 
 Si 37. 
 “ 38. 
 
 “ 39. 
 
fitfLE dal. 
 
 Phyilopoda. 
 
 B.Meisel , litk Boston. 
 

 
 
 
 THE LIBRARY 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
Parker. — Compound Eyes in Crustaceans. 
 
 Fig. 46. 
 
 Fig. 47. 
 
 “ 48. 
 
 Fig. 49, 
 
 “ 50. 
 
 “ 51. 
 
 “ 52. 
 
 “ 53. 
 
 “ 54. 
 
 “ 55. 
 
 “ 56. 
 
 “ 57. 
 
 Fig. 58. 
 
 59 . 
 
 PLATE V. 
 
 Porcellio. 
 
 A transverse section through a retinula in a plane slightly distal to the 
 basement membrane. The single, light, central .spot represents the 
 proximal end of the rliabdome. X 475. 
 
 Iclotea robusta , Kroyer. 
 
 (Figs. 47, 48.) 
 
 A transverse section through the distal end of a retinula. The bodies, 
 one of which is marked x, are spheres of coagulated material which 
 occur in the interommatidial spaces, and which have been brought 
 into prominence by the action of the hardening reagent. X 475. 
 
 A transverse section through three ommatidia in the region of their 
 rhabdomes. X 475. 
 
 Idotea irrorata, M. Edws. 
 
 (Figs. 49-57.) 
 
 The anterior face of a section transverse to the chief axis of the body, 
 and passing through the eye on the right side of the head. X 140. 
 
 A longitudinal section of an ommatidium. The numbers at the left of 
 the figure correspond to the numbers of the following six figures 
 of transverse sections and mark the levels at which the latter were 
 taken. X 475. 
 
 A transverse section through the distal ends of the cones. X 475. 
 
 A transverse section through the middle region of a cone. X 475. 
 
 A transverse section through the middle of a retinula. Near the centre 
 of each cell can be seen a small axis of nerve fibrillse. X 475. 
 
 A transverse section through a retinula composed of seven cells instead 
 of six. This section was cut approximately at the same level as that 
 shown in the preceding figure. X 475. 
 
 A transverse section through a retinula near its proximal end. Each 
 fibrillar axis is much larger at this plane than in that shown in Fig- 
 ure 53 X 475. 
 
 A transverse section of several groups of retinular cells immediately 
 proximal to the basement membrane. X 475. 
 
 A transverse section of four retinular cells at the level in which their 
 nuclei occur. The axis of nerve fibrillae in the plane of this section 
 and in that of the preceding one (Fig. 56) are smaller than they 
 are at the base of the retina (compare Fig. 55). 
 
 Sphceroma. 
 
 (Figs. 58, 59.) 
 
 A transverse section of a retinula at a level slightly distal to the base 
 ment membrane. X 475. 
 
 A transverse section of the fibrous ends of the cells from a single re- 
 tinula. The plane of section is slightly proximal to the basement 
 membrane. The only indication of an axis of nerve fibrillse is the 
 more transparent condition of the central part of the cells, due to the 
 partial absence of pigment granules. X 475. 
 
Parker - Crustacean Eye 
 
 Pl.Y. 
 
 G.H.P dal. 
 
 ISOPODA. 
 
 BMeiseUiiH.Boslon. 
 

 
 
 THE LIBRARY 
 OP THE 
 
 UNIVERSITY OF ILLINOIS 
 
 
 
Parker. — Compound Eyes in Crustaceans. 
 
 PLATE VI. 
 
 Serolis. 
 
 Figures 60 to 64 inclusive represent the structure of the ommatidium in the 
 adult. Figures 65 to 72 are drawn from sections of ommatidia in well ad- 
 vanced embryos. All figures are magnified 475 diameters. 
 
 Fig. 60. 
 
 “ 61. 
 
 “ 62. 
 
 “ 63. 
 
 “ 64. 
 
 “ 65. 
 
 “ 66 . 
 
 67. 
 
 68 . 
 
 “ 69. 
 
 “ 70. 
 
 “ 71. 
 
 A tangential section through the most distal portion of the retina. This 
 section includes a portion of a cone and the tissue lying between it 
 and two adjoining cones. 
 
 A transverse section of a retinula in the region of its rhabdome. The 
 arrangement of the pigment granules and nerve fibrillae is indicated 
 in only one of the four cells. Of the two lines which appear to 
 separate the cone cells (cl. con.) from the rhabdomere (rhb’in.), the 
 one nearer the axis of the ommatidium is the real line of separa- 
 tion ; the other lies within the substance of the rhabdomere itself 
 (compare p. 92). 
 
 A transverse section through a retinula proximal to the rhabdome and 
 in the region of the hyaline cell. As in Figure 61, the pigment 
 granules are drawn in only one of the retinular cells. 
 
 A transverse section through a single retinular cell in the region of its 
 nucleus. The axis of nerve fibrillae is represented by several small 
 axes in the substance of the cell at one side of the nucleus. 
 
 A transverse section of the fibrous ends of the cells of one retinula in 
 their passage through the aperture in the basement membrane. 
 Each cell shows a well marked fibrillar axis, the centre of which is 
 often occupied by a core of pigment. The basement membrane is 
 viewed from its distal face. The irregularly oval body in the upper 
 left-hand corner of the figure is probably a nucleus. It lies on the 
 proximal face of the membrane through which it is seen. 
 
 A longitudinal section through the ommatidium of an advanced embryo. 
 The numbers at the left of the figure correspond to the numbers of 
 the six following figures of transverse sections, and indicate the 
 levels at which the latter were taken. Figure 68 represents a sec- 
 tion so nearly in the same plane as that shown in Figure 67 that its 
 number has been omitted. 
 
 A transverse section at the level of the corneal hypodermis. 
 
 A transverse section through the distal end of a cone. 
 
 A transverse section made in a plane only slightly proximal to that 
 shown in Figure 67. 
 
 A transverse section through the region of the distal retinular nuclei. 
 
 A transverse section through the proximal ends of the cones. 
 
 A transverse section through the retinula in the region of the rhabdome. 
 
 A transverse section at the level of the proximal retinular nuclei. 
 
Parker - Crustacean Eye 
 
 . nl.cm.l. 
 ; nl.con. 
 
 Pl.VI. 
 
 G.H.R del 
 
 Serolis . 
 
 B.MeiseUith.Boslon. 
 
THE LIBRARY 
 OF THE 
 
 UNIVERSITY of ILLINOIS 
 

 
 
 
 
 
 
 
 
 
 
 
 
 « 
 
 
 
 
 
 
Parker. — Compound Eyes in Crustaceans. 
 
 PLATE VII. 
 
 My sis . 
 
 Fig. 73. A longitudinal section of an ommatidium. The numbers at the left of 
 the figure indicate the levels at which the sections for Figures 76-89 
 were taken. X 475. 
 
 “ 74. The distal face of a corneal facet, cleaned in potash and examined in 
 
 water. X 475. 
 
 “ 75. A transverse section of three ommatidia in the plane of the corneal hy- 
 
 podermis. X 475. 
 
 “ 76. A transverse section through the distal end of a cone. X 475. 
 
 “ 77. A transverse section through the proximal end of a cone and the adjoin- 
 
 ing distal retinular cells. X 475. 
 
 u 78. A transverse section similar to that shown in the preceding figure, 
 except that it is depigmented and stained in Kleinenberg’s alum- 
 haematoxylin. X 475. 
 
 Figures 79 to 82 inclusive represent consecutive transverse sections through the 
 region of the proximal retinular nuclei of four adjacent ommatidia. The 
 centre of each ommatidium is indicated by the group of cone cells (cl. con.), 
 and the corresponding ommatidia in different sections are designated by the 
 same Roman numeral. The nuclei around ommatidium II. have been num- 
 bered in Figures 79-81. Figure 79 represents the most distal section, and 
 Figure 82 the most proximal one of the series. 
 
 Fig. 79. The bodies marked x and y are portions of nuclei the rest of which are 
 correspondingly marked in Figure 80. X 475. 
 
 “ 83. A transverse section of the four fibres at the distal end of the rod (com- 
 
 pare p. 102). Depigmented, and stained in Kleinenberg’s alum-haem- 
 atoxylin. X 615. 
 
 “ 84. A transverse section of the rod at a slightly more proximal level than 
 
 that shown in Figure 83. Depigmented, and stained in Kleinenberg’s 
 alum-haematoxylin. X 615. 
 
 “ 85. A transverse section of the retinula somewhat distal to the distal end 
 
 of the rhabdome (compare Fig. 90). Depigmented, and stained in 
 Kleinenberg’s alum-haematoxylin. X 615. 
 
 " 86. A transverse section from the region between the distal end of the rhab- 
 
 dome and the proximal end of the rod (compare 86 in Fig. 90). De- 
 pigmented, and stained in Kleinenberg’s alum-haematoxylin. X 615. 
 
 “ 87. A transverse section through the rhabdome and surrounding retinular 
 
 cells. X 615. 
 
 “ 88. A transverse section, at the level of the basement membrane, through 
 
 the nerve fibres from a single retinula. Depigmented, and stained 
 in Weigert’s haematoxylin. X 615. 
 
 “ 89. A transverse section through the fibres of the optic nerve at a level mid- 
 
 way between retina and optic ganglion. Preparation as in Figure 
 88. X 615. 
 
 “ 90. A longitudinal section through the basal portion of one and parts of two 
 
 adjoining ommatidia. Depigmented, and stained in Kleinenberg’s 
 alum-haematoxylin. X 615. 
 
 “ 91. A section cut in the same plane as that shown in the previous figure, 
 
 but including only the proximal ends of two rhabdomes. Prepara- 
 tion as in Figure 90. X 615. 
 
 “ 92. A cone viewed from the side. Isolated in Muller’s fluid and studied in 
 
 water. X 475. 
 
G.HJ? del. 
 
 Mys i s . 
 
 B.MeiseUilK .B o s to n. 
 

 the LIBRARY 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
Parker. — Compound Eyes in Crustaceans. 
 
 Fig. 93. 
 
 “ 94. 
 
 “ 95. 
 
 “ 96. 
 
 “ 97. 
 
 “ 98. 
 
 “ 99. 
 
 “ 100 . 
 
 “ 101. 
 “ 102. 
 
 PLATE VIII. 
 
 Gonodactylus. 
 
 Part of a tangential section through a superficial portion of the retina. 
 The extreme edges of the section both right and left are immediately 
 beneath the corneal cuticula ; the central portion is farthest from the 
 cuticula. At the right of the middle line are seen 'the ends of the 
 larger ommatidia ; at the left, those of the smaller. X 275. 
 
 A longitudinal section of a large ommatidium. The numbers at the left 
 of the figure correspond to the numbers of six figures of transverse 
 sections (Figs. 96-101), and mark the levels at which the latter were 
 made. Depigmented. X 275. 
 
 A longitudinal section of a small ommatidium containing its natural 
 pigment. X 275. 
 
 A transverse section through the cells of the corneal hypodermis and the 
 distal end of the cone in a large ommatidium. X 275. 
 
 A transverse section through the distal part of a cone in a large omma- 
 tidium. X 275. 
 
 A transverse section through the middle of a cone from a large omma- 
 tidium. X 275. 
 
 A tranverse section through a number of cones at the level of the distal 
 retinular nuclei in the large ommatidia. X 275. 
 
 A transverse section through six retinulse of the large ommatidia in 
 the region of the proximal nuclei. Each retin ula is numbered. The 
 plane of this section is slightly oblique, so that retinula 1 is cut at a 
 relatively higher level than any of the others, and retinula 6 at the 
 ' lowest level. X 475. 
 
 A transverse section of a retinula from one of the larger ommatidia, in a 
 plane not far from the basement membrane. Depigmented, X 475. 
 
 A transverse section of a retinula from one of the smaller ommatidia cut 
 in a plane nearly corresponding to that of Figure 101. X 475. 
 
Parker - Crustacean Eye 
 
 PL.vm. 
 
 nl.c-rrv. 
 
 nl.con. 
 
 nl.con. 
 
 nl.dst. 
 
 ax.n. 
 
 ax.n. 
 
 nl.ms’drm. 
 
 pfff m4Ai 
 
 nl.dst. 
 
 G.H.P del. 
 
 Gonodactylus . 
 
 B.Meisel,litK.B o s to n. 
 

 ?HE UBRAHY 
 
 or THE 
 
 tWVERSiTY Of ILLINOIS 
 
 
 
Parker. — Compound Eyes in Crustaceans. 
 
 PLATE IX. 
 
 Palcemonetes. 
 
 In all Figures on this plate the magnification is 475 diameters. 
 
 Fig. 103. A longitudinal section of an ommatidium. The numbers. at the left of 
 the figure correspond to the numbers of nine of the following fig- 
 ures of transverse sections, and mark the levels at which the latter 
 were taken. 
 
 104. A longitudinal section of an ommatidium which has been depigmented. 
 
 The bodies marked x resulted from the action of the depigmenting 
 reagent. 
 
 “ 105. A facet from the corneal cuticula; cleaned in strong potassic hydrate, 
 and examined from its distal side in water. 
 
 “ 106. A transverse section through the region of the corneal hypodermis. 
 
 “ 107. A transverse section through the distal end of a cone in the region of 
 the nuclei of the cone cells. 
 
 “ 108. A transverse section through the middle of a cone. 
 
 “ 109. A transverse section through parts of four ommatidia in the region of 
 the distal retinular nuclei. 
 
 Figures 110-112 represent three successive transverse sections, each through 
 five ommatidia, in the region of their proximal retinular nuclei. Only the 
 outlines of the nuclei and the five groups of cone cells (cl. con.) are drawn. 
 The nuclei in each ommatidium are numbered from 1 to 7, and as their plan 
 of arrangement is the same in the different ommatidia, corresponding nuclei 
 have been designated by the same number. In some cases the nuclei were 
 cut in two, and consequently appear in two adjoining sections. In such 
 cases the two parts have been marked with the same number. Figure 110 
 is the most distal of the series; Figure 112, the most proximal. 
 
 Fig. 113. A transverse section of the retinula near the distal end of the rhabdome. 
 Depigmented. 
 
 “ 114. A transverse section of four retinulae at the level of the eighth retinular 
 nucleus. 
 
 “ 115. A transverse section through four retinul® in the region of the accessory 
 pigment cells; viewed by reflected light. The retinulae appear as 
 dark masses embedded in a whitish field composed for the most part 
 of the substance of the accessory pigment cells. 
 
 “ 116. A transverse section through a retinula at about the same level as that 
 shown in Figure 115. Depigmented. 
 
 “ 117. A transverse section through the optic nerve fibres at a level slightly 
 proximal to the basement membrane. Depigmented. 
 
Parker - Crustacean Eye 
 
 Pl.IX. 
 
 704 -. 
 
 705 . 
 
 nl.cm. 
 
 nldst. 
 
 nl.ms’drm,.. 
 
 debt. 
 
 nLmstbm. 
 
 
 nlmscbm. 
 
 ( 
 
 ( 
 
 r 
 
 
 ! i 
 
 G.KP del. 
 
 Palamoketes. 
 
 BMeiseLiith. Boston. 
 

Parker. — Compound Eyes in Crustaceans. 
 
 PLATE X. 
 
 In all Figures on this plate the magnification is 475 diameters. 
 
 Cambarus. 
 
 Figures 118-122 represent a series of five successive transverse sections through one 
 and parts of four adjoining ommatidia in the region of their proximal 
 retinular nuclei. Figure 118 represents the most distal section in 
 the series; Figure 122, the most proximal. In these figures, only 
 the outlines of the nuclei and the groups of cone cells are drawn. 
 
 Crangon. 
 
 Fig. 123. A transverse section through a number of ommatidia in the region of 
 their distal retinular nuclei. 
 
 Pahnurus. 
 
 Fig. 124. A transverse section through a retinula in its middle region. The out- 
 lines of the retinular cells cannot be distinguished ; the position of 
 each cell is marked by an irregular light mass in its centre. 
 
 “ 125. A transverse section through 'a retinula in the plane of its eighth 
 nucleus. Depigmented. 
 
 Cancer. 
 
 (Figs. 126-131.) 
 
 Fig. 126. 
 
 A 
 
 “ 127. 
 
 A 
 
 “ 128. 
 
 A 
 
 “ 129. 
 
 A 
 
 “ 130. 
 
 A 
 
 “ 131. 
 
 A 
 
 corneal facet viewed from its distal surface. The cuticula from which 
 this facet was drawn was cleaned by being boiled in a strong aqueous 
 solution of potassic hydrate. It was examined in water 
 transverse section of the distal end of a cone. 
 
 transverse section through three ommatidia at the level of the distal 
 retinular nuclei. The pigment granules have been indicated in only 
 the lower circle of cells. 
 
 transverse section through the distal region of four retinulae. In the 
 two on the right, the pigment granules have not been drawn 
 transverse section through a retinula near the base of the retina, 
 slightly oblique section through the basement membrane. The upper 
 part of the figure represents the retinulae as seen m transverse sec- 
 tion distal to the basement membrane; the part marked mb. bci. 
 represents the region in which the membrane itself appears in section, 
 and the lower half of the figure shows the cut fibres of the optic 
 nerve. The pigment granules are omitted from the right side of the 
 figure. The transition from the retinular cells to the nerve fibres 
 is evident in passing over the section from top to bottom. 
 
Parker - Crustacean Eye 
 
 Pl.X. 
 
 Decapoda 
 
 B.MeiselM.Boston 
 
 H R del. 
 
 727 . 
 
 0 _ O nlnx ' 
 
 cLcon. 
 
 121 . 
 
 ® °&)© 
 
 O 
 
 iCD ^ O O 
 
 ooo 
 
 123 . 
 
 720 . 
 
 722 . 
 
 \ ^ ..d.con. 
 
 o 
 
 m 
 
 131 . 
 

 THE LIBRARY 
 OF THE 
 
 OWVESSITY OF ILLINOIS 
 
Sonder-Abdruck aus: 
 
 Anatomisclier Anzeiger. 
 
 Centralblatt fur die gesamte wissenschaftliche Anatomie. 
 
 Amtliches Organ der Anatomischen Gesellschaffc. 
 
 • Herausgeg. von Prof. K. Bardeleben in Jena. — Yerlag von Gustav Fischer in Jena. 
 Y. Jahrgang (1890), Nr. 13 und 14. 
 
 Nachdruck verboten. 
 
 Is the Oinmatidium a Hair-hearing Sense Bud ? 
 
 By William Patten, Ph. D., Prof, of Biology in the University 
 of North Dakota, Grand Forks, N. D. U. S. A. 
 
 With 4 figures. 
 
 For the past year or two, I have from time to time examined 
 material that promised to shed some light on the nature of the higher 
 sense organs. The facts thus obtained, being of general interest and 
 necessitating a modification of my earlier views on this subject, are 
 herewith summarily presented, together with a few suggestions of a 
 theoretical nature. 
 
 The descriptive part refers principally to the development of the 
 convex eyes and frontal ocelli of Vespa, Aphis and Formica, 
 and to the structure of the eyes in adult Belostoma and T a b a n u s. 
 
 25 
 
354 
 
 The main conclusion is that the convex eye of Arthro- 
 pods is a group of hair-bearing sense buds. 
 
 Each ommatidium of Belostoma contains one very large cell, 
 whose pointed outer end terminates in a double rod lying in the centre 
 of the retinula (fig. 1, g 1 ). In some cases this cell is double, like 
 the retinophorae in the ocelli of Acilius; and there are indications 
 that the six peripheral retinula cells are built on the same plan. 
 There are at least three other cells in the centre of each ommatidium. 
 The lai gest one is spindle-shaped and its outer end may appear either 
 fibrous or granular, or as a slender, hyaline rod that lies side of the 
 double one belonging to the large retinula cell described above (fig. 1, 
 g 2 and A 1 ). Of the two remaining cells to which no rods are 
 attached, one is for the most part a coarse fibre. Its nucleus, if still 
 within the limits of the eye, is probably one of the small red bodies 
 often seen near the base of the ommatidium. Each of these nine or 
 ten retinula cells is continuous with »a bundle of nerve fibrillae 
 (fig. 1, D). 
 
 In Tabanus, at the base of the circle of seven, equal, retinula 
 cells, is an eighth cell, which is very short, but nevertheless provided 
 with a distinct rod and nucleus (fig. 2, b and c). Within the circle is 
 a bundle of axial nerve fibres which can be traced through the base- 
 ment membrane into the optic nerves (fig. 2, b and c). 
 
 The cone cells in Belostoma are very small and their rods 
 are only slightly thickened and refractive in the place where the cone 
 in other Arthropods is present. Each cone cell terminates in 
 a hair-like process which probably abuts against the cornea 
 (fig. 1, r. A). 
 
 The inner ends of the cone cells extend , between the retinula 
 cells, as four delicate fibres along the outer surface of the rhabdom; 
 I could not trace them inwards as far as the basement membrane. 
 Essentially the same condition seems to prevail in Tabanus and in 
 Vespa during the pupal stages. In Homarus also, according to 
 Parker, the ends of the cone cells pass around the spindle. Thus 
 -in these cases, contrary to my former observations on Penaeus, 
 ) th e cone cells are not continuous with the rhabdoms. 
 My observations on Belostoma and Vespa were made independ- 
 ently and some time before the appearance of Parker’s paper. Publi- 
 cation was delayed in order to study earlier stages, but the necessary 
 material could not be obtained. 
 
 In Tabanus, the distal ends of the cone cells form a hya- 
 line rod, the pseudocone, which is expanded and hollow at the 
 
355 
 
 summit, where it is continuous with the overlying corneal facet 
 (fig. 2, ps. c.). 
 
 In Vespa during part of the pupal stage, the cone cells lie at 
 the bottom of a considerable depression, and their outer ends 
 are conspicuously pointed, as in Belostoma. Over the 
 ommatidia, are faintly stained, rounded bodies which I formerly mistook 
 for nuclei of the corneagen (fig. 3, p. s. c.). But further study shows 
 they are chitenous bodies secreted by the cone cells 
 and that they probably correspond to the pseudocone 
 in Tabanus and Belostoma. 
 
 Fig. 1 . Semidiagrammatic view of an ommatidium of Belostoma. 
 
 Fig. 2. Same, of Tabanus. 
 
 Fig. 3. Same, of young pupa of Vespa: A — D. Cross sections of ommatidium 
 and points whence they were taken, a — c. Same of Tabanus. E. Semidiagrammatic 
 view of cone cells in an old Hymenopterous pupa, ax.n ., axial nerve bundle, c.c., 
 crystalline cone cells, c.g., incipient corneagen. <7 1 — 4, incipient ganglion cells, n , nerve 
 branch, o.p., optic pit. o.s., ommatidial spine, o.t ., cuticular tube, formed by circle of 
 cells around the ommatidium. p g., primary pigment cell, ps.c ., pseudocone, r.h ., rudi- 
 mentary hairs on cone cells, t., trachea. 
 
 The presence of these hair-like pseudocones over the cone cells of 
 Belostoma, Tabanus and Vespa cannot but awake the sus- 
 picion that the ommatidia are modified hair-bearing organs, and this 
 suspicion is fully confirmed by the fact that , in the young pupae of 
 Vespa, the first corneal cuticula is actually provided with hair-like 
 spines unquestionably formed by the hardening of the outer ends of 
 
 25 * 
 
356 
 
 the pseudocones (fig. 3, o. s.). This spine-bearing cornea is soon shed 
 and a faceted one formed, each facet, which is in the main the product 
 of two newly formed corneagen cells, often containing in the centre 
 the remnants of an ommatidial spine! The pseudocone disappears 
 soon after the spine-bearing cornea is shed. Just outside of the eye 
 of Vespa are many hair-bearing cells over which, in the first pupal 
 cuticula, temporary spines are formed resembling those over the omma- 
 tidia. This cuticle is apparently pushed off by the growth of perma- 
 nent hairs, which at first are protoplasmic outgrowths or secretions 
 which resemble in almost every particular the pseudocone of Vespa! 
 In Aphis similar hair cells are found in abundance between the 
 ommatidia. 
 
 Now if the ommatidia are hair-bearing sense buds, we ought to 
 find (1) some resemblance between isolated hair cells and retino- 
 phorae, (2) we ought to find isolated hair cells acting as rudimentary 
 ommatidia. Such is really the case, for, as regards the first point, 
 the isolated hair cells of Vespa are beyond all question double 
 cells and contain a coiled canal, which for several reasons I believe 
 to be continuous with a nerve tube (fig. 4 C.). After the first pupal 
 moult, the larger component cell forms a long, protoplasmic process 
 which is finally converted bodily into one of the bristles so abundant 
 on the forehead and about the eyes. Briefly stated, these double hair 
 cells resemble the retinophorae of Molluscs and Arthropods: (1) in their 
 axial nerve canals, (2) in the imperfect union of their twisted, com- 
 ponent cells, and (3) in the position, size, and color of their nuclei 
 (fig. 4 (7.). As regards the second point, I have found hair cells 
 between the ommatidia in the convex eyes of Aphis, Vespa 
 and Belostoma; in the last two cases, the protruding 
 hairs were absent or very rudimentary, and in all 
 three, the cells were surrounded by a layer of pigment 
 so that they bore a striking resemblance to very 
 simple ommatidia and probably functioned as such! 
 
 The development and permanent condition of the corneagen bears 
 directly on the views here advanced; and I consider it an important 
 fact that I am now able to show, after the examination of new ma- 
 terial, that the corneagen of Vespa is not formed by the union of 
 two folds over the whole eye, as I formerly suspected, but by the 
 growth of two cells over the distal end of each ommatidium (fig. 3, 
 c. g. and E). Even more important is the fact that in the oldest stages 
 of Vespa there are minute openings in the corneagen, through which 
 protrude hair-like projections of the cone cells (fig. 3 E .), making a 
 
357 
 
 small scar or indentation in the centre of each facet, like those de- 
 scribed by me in P e n a e u s. 
 
 In both, B e lost o ma and Tabanus, a corneagen is present, but 
 it is evidently in a very primitive condition ; there is a large, circular 
 opening between every two cells, for the passage outward of the pseudo- 
 cones , and each pair of cells forms the wall of a deep cup closed 
 by the facet only (figs. 1 and 2, c. g.). As this condition is essentially 
 a repetition of the early stages of Vespa, we have a satisfactory de- 
 monstration that the pseudocone type is the most primi- 
 tive. This fact, together with the hair-like nature of the pseudo- 
 cones, the resemblance of isolated hair cells to retinophorae or even 
 to ommatidia, and the presence of primitive corneal spines in the eyes 
 of Vespa, render it almost certain that the pseudocone omma- 
 tidium is nothing but a hair-bearing sense bud. In accordance with 
 this view, the pseudocone may be regarded as one or more hairs 
 standing in a pit, the mouth of which is closed by the expanded ends 
 of the hairs, now converted into corneal facets. The retinula cells 
 would there correspond to the so called „ganglion cells‘ c which at one 
 time probably extended, as fine processes, into the pseudocone. Indeed 
 this condition seems to be retained in some of the Diptera described 
 by Ciaccio (Fig. 4 B.). 
 
 Fig. 4. A. Diagram of an “olfactory” hair-pit. Partly from Lubbock after Hauser. 
 B . Hypothetical primitive ommatidium , a combination of the young stage in Vespa 
 whit the adult stage in Tabanus. C. Hair cell from the neighborhood of the eyes in 
 V e s p a ; partly diagrammatic. D. Diagram to illustrate conversion of retinophorae into 
 ganglion cells. E. A hypothetical ommatidium like sense bud. 
 
358 
 
 I wish to say here, in regard to Watase’s recently published 
 views on the nature of the compound eye, Johns Hopkins Circulars, 
 1889—90, that the large cell in Belostoma is like the ganglion 
 cell described by him in the lateral eyes of Limulus, but in my 
 opinion the facts here presented show that there is no homology be- 
 tween them, and that in Limulus this cell cannot have the morpho- 
 logical importance which he attributes to it. Again, the retinula cells re- 
 semble in general appearance the gigantic cells at the bottom of the 
 larval ocelli of Acilius, but development shows that in Vespa 
 there is no invagination of the ommatidia and no bending of the re- 
 tinula cells, so that the rhabdomeres and the segments of the true 
 crystalline cone must be formed along the sides of their respective 
 cells and not on their outer ends. It is impossible, I believe, to har- 
 
 ( monize these facts with Watase’s views, the essential features of which 
 I had already independently formulated and was ready to maintain, 
 when these observations rendered them untenable. 
 
 I was formerly in doubt as to the origin of the corneagen of the 
 frontal ocelli (Eyes of Vespa, p. 212), but I am now convinced, after 
 the examination of new material, that in Vespa, Aphis and For- 
 mica it arises by a process of delamination, similar to that which 
 gives rise to the corneagen of the convex eye ; consequently it is very 
 different from that of the larval ocelli of Acilius. This fact 
 strengthens the idea suggested in my paper upon Eyes of Acilius, 
 p. 167, that the frontal ocelli were closely related to the convex eyes. 
 Moreover the development of the corneagen indicates that the retina 
 of the frontal ocelli, like the ommateum , is formed by the union of 
 independent organs, each one surrounded by indifferent cells. As I 
 have already shown that the larval ocelli of Acilius are also formed 
 by the union of sense buds A ), it becomes highly probable that all the 
 higher sense organs of Arthropods are formed by the aggregation of 
 sense buds, and, for reasons to be presented elsewhere, it is also prob- 
 able that the same condition prevails in the eyes and nose of Verte- 
 brates. Moreover the cephalic sense buds of fishes may be compared 
 with the sense hairs, pits, and perforated spines in the cephalothorax 
 of Arthropods. For example in Vespa and Aphis the circle of cells 
 surrounding each ommatidium forms, at first, a long chitenous tube, 
 open at the top; if the rudimentary hairs of the cone cells actually 
 
 1) This fact has an important bearing on Watase’s speculations, but 
 he has entirely ignored it, as well as my other observations on the po- 
 sition and character of retinal rods, and the nerve fibrillae in them. 
 
359 
 
 pierced the corneal facet, a sense bud not unlike some found in fishes 
 etc. would be formed (fig. 3, o. t. and fig. 4 E.). 
 
 It is almost certain that the four central retinula cells of Belostoma 
 are in various of stages withdrawal to form ganglion cells, and that the axial 
 fibres in the retinula of Tab anus are the outer ends of cells that have 
 completed this process! Theoretically an indefinite number of retinophorae 
 may withdraw in this way from the same point, each leaving its 
 attenuated outer end between two newly formed sense cells (compare 
 V espa, Acilius, Belostoma, Tabanus, P ecten); a string 
 of double ganglion cells might thus be formed, the whole series 
 terminating at one end in a perfect ganglion cell and at the other in 
 a newly formed retinophora (fig. 4 D.). This telescoping of indi- 
 vidual, epithelial cells, or of a small number of them, seems to me 
 to be a true phylogenetic process and invagination a purely ontogenetic 
 one. Invagination probably occurs only in compound 
 sense organs and there as an incidental result of the 
 rapid inwandering of ganglion cells which thus causes 
 an enlargement of the inner surface of the sensory 
 layer and consequently a warping of the whole organ. 
 In the convex eye of V e s p a , the temporary invagination produced in 
 this way is very deep (see Eyes of Vespa), yet it is not so deep but 
 that, when the warping ceases, the eye returns to its natural con- 
 dition. But we can readily see how an increase in the strength and 
 duration of the warping might make a return to the original condition 
 impossible; if so, this convex eye, might, in a single generation, 
 be converted into an open pit or a closed vesicle; and it is obvious 
 that the advantage thus gained in shape and protection would be a 
 purely incidental result of its increasing complexity. Thus it is 
 not at all necessary toassume, as is usually done, that 
 the adult ancestors of animals with vesicular eyes had 
 eyes in progressive stages of invagination. I believe we 
 may safely assume that primitive sense organs, ganglia, nerve centres etc. 
 have been formed, phylogenetically, by the telescoping of in- 
 dividual epithelial cells; this process when repeated ontogenetic- 
 ally gives rise to invaginations, for the reasons given above. The 
 present condition of the more complex sense organs is a resultant of 
 the phylogenetic and the ontogenetic process. 
 
 This explanation may be extensively applied to explain invagin- 
 ation and evagination, and also to account for the sudden appearance 
 of new types of organs. 
 
 Grand Forks, N. Dak., May 15 th 1890. 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 f 
 
 ' 
 
 *• » ~ 
 
 
 
 
 
 ' 
 
 
 
 
 
 
 
 © 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 * 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 , ' , ■ 
 
 
 
A-. 
 
 
 Deber den Ban der Phalangidenangen. 
 
 INAUGURAL -DISSERTATION 
 
 ZUR 
 
 ERLANGUNG DER DOCTORWURDE 
 
 YON DER PHILOSOPHISCHEN F1CULTAT 
 
 DER 
 
 FRIEDRICH -WILHELMS- UNIVERSIT AT Z U BERLIN 
 
 GKNEHMIGT 
 
 UXD 
 
 NEBST DEN BEIGEFUGTEN THESEN 
 
 OEFFENTLICH ZU VERTHEIDIGEN 
 
 am 13. Januar 1894 
 
 von 
 
 Frederick Purcell 
 
 aus Capstadt. 
 
 OPPONENTEN: 
 
 Hr. Cand. phil. E. Menne. 
 
 - Cand. phil. F. Schaudinn. 
 
 - Cand. phil. T. Montgomery. 
 
 BERLIN. 
 
 Buchdruckerei von Gustav Schade (Otto Francke). 
 Linienstrasse 158. 
 
Seiner theuren Mutter 
 
 in Liebe und Dankbarkeit 
 
 gewidmet 
 
 vom 
 
 Verfasser. 
 
Einleitung. 
 
 Wiederholt wurde in den letzten Jahren von ver- 
 schiedenen, darunter auch recht guten Beobachtern, der 
 Bau der Arachnidenaugen studirt. Zumeist betreffen die 
 Untersuchungen die Skorpione und Spinnen. Den Pha- 
 langiden wurde nach dieser Hinsicbt weniger Aufmerk- 
 samkeit geschenkt und die beiden einzigen Forscher, 
 Grenacher und Patten, welche sie mit Hilfe der 
 neuen Metboden untersuchten, gelangten beziiglich der 
 Auffassung der Phalangidenaugen zu vollig entgegen- 
 gesetzten Resultaten. 
 
 Der erste, welcher sich, soweit mir bekannt ist, mit 
 der Anatomie der Phalangidenaugen beschaftigte, war 
 Treviranus (1816 No. 26), dem dann einige Zeit 
 spater Tulk (1843 No. 27) folgte. Beide Arbeiten sind 
 im alten Style geschrieben und haben nur noch histori- 
 sches Interesse. 
 
 Ley dig (1853/62 No. 13/14) verdanken wir einige 
 gelegentliche Bemerkungen fiber verscbiedene Theile des 
 Auges; eine eigentliche Beschreibung wurde jedoch nicht 
 von ihm gegeben. 
 
 Erst Grenacher (1879 No. 6), welcher in seiner 
 grossen Monographic auch das Auge von Phalangium 
 opilio L. behandelte und es durch einige Abbildungen 
 erlauterte, giebt die erste eingehende und, abgesehen 
 von einem wichtigen Punkte, die genaueste Darstellung, 
 die wir besitzen. 
 
6 
 
 1887 veroffentlichte Patten (20) eine kurze, andert- 
 halb Seiten umfassende vorlaufige Mittheilang ohne Ab- 
 bildangen fiber eine vermuthlich amerikanische Species 
 von Phalangium. 
 
 Endlich habe ich selbst (22) eine kurze vorlaufige 
 Notiz fiber die Anatomie und Entwicklung des Auges 
 von Leiobunum rotundum Latr. (L. kemisphaericum 
 Herbst) gegeben, welche sich aber nur auf einige 
 wenige der bemerkenswerthesten Zfige beschrankte. 
 
 Die vorliegende Untersuchung beabsichtigt, eine 
 moglichst genaue und vollstandige Beschreibung der 
 Retinaelemente zu geben. Die Linse und der Glas- 
 korper werden besonders aus dem Grunde, weil sie von 
 Grenacher schon sehr genau beschrieben sind, weniger 
 eingehend behandelt werden. 
 
 Die Praparate, welche als Beleg ffir die vorliegen- 
 den Ausffihrungen dienen, wurden bei Gelegenheit der 
 zweiten Jahresversammlung der Deutschen Zoologischen 
 Gesellschaft demonstrirt. Die Beobachtungen sind an 
 alien denjenigen Species, welche ich mir in der Um- 
 gegend von Berlin in genfigender Menge verschaffen 
 konnte, gewonnen worden. Diese acht Species gehoren 
 sechs Genera*) an. 
 
 *) Die Bestiminung der Species geschah mit Hilfe des aus- 
 gezeichneten Buches von Simon (24), dessen NomeDklatur ich, 
 ausser in dem folgenden Punkte, angewendet habe. Das Genus 
 Phalangium, wie ich es hier verstehe und welches in meiner Arbeit 
 durch Ph. opilio L. und Ph. brevicorne C. K. vertreten ist, stimmt 
 iiberein mit dem Genus Cerastoma von C. L. Koch und Anderen. 
 Es ist charakterisirt durch die hornahnliche Verlangerung, welche 
 sich an der Basis des zweiten Gliedes der Cheliceren beim Mannchen 
 findet. Simon dagegen, welcher keinen Unterschied zwischen den 
 Weibchen von Cerastoma und Opilio fand, vereinigte beide Genera 
 unter dem Namen Phalangium. Ich habe nun aber gefunden, dass 
 die Species von Koch’s Gattung Cerastoma (Ph. opilio und brevicorne) 
 sehr nahe Leiobunum verwandt sind, aber betrachtlich von Opilio 
 
7 
 
 Dieselben lassen sich nacb dem Bau ihrer Rhab- 
 dome in zwei natiirliche Gruppen unterscheiden, welche 
 ich durch den Namen eines charakteristischeri Genus 
 gekennzeichnet babe: 
 
 a) die Leiobunum- Gruppe; sie umfasst: 
 
 1. Leiobunum rotundum Latr. (L. hemisphaeri- 
 cum Herbst), 
 
 2. Phalangium opilio L. (P. cornutum L.), 
 
 3. Pbalangium brevicorne C. Koch, 
 
 4. Platybunus triangularis Herbst; 
 
 b) die Acantholophus-Gruppe; sie umfasst: 
 
 5. Opilio parietinus de Geer, 
 
 6. Acantholopbus bispidus Herbst, 
 
 7. Oligolophus palpinalis Herbst (terricola C. 
 Koch), 
 
 8. Oligolophus tridens C. Koch. 
 
 Jede von diesen Arten wurde von mir mit mog- 
 lichster Genauigkeit gepriift, und alle Angaben in der 
 folgenden Darstellung gelten fur alle Species, falls das 
 Gegentheil nicht besonders bervorgeboben ist. 
 
 Es ist mir eine angenehme Pflicht, Herrn Geheim- 
 rath Professor F. E. Schulze fur das lebhafte Interesse, 
 das er meiner Arbeit zuwandte, sowie fur die Ueber- 
 lassung der reicben Hulfsmittel des Berliner Zoologiscben 
 Instituts meinen aufrichtigen Dank auszusprechen. Auch 
 Herrn Professor E. Korschelt bin ich fur vielfachen 
 freundlicben Rath zu berzlichem Dank verpflichtet. 
 
 parietinus de Geer abweichen, wahrend die letztere Art wiederum 
 sehr nahe Acantholophus uud Oligolophus verwandt ist. Aus diesen 
 Griinden bin ich gezwungen, die beiden erstgenannten Species 
 (Ph. opilio und brevicorne) von Opilio parietinus wieder zu trennen, 
 und ich habe statt Koch’s Namen Cerastoma den Namen Phalan- 
 gium L., fur welche Gattung Ph. opilio L. typisch ist, gebraucht, 
 wahrend Opilio Herbst fur die Gattung bleibt, fur welche 0. pa- 
 rietinus de Geer den Typus darstellt. 
 
8 
 
 Untersuchungsmetlioclen. 
 
 Yon den technischen Schwierigkeiten, mit welchen 
 bekanntlich eine Untersuchung der Retina zu kampfen 
 hat, ist die grosste diejenige, eine geeignete Konser- 
 virungsfliissigkeit zu finden. 
 
 Die gewohnlichen kalten wassrigen Losungen 
 sind wenig brauchbar, wahrscheinlich, weil sie viel zu 
 langsam eindringen und die rasch sich verandernden 
 Gewebe des Auges erst einige Zeit nach dem Tode er- 
 reichen. 
 
 Warme wassrige Losungen zwischen 45° C. 
 und 95° C. dringen zwar rasch ein, aber die wichtigen 
 Zellgrenzen werden oft undeutlich wiedergegeben und 
 die Rhabdome werden durch die Hitze stark verlangert, 
 so dass ihre Bilder zum Studium weniger geeignet sind. 
 
 Um diese Schwierigkeiten zu iiberwinden und um 
 ein rascheres Eindringen ohne Anwendung von Hitze 
 zu bewirken, bin ich fast ausschliesslich nach folgendem 
 Princip vorgegangen. 
 
 Die gewohnlichen wassrigen Reagentien (Pikrin- 
 saure, Flemming’sche Losung etc.) werden mit gleichen 
 Mengen von absolutem Alkohol vermischt, wodurch sich 
 eine Fliissigkeit ergiebt, die 50 °/ 0 Alkohol enthalt und 
 welche die Fahigkeit hat, schnell in das Gewebe einzu- 
 dringen. 
 
 Diese Mischungen habe ich gewohnlich in kaltem 
 Zustande, nur fur bestimmte Zwecke auch warm ange- 
 wandt. 
 
 Alkoholische Pikrinsaurelosung, welche ich 
 durch Mischung von absolutem Alkohol und einer 
 gesattigten wassrigen Pikrinsaurelosung zu gleichen 
 Raumtheilen erhielt, ist die bei weitem beste Kon- 
 servirungsflussigkeit, welche ich kenne, fur die Dar- 
 stellung der feinsten Strukturen in der Retina. Der 
 
9 
 
 abgeschnittene Cephalothorax bleibt in der Mischung 2 
 bis 4 Stunden und wird nacbber mit 63proc. Alkohol 
 ausgewaschen. Die Kerne, Zellgrenzen, die Waben- 
 struktur des Protoplasmas und die Rhabdome bleiben 
 ausgezeichnet erhalten. Das hohe Lichtbrechungsver- 
 mogen der Rhabdome und die Eigenschaft der ver- 
 scbiedenen Theile der letzteren, sick verschieden zu 
 farben, sowie besonders die Losbarkeit des Pigments 
 werden dadurck nickt beeintrachtigt oder gemindert. 
 Gewisse Nacktkeile des Reagens beruken in der sckleckten 
 Konservirung der Nervenfasern. Infolge dessen mussten 
 neben dieser Metkode nock andere angewendet werden. 
 
 Da die Nervenfasern durck kalte Flussigkeiten 
 iiberkaupt sekr sckleckt zu konserviren sind, so muss 
 man die betreffenden Reagentien warm anwenden und 
 zwar bei Temperaturen zwiscken 35° C. und 95° C. 
 (z. B. alkokol. Pikrinsaure bei 45° C.; Pikrinsckwefel- 
 saure bei 56° C.). 
 
 Zum Studium der Rhabdome eignet sick am besten 
 die Hartung in kalter, alkoholischer Pikrinsaure und die 
 Farbung mit Hamatoxylin. 
 
 Um bei gewissen Species (z. B. Acantkolopkus) 
 die Wabenstruktur deutlick zu machen, muss man, so 
 lange das Objekt in der Flussigkeit sick befindet, die- 
 selbe auf 35° C. erwarmt kalten. 
 
 Ferner babe ich, um eine Differenzirung zwiscken 
 dem centralen Rhabdomer und den peripheren Rhab- 
 domeren zu bekommen, die Objekte 20 Minuten bei 
 45° C. bis 50° C. in 50proc. Alkokol, der bei derselben 
 Temperatur mit Pikrinsaure gesattigt war, gehartet. 
 Das Pigment des Auges, welches durch die warme 
 Flussigkeit gelost wird, farbt die Kerne und gewisse 
 Theile des Rhabdoms (besonders das centrale Rhab- 
 domer) tief braun oder sckwarz, wahrend die iibrigen 
 Theile des Rhabdoms ungefarbt bleiben oder hochstens 
 
10 
 
 einen hellbraunen Ton annehmen. Aus solchen Pra- 
 paraten darf natiirlich das Pigment nicht entfernt werden 
 und eine weitere Farbung ist iiberfliissig. 
 
 Uebersichtsbilder, welche die richtigen topo- 
 graphischen Verhaltnisse der Theile des Auges zeigen, 
 sind sehr schwer zu gewinnen, weil bei Anwendung 
 von kalten Losungen der Glaskorper schrumpft, bei An- 
 wendung von warmen die Rhabdome und Retinazellen 
 sich verlangern. Am besten eignet sich hierfiir viel- 
 leicht eine Mischung von Flemming’scher Losung und 
 Alcohol absolutus zu gleichen Theilen, welche man 
 bei gewohnlicher Temperatur anwendet*, ferner auch 
 warme alkoholische Mischungen mit Flemmi ng’scher 
 Losung, Pikrinsaure etc. bei einer Temperatur von 
 56° C. fur 40 Minuten. 
 
 Das Pigment wurde mittelst einer Mischung von 
 2 Theilen 80 °/ 0 Alkohol, 1 Theil Glycerin und 2 — 3 °/ 0 
 Salzsaure (nach Grenachers Angabe No. 8) entfernt. 
 Die Fahigkeit dieser Mischung, das Pigment zu losen, 
 wird sehr durch die Methode der Hartung beeinflusst. 
 Sie entfernt das Pigment vollstandig in % — 1 / 2 Minute 
 aus diinnen Schnitten (3,5 [i) t welche in kalter alkoholi- 
 scher Pikrinsaure gehartet sind, ohne das Gewebe im 
 geringsten zu schadigen. 
 
 Dagegen ist fur solche Praparate, welche in warmen 
 Fliissigkeiten gehartet sind, eine langere Einwirkung und 
 oft ein gelindes Erwarmen nothig, wobei die Schnitte 
 mehr oder weniger stark leiden. Man farbt am besten 
 mit Hamatoxylin. Schnitte von 3,5 ^ Dicke werden 
 nach der Entfernung des Pigments 20 Minuten in Dela- 
 field’sches Hamatoxylin gebracht, dann einige Stunden 
 in gewohnlichem Wasser (nicht salzsaurem Alkohol) aus- 
 gewaschen. 
 
 Es ist infolge der Kleinheit der Elemente der 
 Retina unbedingt nothwendig, liickenlose Serien von 
 
11 
 
 Schnitten zu bekommen, welche nicht mebr als 3,5 
 dick sind. Es wiirde mir dieses ohne Anwendung der 
 Losung von Mastix und Kollodium (in Alkohol und 
 Aether) unmoglich gewesen sein. Diese Methode ver- 
 danke ich der Freundlichkeit des Herrn Professor Karl 
 Heider*), der sie zuerst in seiner Hydrophilus -Arbeit 
 erwahnt. 
 
 Die Oberflache des Paraffinblockes wird jedesmal 
 vor dem Schneiden mit dieser Losung iiberstrichen. Das 
 diinne Hautchen von Mastix-Kollodium, welches nach 
 der Verdunstung des Alkohols und Aethers zuriickbleibt, 
 geniigt, um ein Zerreissen oder Brechen des Schnittes, 
 wenn das . Messer durch das harte Chitin des Korpers 
 geht, zu verhindern. Das Schneiden wird dadurch sehr 
 erleichtert, dass man zuerst mit dem Mikrotom einen 
 grossen Theil der Linse und der vorderen chitinosen 
 Wand des Augenhockers wegschneidet. Das Objekt wird 
 dann noch einmal in Paraffin eingebettet, bevor man die 
 definitiven ScKnitte, welche quer zur Sehachse liegen, 
 anfertigt. 
 
 Die Untersuchung wurde in einem Medium vor- 
 genommen, welches einen niedrigen Brechungsindex hat, 
 wie z. B. Wasser, Alkohol und besonders einer Losung von 
 essigsaurem Kali. Die Praparate halten sicb in letzte- 
 rem Medium mehrere Jahre; jedoch wird die Farbung 
 gewohnlich nach einigen Wochen theilweise ausgezogen. 
 Sie konnen aber dann leicht, wenn es nothwendig ist, 
 zu jeder Zeit wieder gefarbt werden. Canadabalsam ist 
 wegen seines hohen Brechungsindex wenig geeignet, weil 
 die feineren Details verschwinden oder sehr undeutlich 
 werden. 
 
 In Bezug auf die oben angegebene Herstellungs- 
 methode der Sehnitte (alkoholische Pikrinsaure — Gre- 
 
 *) K. H eider, Die Embryonal -Entwicklung von Hydrophilus 
 piceus. I. Theil. Jena 1889. 
 
12 
 
 nacher’sche Fliissigkeit — Hamatoxylin, — essig- 
 saures Kali) ist es wichtig zu bemerken, dass der Zu- 
 stand der definitiven Praparate bei verschiedenen Indi- 
 viduen derselben Species sebr verschieden ist. Es ist des- 
 halb zu ratherr, von jeder Art, die man untersucht, eine 
 grossere Zahl von Scbnittserien anzufertigen. Yon meinen 
 Schnittserien erwies sich zwar gewohnlich die grossere 
 Zahl als gut, dock fand ich unter ihnen eine oder zwei, 
 deren Erhaltungszustand und Farbung ganz musterbaft 
 waren, und welche die iibrigen hierin weit iibertrafen. 
 Nachdem ich mir liickenlose Schnittserien mit gut er- 
 haltenen Zellgrenzen verschafft hatte, wurden die Zell- 
 grenzen in jenem Theil der Retina, welcher genau quer 
 getroffen war, mit Hilfe eines Zeichenapparates entworfen. 
 Jede Retinula in einer der Zeichnungen wurde dann mit 
 einer Zahl versehen. Durch sorgfaltige Vergleichung 
 mit der Zeichnung des nachsten Schnittes lasst sich mit 
 absoluter Sicherheit jede Retinula in letzterem iden- 
 tificiren und jede mit der ihr zukommepden Zahl ver- 
 sehen. 
 
 Nachdem derart mit alien Zeichnungen von sammt- 
 lichen Schnitten verfahren ist, kann man schliesslich den 
 Querschnitt durch eine und dieselbe Retinula in jeder 
 Region genau feststellen und studiren. 
 
I. Orientirung fiber den groberen Bau der 
 Phalangiden - Augen. 
 
 Die Phalangiden besitzen im Gegensatz zu den 
 meisten Arachniden nur zwei Augen. Dieses Augen- 
 paar ist bekanntlich in einem Hocker auf der Dorsal- 
 flache des Cephalothorax enthalten. 
 
 Die Sebachsen bilden einen rechten Winkel mit ein- 
 ander und liegen in einer Transversalebene des Korpers, 
 sodass sie seitwarts und aufwarts gerichtet sind. Da die 
 Basen der beiden Augen zum grossten Tbeil in der 
 Medianebene sich beruhren, so sind sie hier abgeplattet, 
 wahrend der ubrige Theil der Oberflache des Augen- 
 bulbus gewolbt ist. 
 
 Jedes Auge ist durch eine Scheidewand, die pra- 
 retinale Zwischenlamelle Graber’s (5), welche 
 eine direkte Fortsetzung der Basalmembran der Hypo- 
 dermis ist, in einen ausseren und inneren Abschnitt ge- 
 schieden. Der erstere funktionirt als dioptrischer Apparat 
 und besteht aus dem Glaskorper mit seinem Abschei- 
 dungsprodukt, der Linse; Linse und Glaskorper sind von 
 einer Zone pigmentirter Hypodermiszellen resp. p i g- 
 mentirter Cuticula umgeben. 
 
 Der innere Abschnitt dagegen bildet den percipi- 
 renden oder nervosen Theil des Auges, der sich haupt- 
 sachlich aus einer machtigen Retina und einer viel 
 diinneren Nervenfaserschicht, die proximal von ihr 
 liegt, zusammensetzt. Die Oberflache dieses Ab- 
 
14 
 
 schnittes ist, mit Ausnahme des abgeplatteten medianen 
 Theiles, von einer Membran (der Perireti n aim embran 
 Mark’s) und ihrer diinnen zelligen Matrix umgeben. 
 Da diese Membran nicbt zwischen die Augen dringt, 
 sondern am Rande der erwahnten Beruhrungsstelle beider 
 Augen kontinuirlich von einem zum anderen Auge iiber- 
 geht, so bilden die periretinalen und die oben genannte 
 praretinalen Membranen beider Augen zusammen eine 
 einzige Kapsel (die Retinalkaps el Mark’s), welche die 
 beiden Retinae sammt Nervenfaserschichten in einer ge- 
 meinsamen Hulle einschliesst. Auf der ventralen Seite 
 ist die Kapsel von einer Anzahl Locher durchbrochen, 
 welche in zwei parallelen Reihen von gewohnlich je acht 
 angeordnet sind, durch deren jedes ein Sehnerv hin- 
 durchtritt. 
 
 II. Die Linse nnd die angrenzende Cuticula. 
 
 Die Linse ist bekanntlich eine verdickte, durch- 
 sichtige Partie der chitinosen Cuticula des Korpers. 
 Ihre stark konvexen Flachen gehoren, wie Grenacher 
 sagt, Kugelflachen von ungleichen Radien an, und zwar 
 ubertrifft derjenige der ausseren den der inneren. 
 
 Die an die Linse angrenzende Cuticula lasst deut- 
 lich drei chitinose Schichten erkennen, von denen die 
 ausserste sehr diinn, stark lichtbrechend, homogen ist 
 und sich nicht farbt. Die mittlere erscheint viel dicker 
 und meist pigmentirt, besonders in der unmittelbar der 
 Linse benachbarten Partie, welche dadurch wie eine 
 breite dunkle Zone erscheint. Die mittlere Lage zeigt 
 nur eine undeutliche Schichtung, wahrend diese bei der 
 innersten Lage sehr deutlich ausgepragt ist. 
 
15 
 
 Von diesen drei Schichten geht nur die ausserste 
 unverandert beziiglich ihrer Dicke und sonstigen Be- 
 schaffenheit in die aussere Lage der Linse iiber; die 
 mittlere und innere Schichten dagegen werden stark 
 verdickt und bilden die Masse der Linse. 
 
 In der letzteren lassen sich diese beiden inneren 
 Schichten nicht mehr von einander unterscheiden, zumal 
 sie auch in ihrer Beschaffenheit jetzt sich ahnlich er- 
 weisen, indem sie mit Hamatoxylin sich intensiv farben, 
 gerade so wie die weiche neugebildete Cuticula wahrend 
 der Hautung. Die Cuticula des Korpers nimmt sonst 
 kaum eine Farbung an. Auch die Substanz der Linse 
 ist gegen Reagentien weniger widerstandsfahig; so wird 
 sie z. B. durch heisse Pikrinschwefel- und Pikrinsalpeter- 
 saure stark angegriffen, obwohl dies bei gewohnlicher 
 Cuticula durchaus nicht der Fall ist. 
 
 In der Cuticula linden sich zwei Arten von Poren- 
 kanalen, grobere und feinere. Von diesen konnte ich 
 die groberen, ebensowenig wie Grenacher, in der Linse 
 auffinden, obwohl sie Leidig (13, S. 434) beschreibt und 
 abbildet. Die anderen Porenkanale, weiche ausserst fein 
 sind und die ganze Dicke der Cuticula in grosser An- 
 zahl durchsetzen, sind auch in der ganzen distalen Partie 
 der Linse deutlich zu unterscheiden, dagegen konnte ich 
 sie nicht im Centrum und in der proximalen Partie er- 
 kennen. 
 
16 
 
 III. Glaskorper. 
 
 Zu der genauen Beschreibung, welche Grenacher 
 vom Glaskorper giebt, kann ich nichts Neues hinzu- 
 fugen. Er ist ein durchsichtiger Theil der Hypodermis, 
 in welche er kontinuirlich ubergeht. 
 
 Die kleinen Kerne liegen am proximalen Ende ihrer 
 Zellen. Nur an der Grenze, wo der Uebergang in die 
 Hypodermis stattfindet, verschiebt sich ihre Lage nach 
 der Mitte und dem distalen Ende. Hier sind auch die 
 Zellen pigmentirt und ihre Kerne erscheinen lang ge- 
 streckt. Dieser Ring von Pigmentzellen, der direkt 
 unter der oben erwahnten Pigmentzone der Cuticula 
 liegt, bildet mit der letzteren zusammen eine Pigment- 
 wand, welche verhindert, dass die Lichtstrahlen auf 
 anderem Wege als durch die Linse in das Auge ein- 
 dringen. 
 
 Der Glaskorper umschliesst die proximale Flache 
 der Linse kapselartig. Am starksten entwickelt ist er 
 bei Platybunus, wo seine Dicke mehr als 2 / 3 derjenigen 
 der Linse betragt. Bei Phalangium ist er ein halbes 
 Mai so dick, bei den iibrigen Genera dagegen betracht- 
 lich diinner, etwa 1 / 5 so dick, wie die Linse. 
 
17 
 
 IV. Retina. 
 
 Die Retina liegt direkt unter dem Glaskorper, in- 
 dem sie von ihm nur durch die praretinale Membran 
 getrennt ist. Ihre distale Flache ist also konkav und 
 mit der inneren Wdlbung der Linse koncentrisch. 
 
 Wie schon Grenacher (6) beschreibt, setzt sich 
 die Retina aus einer einzigen Lage sehr langer Nerven- 
 endzellen zusammen, von denen eine jede an ihrem 
 distalen Ende einen stark lichtbrechenden Korper, das 
 Rhabdomer, abscheidet, an ihrem proximalen Ende da- 
 gegen, in dessen Nahe der Kern liegt, in eine Nerven- 
 faser iibergeht. 
 
 Wahrend der distale, den Sehstab tragende Theil 
 gerade und mehr oder weniger senkrecht zur proximalen 
 Flache des Glaskorpers steht, biegt der proximale Ab- 
 schnitt der meisten Zellen allmahlich nach der Stelle 
 um, wo der zunachst befindliche Opticus durch die 
 Retinalkapsel dringt. 
 
 Da diese Stellen nicht in der Sehachse liegen, son- 
 dern ventral von ihr, so erscheint die Retina in den 
 meisten Fallen bilateralsymmetrisch gebaut, was ferner 
 auch durch den Umstand bewirkt wird, dass der Breiten- 
 durchmesser zwischen der hinteren und vorderen Flache 
 grosser ist als zwischen der medialen (dorsalen) und late- 
 ralen Flache. Ein zur Sehachse vertikal gefiihrter Schnitt 
 durch die Retina zeigt sie deshalb von ovalem Umriss. 
 
 Es diirfte nicht uberfliissig sein zu bemerken, dass 
 die bilaterale Symmetric auf die Retina beschrankt ist 
 und nicht fur die Linse und den Glaskorper gilt, da 
 diese letzteren und also auch die distale Flache der 
 Retina selbst vollig kreisformig sind. Leiobunum rotun- 
 
 p- 2 
 
18 
 
 dum unterscheidet sich von den anderen Arten dadurch, 
 dass bei der grossten Zahl der Retinazellen ihr proxi- 
 maler Theil nicht nur ventralwarts, sondern auch nach 
 vorn gekrummt ist, wodurch .die Retina asymmetrisch 
 gebaut erscheint. 
 
 Als eine der wichtigsten und bemerkens- 
 werthesten Eigenschaften der Retina ist die 
 konstante Anordnnng ihrer Elemente zu Gruppen 
 (Retinulae) von je vier Zellen hervorzuheben, 
 sowie die Vereinigung der Sehstabe dieser vier 
 Zellen zu einem einzigen Stuck, dem Rhabdom, 
 welches mithin aus 4 Theilen oder Rhabdomeren 
 gebildet ist. 
 
 Die ganze Retina setzt sich aus diesen Retinulae 
 zusammen, welche in direkter Beriihrung mit einander 
 stehen, da keine Pigment- oder andere Zellen zwischen 
 ihnen vorkommen. 
 
 1. Die Form der Retinulazellen. 
 
 Die Zellen einer Retinula sind immer so zu ein- 
 ander angeordnet, dass eine in der Achse liegt und von 
 den anderen drei umgeben wird. Ich bezeichne die erstere 
 als die centrale und die letzteren als die peripheren 
 Zellen. Diese beiden Zellarten unterscheiden sich so- 
 wohl durch ihre Gestalt als auch durch den Bau ihrer 
 Rhabdomere betrachtlich von einander. 
 
 Die peripheren Zellen sind die grossten und er- 
 scheinen in alien Regionen, das distale Ende ausge- 
 nommen, fast gleichmassig dick; nur am distalen Ende 
 werden sie bei den meisten Arten etwas diinner oder 
 (bei der Acantholophus-Gruppe) sogar zugespitzt. Sie 
 sind prismatisch, fiinf- bis sechsseitig oder etwas abge- 
 rundet und sind an ihrem proximalen Ende nicht durch 
 den Kern aufgetrieben. 
 
19 
 
 Die centrale Zelle dagegen variirt in den ver- 
 schiedenen Abschnitten betrachtlich in ihrer Form. Da 
 sie stets ihre Lage zwischen den drei peripheren Zellen 
 beibehalt, so nimmt sie im grossten Theil ihrer Lange 
 ausser an den Enden eine dreiseitige Gestalt an. Der 
 grosste Theil des nicht stabchentragenden Abschnittes 
 der Zelle ist (ausser bei Opilio) schlank, dreiseitig und 
 halb so dick im Durchmesser wie die peripheren Zellen 
 oder noch dunner. Am proximalen Ende aber ist sie 
 durch den hier liegenden Kern stark aufgetrieben und 
 fiinf- bis sechsseitig. Auch gerade proximal vom Rhab- 
 dom verdickt sich die centrale Zelle betrachtlich und 
 wird ungefahr ebenso dick wie die peripheren, behalt 
 aber ihre dreiseitige Form bei. In der Rhabdomregion 
 werden die drei flachen Seiten konkav und bilden dann 
 die drei Langsrinnen, welche um so tiefer werden, je 
 mehr man sich dem distalen Ende nahert. In jeder 
 Rinne liegt eine periphere Zelle. Bisweilen erstrecken 
 sich diese Rinnen auch weiter proximalwarts vom 
 Rhabdom. 
 
 Durch gelungene Maceration in Haller’ scher 
 Fliissigkeit kann man eine Anzahl von isolirten Reti- 
 nulae erhalten, deren vier Zellen noch durch ihre Rhab- 
 domere zusammenhangen. 
 
 Bei Opilio parietinus weicht die Gestalt der cen- 
 tralen Zelle von der typischen nur in so weit ab, als 
 der Abschnitt zwischen Rhabdom und Kern nur wenig 
 dunner ist als die peripheren Zellen und oft die Langs- 
 rinnen sich auf der ganzen Lange finden. 
 
 2. Der Sehstab-tragende Tlieil der Zellen und seine Be- 
 ziehung zum Rhabdom. 
 
 Die centrale Zelle erzeugt ein axiales Rhabdomer, 
 welches die seitliche Grenzflache der Zelle an drei Stellen 
 erreicht, namlich am Boden jeder der Langsrinnen, um 
 
20 
 
 hier mit den drei peripheren Rhabdomeren zu ver- 
 schmelzen. Die Rhabdomere der peripheren Zellen da- 
 gegen liegen excentrisch, der centralen Zelle genahert 
 und erreichen die Zellen-Oberflache nur an einer Stelle, 
 namlich da, wo sie mit der centralen Zelle in Beriihrung 
 kommen. 
 
 Durch diese Art von Rhabdombildung wird das Pro- 
 toplasma der centralen Zelle in der Rhabdomregion in drei 
 Plasmastrange getheilt, welche nur proximal vomRhabdom 
 zusammenhangen. Ein Querschnitt muss demnach genau 
 ein Bild geben, als ob das Rhabdom von sechs Zellen 
 umgeben ware, wie es Patten (20) wirklich ange- 
 nommen hat. In der That, wenn man solche Schnitte, 
 wie die von Opilio oder die vom Acantholophus be- 
 trachtet, so ist es beim ersten Anblick wirklich schwer 
 zu glauben, dass nur vier und nicht sechs Zellen vor- 
 handen sind. 
 
 Durch zahlreiche Schnitte, welche ich durch das 
 proximale Ende des Rhabdoms bei jeder der acht Arten 
 fiihrte, habe ich mich vollig iiberzeugt, dass es wirklich 
 so ist, wie ich oben angegeben habe, dass namlich drei 
 von den scheinbaren Zellen nur die distalen Theile der 
 centralen Zelle der Retinula sind. Besonders klar ist 
 dies auf solchen Praparaten zu erkennen, auf welchen 
 die Zellgrenzen sehr scharf hervortreten, wie es in dieser 
 Region gewohnlich der Fall ist. 
 
 Die drei Plasmastrange der centralen Zelle 
 spielen jedenfalls eine nicht unwichtige Rolle in der 
 Physiologie des Auges, da ihnen bei der Acantholophus- 
 Gruppe hauptsachlich die Aufgabe zufallt, den Zugang 
 des Lichtes zu den Rhabdomen zu reguliren. Ich werde 
 sie deshalb ausfiihrlich beschreiben. 
 
 Da jeder der Plasmastrange zwischen einem Rhab- 
 dom, zwei peripheren Zellen und einer angrenzenden Re- 
 tinula liegt, so konnen wir vier korrespondirende Be- 
 
21 
 
 grenzungsflachen unterscheiden, von denen drei stets 
 vorhanden sind, die vierte dagegen mitunter fehlen kann. 
 
 Diese letztere ist diejenige, welche an das Rhabdom 
 grenzt und deren Breite, wie spater gezeigt wird, mit 
 der Breite des centralen Rhabdomers an derselben Stelle 
 ubereinstimmt. Folglicb ist bei solchen Arten, welche 
 ein gut entwickeltes centrales Rhabdomer besitzen, diese 
 Grenzflache immer zu sehen, die Strange sind mindestens 
 vierseitig (niemals dreiseitig) und verhindern vollkommen, 
 dass die drei peripheren Zellen einander beriihren. 
 
 Bei der Acantholophus-Gruppe finden wir andere 
 Verhaltnisse. Hier ist das centrale Rhabdomer in der 
 Mittelregion des Rhabdoms sehr reducirt und kann 
 stellenweise sogar ganz fehlen. Ebenfalls verliert auf 
 dieser gleichen Region jene Grenzflache der Plasma- 
 strange an Breite und verschmalert sich bis zu ausserster 
 Feinheit oder sie verschwindet ganz. Im letzteren Falle 
 stehen die Plasmastrange der centralen Zelle sogar nicht 
 mehr in Beriihrung mit dem Rhabdom an der betreffen- 
 den Stelle, werden sehr fein und sind daher manchmal 
 schwer zu erkennen. In beiden Fallen nehmen die 
 Strange an der betreffenden Stelle eine charakteristische 
 dreiseitige Form an, indem sie mit scharfen Kanten ver- 
 sehen sind. Dadurch erinnern sie in der Gestalt sehr 
 stark an den nicht rhabdomtragenden Abschnitt der cen- 
 tralen Zelle selbst. 
 
 Es verdient bemerkt zu werden, dass die Strange 
 der centralen Zelle in der Rhabdomregion in alien Fallen 
 ohne Ausnahme an der Bildung der seitlichen Grenz- 
 flache des ganzen distalen Abschnitts der Retinula An- 
 theil nehmen, wie dunn sie auch sein mogen. 
 
 Auf Querschnitten durch den proximalen Abschnitt*) 
 
 *) Wie weiter unten (S. 23) erklart wird, zerfallt das Rhabdom 
 in einen proximalen und einen distalen Abschnitt. 
 
22 
 
 des Rhabdoms konnen die Strange der Centralzelle 
 leicht daran erkannt werden, dass sie konstant in den 
 Winkeln liegen, welche von den Strahlen des Rhabdoms 
 gebildet werden, und hier viel diinner sind, als eine der 
 peripheren Zellen. Bei Platybunus ist dieses, letztere 
 auch im distalen Abschnitt des Rhabdoms der Fall, in-' 
 dem die Plasmastrange bei dieser Gattung uberall fast 
 dieselbe geringe Dicke behalten. Bei alien anderen 
 Formen indessen erscheint der distale Abschnitt der 
 Strange, welcher ungefahr ein Drittel des ganzen Stranges 
 ausmacht, keulenformig verdickt. Dadurch ist eine ent- 
 sprechende Grossenabnahme der distalen Enden der peri- 
 pheren Zellen bedingt. 
 
 Bei der Acantholophus-Gruppe ist dieser Process so 
 in’s Extreme gesteigert, dass die Grossenverhaltnisse 
 zwischen den beiden Zellarten genau entgegengesetzt 
 sind denen, welche man in dem proximalen Abschnitt 
 des Rhabdoms findet. 
 
 Da die distale Region aller Zellen intensiv pigmen- 
 tirt ist, so folgt daraus, dass das aussere Ende des 
 Rhabdoms, welches dem Lichte ausgesetzt ist, in einen 
 Pigmentmantel eingehullt ist. Derselbe gehort bei Platy- 
 bunus hauptsacblich den peripheren Zellen an, bei Leio- 
 bunum und Phalangium findet er sich zur Halfte in der 
 centralen Zelle, zur Halfte in den peripheren Zellen, 
 wabrend er in der Acantholophus-Gruppe fast vollig der 
 centralen Zelle zukommt. 
 
 3. Bau des Rhabdoms. 
 
 Wie oben schon erwahnt wurde, tragen alle Zellen 
 einer Retinula zur Bildung des Rhabdoms bei, indem 
 jede ein Rhabdomer abscheidet; das der centralen Zelle 
 aber unterscheidet sich betrachtlich im Bau und auch in 
 der Beschaffenheit von denen der peripheren Zellen. 
 Das centrale Rhabdomer farbt sich stets hellblau mit 
 
23 
 
 Hamatoxylin, die peripheren dagegen entweder ganz 
 dunkelblau oder theils dunkel, theils hellblau. 
 
 Frisch im Blute des Thieres untersucht, erscheinen 
 die Rhabdome farblos. Ihre Lange variirt sehr in der- 
 selben Retina, die randstandigen sind gewohnlich kiirzer, 
 oft nur halb so lang als die mehr centralliegenden. 
 Dieser Unterschied stimmt im Allgemeinen mit der Lange 
 der Zellen iiberein und ist auf Grenacher’s Figur 15 (6) 
 gut zu erkennen. 
 
 Die Rhabdome konnen cylindrisch, abgeflacht oder 
 mit drei oder weniger hohen Langsleisten versehen sein. 
 Das letztere Verhalten ist das haufigste und in diesem 
 Falle erscheint der Querschnitt dreistrahlig. In den voll- 
 kommeneren Augen (Platybunus, Phalangium), in denen 
 der Glaskorper am starksten entwickelt und sehr dick 
 ist (vergl. Grenadier’s Fig. 15) sind die Rhabdome 
 klein, kompakter und sehr zahlreich, wahrend sie bei 
 den Arten mit weniger entwickeltem, relativ dunnem 
 Glaskorper (Leiobunum, Acantholophus, Opilio, Oligo- 
 lophus) sowohl relativ wie absolut grosser und weit 
 weniger zahlreich sind und viel mehr die Tendenz zeigen, 
 mittelst Leisten sich seitlich auszubreiten. 
 
 Bei alien Arten konnen wir einen distalen von 
 einem proximalen Abschnitt unterscheiden. 
 
 Der distale Abschnitt differirt in seiner Gestalt und 
 in seinem inneren Bau sehr auffallig von dem proxi- 
 malen, weit grossern Abschnitt. Er betragt gewdhnlich 
 ein Drittel, bei Platybunus sogar nur ein Sechstel des 
 ganzen Rhabdoms. Besonders charakteristisch ist fiir 
 ihn die sehr unregelmassige und wechselnde Gestalt 
 und Struktur, welche er nicht nur bei den verschiedenen 
 Species zeigt, sondern sogar in der Retina eines und 
 desselben Thieres erkennen lasst. Er ist in der That 
 der am meisten der Variation unterworfene Theil des 
 ganzen Rhabdoms. 
 
24 
 
 Wie schon oben angefi'ihrt, lassen sich die von mir 
 untersuchten Arten nach der Struktur des Rhabdoms 
 in zwei nattirlicbe Gruppen eintheilen, deren Angehorige 
 ich jetzt zunachst charakterisiren und beschreiben 
 werde. 
 
 A. Die Leiobunum-Gruppe. 
 
 Bei den Arten dieser Gruppe ist der proximale Ab- 
 scbnitt des Rhabdoms cylindrisch oder mehr oder weniger 
 dreistrahlig. In derselben Retina zeigt er sich stets von 
 sehr gleichmassiger Gestalt. Da die peripheren Rhab- 
 domere sich iiberall in der ganzen Lange mit Hamatoxylin 
 dunkelblan farben, so konnen sie leicht selbst am distalen 
 Abschnitt des Rhabdoms erkannt werden. Die Rhabdome 
 bleiben normaler Weise getrennt und treten nicht mit den 
 benachbarten in Verbindung; nur hier und dort findet 
 man bei alien Arten ein oder zwei Paare miteinander 
 vereinigt. 
 
 Leiobunum rotundum Latr. 
 
 Diese Species ist zum Studium bei Weitem am besten 
 geeignet, weil die beiden Arten von Rhabdomeren in 
 besonders deutlicher Ausbildung vorhanden sind und sich 
 sehr leicht in alien Regionen unterscheiden lassen. Es 
 ist mir deshalb besonders bei dieser Art moglich ge- 
 wesen, einen tieferen Einblick in den Bau zu gewinnen 
 und damit einen Schliissel zum Verstandniss der weniger 
 giinstigen Formen zu erhalten. 
 
 Der proximale Abschnitt ist mit drei Langs- 
 leisten versehen, so dass er im Querschnitt deutlich 
 dreistrahlig erscheint. Die Strahlen (oder Leisten) sind 
 am hochsten nahe dem proximalen Ende und werden 
 nach dem distalen Abschnitt zu niedriger. Sie sind 
 hauptsachlich von den peripheren Rhabdomeren gebildet, 
 und nur ein kleiner Theil von jedem kann auch vom 
 centralen Rhabdomer herriihren. Das centrale Rhab- 
 
25 
 
 domer ist durcli die ganze Lange des Rhabdoms gut 
 entwickelt. Nahe an seinem proximalen Ende ist es 
 ungefahr einem der peripheren Rhabdomere an Grosse 
 gleich, wird in der Mitte am diinnsten und nimmt dann 
 gegen das distale Ende wieder an Umfang zu. Hier- 
 durch unterscheidet es sich von den peripheren Rhab- 
 domeren, welche proximal am dicksten, distal dagegen am 
 diinnsten sind. 
 
 Der distale Abschnitt, welcher gewohnlich seit- 
 lich komprimirt ist, verdankt seinen Unterschied im Bau 
 hauptsachlich dem zunehmenden Umfange des centralen 
 Rhabdomers unter entsprechender Abnahme der drei 
 peripheren Rhabdomere. Dabei verschwinden einige der 
 Leisten, welche von den letzteren gebildet werden. Diese 
 Veranderung beginnt an der Grenze zwischen dem 
 distalen und proximalen Abschnitt. 
 
 Obgleich die Rhabdome normal ganz getrennt sind, 
 so kommt es doch hin und wieder vor, dass zwei be- 
 nachbarte durch eine Briicke von Rhabdom-Substanz 
 verbunden sind. Diese Yerbindung ist fast stets auf 
 den distalen Abschnitt beschrankt und entsteht durch 
 das Zusammentreffen und Verschmelzen zweier Strahlen 
 (Leisten), die natiirlich dem centralen Rhabdomer ange- 
 horen, da ja die der peripheren Rhabdomere in diesem 
 Abschnitt sehr an Grosse reducirt sind. Ich habe in 
 derselben Retina niemals mehr als fiinf Paare von Rhab- 
 domen gefunden, welche in dieser Weise vereinigt waren. 
 Im proximalen Abschnitt sind solche Verbind ungen viel 
 seltener, und nur zwei Mai konnte ich einen derartigen 
 Fall beobachten. In dem einen Fall wurde die Yerbin- 
 dung durch centrale, im anderen durch periphere Rhab- 
 domere dargestellt. 
 
 Zuweilen findet man Retinulae mit einer iiber- 
 zahligen peripheren Zelle, so dass sie aus einer 
 centralen und vier peripheren Zellen bestehen. Dieses 
 
26 
 
 Vorkommniss liess sich in ein and derselben Retina 
 selten an mehr als zwei oder drei Retinulae nachweisen. 
 
 Ich habe schon betont, wie leicht sich das centrale 
 und die peripheren Rhabdomere bei dieser Species von 
 einander unterscheiden lassen. Die Grenzlinie zwischen 
 einer hell- und dunkelgefarbten Partie des Rhabdoms ist 
 scharf und deutlich, und oft, besonders im proximalen 
 Abschnitt, durch eine noch dunklere Farbung, als sie die 
 peripheren Rhabdomere annehmen, ausgezeichnet. Sie 
 stimmt in Bezug auf ihre Lage stets genau uberein mit 
 einer der Grenzen zwischen der centralen und den peri- 
 pheren Zellen und erscheint als direkte Fortsetzung dieser 
 Zellgrenze in das Rhabdom hinein. Dass dieses wirk- 
 lich der Fall ist, lehrt ein Vergleich von zwei auf ein- 
 ander folgenden Scbnitten, von denen der eine kein 
 Rhabdom enthalt, wahrend der andere durch sein 
 ausserstes proximales Eude geht. 
 
 Hier ist es vollig klar, dass die Grenzen zwischen 
 den hell- und dunkelgefarbten Theilen des Rhabdoms 
 zusammenfallen mit denen zwischen der centralen Zelle 
 und den peripheren Zellen. Und wenn man weiter be- 
 denkt, dass jene Flache der Plasmastrange der Central- 
 zelle, die dem Rhabdom anliegt, stets und in alien 
 Regionen genau einer freien seitlichen Grenzflache des 
 hellgefarbten Theiles des letzteren entspricht, so kann 
 kein Zweifel dariiber obwalten, dass die hell- und 
 dunkelgefarbten Theile des Rhabdoms von Leiobunum 
 identisch sind mit den Rhabdomeren der centralen, bezw. 
 peripheren Zellen. 
 
 Ich habe noch eines anderen Unterschiedes zwischen 
 den Rhabdomeren zu gedenken, welcher sich in solchen 
 Praparaten findet, die bei 45° A), in einer gesattigten 
 alkoholischen Pikrinsaurelosung gehartet sind (siehe 
 S. 9). Ein Theil des Augenpigments wird durch die 
 warme Flussigkeit gelost und farbt das ganze centrale 
 
27 
 
 Rhabdomer tiefbraun oder fast schwarz, wahrend die 
 peripheren Rhabdomere fast farblos bleiben oder doch 
 nur hellbraun werden. 
 
 Phalangium opilio L. und P. brevicorne C. K. 
 
 schliessen sich in Betreff des Baues ihrer Rhabdome 
 ganz an Leiobunum an. Die Rhabdome sind aber ge- 
 wohnlich nicht mit so bohen Leisten versehen, wie bei 
 letzterer Art. Sie sind mehr dreiseitig und kompakter, 
 ein Umstand, der Hand in Hand gekt mit ihrer ge- 
 ringeren Grosse und grosseren Zahl. 
 
 Der distale Abschnitt ist ganz ahnlieh gestaltet wie 
 bei Leiobunum und zeigt dasselbe Anwachsen des cen- 
 tralen Rhabdomers und entsprechend dieselbe Abnahme 
 der peripheren Rhabdomere, indem die letzteren tief in 
 die Substanz des centralen Rhabdomers eindringen. 
 
 Platybunus triangularis Herbst. 
 
 Hier sind die Rhabdome manchmal etwas dreiseitig, 
 gewohnlich indessen vollig cylindrisch, und der distale 
 Abschnitt ist sehr kurz, indem er weniger als den 
 sechsten Theil des ganzen Rhabdoms ausmacht. Die 
 Rhabdome sind deutlich dreitheilig mit einer dunkel 
 sich farbenden dreistrahligen Grenzlinie, welche die drei 
 Theile scheidet. Das centrale Rhabdomer erscheint fast 
 ganz unterdriickt und ist gewohnlich nur am distalen 
 Ende sichtbar, wo es in die Plasmastrange der centralen 
 Zelle auswachst, wodurch dieses Ende etwas dicker er- 
 scheint und sich von dem ubrigen Theil des cylindrischen 
 Rhabdoms unterscheidet. Die Plasmastrange der cen- 
 tralen Zelle sind sehr fein und diinn und werden an 
 ihrem ausseren Ende nicht verdickt. Das Rhabdom ist 
 fast nur, auch im distalen Abschnitt, von peripheren 
 Zellen umgeben, wodurch sich diese Species von alien 
 ubrigen unterscheidet. 
 
28 
 
 B. Die Acantholophus-Gruppe. 
 
 Das charakteristische Merkmal dieser Gruppe besteht 
 darin, dass der distale Abschnitt des Rhabdoms mit 
 alien oder den meisten der benachbarten Rhabdome 
 diirch Brucken von Rhabdomsubstanz verbunden ist, so 
 dass ein unregelmassiges Netzwerk auf der ganzen 
 distalen Flache der Retina entsteht. Der distale Ab- 
 schnitt setzt sicli ans dem centralen und den peripheren 
 Rhabdomeren zusammen, doch farben sich die letzteren 
 in diesem Abschnitt hellblau wie das erstere, weshalb 
 eine scharfe Unterscheidung der beiden Arten von ein- 
 ander nicht moglich ist. Das Protoplasma saramt 
 Pigment, welches dieses Ende umhullt, gehort fast ganz 
 der centralen Zelle an. 
 
 Den proximalen Abschnitt des Rhabdoms bilden 
 fast ausschliesslich die sich dunkelblau farbenden peri- 
 pheren Rhabdomere, da das centrale Rhabdomer bier nur 
 sehr klein ist. Die Vielgestaltigkeit dieses Abschnittes 
 ist charakteristisch, indem man in derselben Retina so- 
 wohl die dreistrahlige als auch die platte Form stets 
 finden kann, wenn auch das Verhaltniss, in welchem 
 beide Formen vorkommen, bei verschiedenen Individuen 
 derselben Species in weiten Grenzen variirt. 
 
 Wenn auch beide Formen in jedem Theil der Re- 
 tina zu treffen sind, so findet man doch die abgeplatteten 
 zum grossten Theil in der vorderen lateralen Region 
 des Auges, die dreistrahligen dagegen am haufigsten in 
 der gegenuberliegenden medialen hinteren Region. 
 Ferner sind die meisten der abgeplatteten Rhabdome so 
 gestellt, dass ihre breiten Flachen parallel mit der Trans- 
 versalebene des Korpers liegen. Daher zeigen Schnitte, 
 die rechtwinklig zu dieser Ebene gefiihrt sind, von den 
 Rhabdomen in der vorderen Region nur die schmalen 
 Kanten derselben. 
 
29 
 
 Opilio parietinus de Geer 
 
 besitzt grossere Rhabdome als irgend eine von den 
 anderen von mir untersnchten Arten. 
 
 Der proximale Abschnitt der Rhabdome ist di- 
 morph, bezw. trimorph. Er erscheint abgeplattet, von 
 oblongem Querscbnitt oder er zeigt drei Langsleisten 
 und erscheint dann im Querschnitt dreistrahlig; endlich 
 konnen auch nur zwei Leisten vorhanden sein, so dass 
 der Querschnitt zweistrahlig erscheint. Die letztere 
 Form leitet sich von der dreistrahligen durch Reduction 
 eines Strahles ab, kommt aber im Ganzen seltener vor. 
 Schliesslich kann dasselbe Rhabdom in verschiedenen 
 Regionen des proximalen Abschnittes verschieden ge- 
 formte Querschnitte zeigen. 
 
 Diese verschiedenen Formen sind in der Retiua 
 nicht in gleicher Zahl vorhanden, vielmehr schwankt 
 dieselbe sehr. Bei einigen Augen sind bei weitem die 
 meisten Rhabdome abgeplattet und nur wenige drei- 
 strahlige werden im hinteren medialen Theil der Retina 
 gefunden. Bei anderen Augen wieder ist das Gegen- 
 theil der Fall, indem die dreistrahlige Form stark uber- 
 wiegt und die abgeplatteten nur in kleiner Anzahl im 
 vorderen lateralen Theil zu finden sind. Ferner konnen 
 beide Formen in ungefahr gleicher Zahl vorhanden sein 
 und mehr oder weniger durcheinander gemischt liegen; 
 oder aber das Vorkommen der abgeplatteten Formen 
 beschrankt sich auf den vorderen lateralen Theil und 
 das der dreistrahligen auf die hintere mediane Region 
 der Retina. 
 
 Die Dicke der Rhabdome im proximalen Abschnitt 
 ist uberall fast dieselbe, und niemals sind sie in der 
 Mitte, wie bei den anderen Species dieser Gruppe, ein- 
 geschniirt. 
 
 Sehr selten trifft man in diesem Abschnitt eine 
 
30 
 
 Verbindung zwiscben benachbarten Rhabdomen. In einem 
 ganz besonderen Ausnahmefall allerdings war ungefahr 
 die Halfte aller Rhabdome in der Retina zu Gruppen 
 vereinigt; dieses ist indessen ganz abnormal. In alien 
 Fallen wurde die Verbindung durch die peripheren, nie- 
 mals durch das centrale Rhabdomer hergestellt. 
 
 Der distale Abschnitt des Rhabdoms farbt sich 
 ganz hellblau, ist vollig unregelmassig gestaltet und 
 steht mit den anliegenden Rhabdomen mittelst Brucken 
 von Rhabdomsubstanz, die sowohl von der centralen als 
 auch von den peripheren Zellen erzeugt wird, in Ver- 
 bindung. 
 
 Um einige Beispiele anzufiihren, so kann die Bil- 
 dung der Brucke durch zwei centrale Zellen veranlasst 
 worden sein, und dann sind die beiden Rhabdome durch 
 ihre centralen Rhabdomere verbunden. In anderem 
 Falle wird die Brucke durch zwei periphere Zellen er- 
 zeugt und dann erfolgt die Verbindung durch periphere 
 Rhabdomere. Endlich kann das centrale Rhabdomer 
 eines Rhabdoms vereinigt sein mit einem peripheren 
 eines anderen, wie es am haufigsten zu finden ist. 
 
 Auf diese Weise entsteht ein unregelmassiges Netz- 
 werk von Rhabdomsubstanz, an deren Bildung alle 
 Rhabdome in der Retina Antheil nehmen. 
 
 Die Zusammensetzung des Rhabdoms aus centralen 
 und peripheren Rhabdomeren und die Lagenbeziehung 
 derselben zu einander sind keineswegs so klar wie bei 
 Leiobunum. Der proximale Abschnitt farbt sich zum 
 grossten Theil dunkelblau und scheint fast ganz aus peri- 
 pheren Rhabdomeren zu bestehen. Indessen bemerkt 
 man gewohnlich bei sorgfaltiger Priifung auf Quer- 
 schnitten einige wenige schmale, hellblaue Querstreifen, 
 welche von der anliegenden Flache eines Plasmastranges 
 der Centralzelle ausgehen und stets genau mit der 
 letzteren in der Breite ubereinstimmen. Von diesen 
 
31 
 
 hellblauen Streifen sind niemals mehr als drei vorhanden. 
 Sie konnen sich quer durch das Rhabdom erstrecken 
 and mit zwei gegeniiberliegenden Strangen der centralen 
 Zelle in Verbindung treten, oder sie ziehen nur halb so 
 weit, nur bis zur Mittellinie. Schnitte durch das 
 ausserste proximale Ende zeigen, dass das centrale Rhab- 
 domer wirklich vorhanden ist und sich hellblau farbt. 
 
 Erinnern wir uns nun an die Verhaltnisse bei 
 Leiobunum, wie sie besonders in solchen Schnitten dar- 
 gestellt sind, wo das centrale Rhabdomer durch die 
 sich eindrangenden peripheren Rhabdomere in zwei oder 
 drei getrennte Stucke gespalten ist, wodurch die peri- 
 pheren in direkte Beriihrung miteinander treten, — und 
 denken wir ferner daran, dass die Breite des centralen 
 Rhabdomers bei Leiobunum stets genau mit der an- 
 liegenden Flache der Centralzelle iibereinstimmt — , so 
 kann es keinem Zweifel unterliegen, dass die schmalen 
 Streifen von hellblauer Substanz bei Opilio nichts 
 anderes vorstellen als den Rest des stark reducirten 
 centralen Rhabdomers. 
 
 Im distalen Abschnitt ist es ganz unmoglich, in- 
 folge der gleichmassigen Farbung die relative Grosse 
 der beiden Arten von Rhabdomeren zu bestimmen. 
 Dass aber beide sicher vorhanden sind, lehrt die Bil- 
 dung der Briicken, wie oben gezeigt wurde. Aller 
 Wahrscheinlichkeit nach wird der grosste Theil des 
 distalen Abschnittes vom centralen Rhabdomer gebildet, 
 wie wir vielleicht aus der sehr starken Entwicklung der 
 Centralzelle in diesem Abschnitt schliessen diirfen. 
 
 Acantholophus hispidus Herbst. 
 
 Bei dieser Art treffen wir wieder einen Bau, der 
 dem von Opilio beschriebenen sehr ahnlich ist und nur 
 in wenigen Punkten, die hier erwahnt zu werden ver- 
 dienen, davon abweicht. 
 
32 
 
 Eine neue Erscheinung bei dieser und den folgen- 
 den Arten ist die kegelformige Gestalt des Rhabdoms, 
 welche durch die allmahliche Yerjungung des proximalen 
 Abschnittes gegen seine distale Region zu entsteht. 
 Diese letztere Region, d. h. also jene, welche der 
 Grenze zwischen dem proximalen und dem distalen 
 Abschnitt nahe liegt, ist der dunnste Theil des ganzen 
 Rhabdoms. Allerdings kommen betrachtliche Ab- 
 weichungen vor, indem er manchmal ausserst diinn, 
 manchmal (im Durchmesser) mehr als halb so gross 
 als das proximate Ende ist. 
 
 Der proximale Abschnitt kann jede der oben fur 
 Opilio beschriebenen Gestalten besitzen. Ausser der 
 dreistrahligen kommt durch Zurucktreten der Leisten 
 eine sich oft findende dreiseitige Form zu Stande. 
 Wenn beide in demselben Auge zusammen vorkommen, 
 so ist die letztere Form immer im hinteren Theil der 
 Retina zu linden, wahrend die dreistrahlige sowie die 
 abgeplattete Form vorzugsweise dem vorderen Theil zu- 
 kommen. 
 
 Das centraleRhabdomer ist ziemlich gut entwickelt am 
 aussersten proximalen Ende, wo es leicht auf Querschnit- 
 ten erkannt werden kann. Nach der Mitte zu nimmt es an 
 Grosse sehr rasch ab, bis es zuletzt ganz verschwindet. 
 In derselben Region kommen die ausserst feinen Plasma- 
 strange der centralen Zelle in einiger Entfernung vom 
 Rhabdom zu liegen, das hier naturlich ganz aus den 
 peripheren Rhabdomeren besteht. Die peripheren Rhab- 
 domere farben sich nicht uberall dunkelblau in ihrem 
 proximalen Abschnitt, sondern ein Theil im Centrum 
 eines jeden Strahles erscheint gewohnlich hellblau, fast 
 wie das centrale Rhabdomer, ein Umstand, welcher zu- 
 erst bei der genauen Bestimmung der Grenzen des 
 letzteren etwas Schwierigkeit bereitet. 
 
 Indessen eine Yergleichung solcher Praparate mit 
 
33 
 
 anderen, welclie mit gesattigter warmer alkoholischer 
 Pikrinsaure behandelt sind, lasst leicht erkennen, dass 
 nur jene hellblauen Streifen zum centralen Rhabdomer 
 gehoren, die den Strangen der centralen Zelle anliegen, 
 wahrend die anaeren mit der feinen Mittellinie den 
 peripheren Rhabdomeren zuzurechnen sind. Diese Ver- 
 haltnisse werden spater in dem Kapitel iiber die feinere 
 Histologie des Rhabdoms genau erortert werden. 
 
 Oligolophus palpinalis Herbst 
 
 schliesst sich hinsichtlich des Baues seiner Rhabdome 
 an Acantholophus eng an. Das centrale Rhabdomer er- 
 scheint sogar noch mehr durch die peripheren im pro- 
 ximalen Abschnitt verdrangt, wo kaum eine Spur von 
 ihm zu entdecken ist. In der hinteren Region der Re- 
 tina sind die Rhabdome dreiseitig oder mehr oder 
 weniger abgerundet, so dass der proximale Abschnitt 
 hier eine konische Gestalt erhalten kann. 
 
 Oligolophus tridens C. K. 
 
 Bei dieser Art treffen wir wieder ganz dieselben 
 Verhaltnisse wie bei Opilio parietinus, nur mit dem 
 Unterschiede, dass die Rhabdome kegelformig sind. 
 Bei einigen Individuen ist das Netzwerk von Rhabdom- 
 substanz nicht so vollkommen ausgebildet, wie bei den 
 drei vorigen Arten, indem eine kleine Anzahl von 
 Rhabdomen getrennt bleiben kann. 
 
 Aus den vorhergehenden Beschreibungen lasst sich 
 unter anderen folgender Schluss ziehen: Das Rhab- 
 
 dom besteht bei alien Species aus zwei chemisch 
 verschieden beschaffenen Theilen. Zu dem einen 
 gehort das ganze centrale Rhabdomer und, bei 
 der Acantholophusgrup pe, auch der distale Ab- 
 p. 3 
 
34 
 
 schnitt der peripheren Rhabdomere, wahrend 
 der andere Theil bei der Leiobunumgrupp e von 
 den ganzen peripheren Rhabdomeren, bei der 
 Acantholophusgruppe aber nur yon ihrem pro- 
 ximalen Abscbnitt gebildet wird. 
 
 Ferner ist der sich dunkelblau farbende Theil stets 
 stark entwickelt und bildet den bei weitem grosseren 
 Theil des proximalen Abschnittes des Rhabdoms. Da- 
 gegen variirt der sich hellblau farbende Theil des Rhab- 
 doms sowohl in der Form wie in der Masse sehr, und 
 er kann sogar stark reducirt sein (Platybunus). Er ist 
 am starksten entwickelt in dem distalen Abschnitt des 
 Rhabdoms. 
 
 Bei dieser Gelegenheit mochte ich noch auf eine 
 bemerkenswerthe Analogie der Phalangidenaugen mit 
 den Augen der Cephalopoden hinweisen, welche die auf- 
 fallende Verbindung der Rhabdome zu einem unregel- 
 massigen Netzwerk betrifft, wie sie in der Acantho- 
 lophusgruppe vorhanden ist. Diese letztere Erscheinung 
 steht im Bereiche der Arthropoden soweit bekannt ganz 
 vereinzelt da. Dagegen findet sich merkwiirdiger Weise 
 bei den Cephalopoden eine ahnliche Erscheinung, wie 
 von Grenacher (8) nachgewiesen wurde. Bei Octopus, 
 Eledone und Sepia verschmelzen n&rulich die gewohn- 
 lich aus vier Rhabdomeren gebildeten Rhabdome in der 
 Weise mit einander, dass in verschiedenen Regionen 
 der Retina ein unregelmassiges Netzwerk von Rhabdom- 
 substanz zu Stande kommt, in dessen Maschen noch 
 Protoplasma der zugehorigen Zellen liegt. 
 
 Von einer anderen merkwurdigen Analogie dieser 
 Cephalopodenaugen mit den Spinnenaugen wird weiter 
 unten (S. 57) noch die Rede sein. 
 
35 
 
 4. Feinere Struktur ties Rhabdoms. 
 
 Wenn wir einen Querschnitt clurch ernes der ab- 
 geplatteten Rhabdome von Opilio parietinus genauer 
 priifen und ihn mit einem ahnlichen Rhabdom, das man 
 von der schmalen Seite sieht, vergleicht, so sehen wir 
 dass es aus parallelen Schichten, in diesem Falle aus 
 sechs, aufgebaut ist. Diese Schichten sind ihrerseits 
 wieder dadurch, dass sie von feinen Linien oder La- 
 mellen quer durchzogen werden, in Maschen oder Kast- 
 chen getheilt. 
 
 Die aussersten Schichten stehen an dem Seiten- 
 rande des Rhabdoms in kontinuirlicher Verbindung mit 
 einander und konnen deshalb auch als eine einzige 
 Schicht, welche die inneren Lagen umgiebt, angesehen 
 werden. Die vier inneren Schichten verhalten sich in 
 ganz gleicher Weise, d. h. sie gehen ebenfalls an den 
 Seitenrandern in einander iiber. Ein so gebautes Rhab- 
 dom kann als eine Rohre mit einer dreischichtigen 
 Wandung betrachtet werden, die seitlich so stark zu- 
 sammengedriickt ist, dass ihr Lumen verschwindet*). 
 Dieser Vergleich passt dadurch noch besser, dass oft 
 infolge des Einwirkens der Konservirungsfliissigkeiten 
 ein kiinstlicher Spalt gebildet wird, welcher die Wande 
 der zusammengedriickten Rohre trennt. 
 
 Die einzige Zwischenlamelle, in der dieser Spalt 
 auftritt und die das verschwundene Lumen der Rohre 
 reprasentirt, werde ich die Mittellamelle oder Mittel- 
 linie nennen, zum Unterschiede von den Seiten- 
 lamellen oder Seitenlinien zwischen den drei 
 
 *) Dieser Vergleich client lediglich zur Erleichterung der Be- 
 schreibung. Es soil keineswegs damit gemeint sein, dass das Rhab- 
 dom etwa einmal in seiner Entwicklung eine wirkliche Rohre 
 darstellte. 
 
36 
 
 Schichten, welche die Wandung der vermeintlichen 
 Rohre bilden. 
 
 Wenn wir uns vorstellen, dass eine der abgeplatteten 
 Wiinde nach aussen hin eine Langsfaltung erfiihre, so 
 wiirden wir eine dreistrablige Figur erhalten, welche 
 genau in ihrem Baue mit einem dreistrahligen Rbabdom 
 iibereinstimmt. Man kann deshalb das letztere be- 
 trachten als eine Rohre, welche eine dreischichtige Wand 
 hat und welche seitlich von drei verschiedenen Stellen 
 her so weit zusammengedriickt wurde, bis ihr Lumen 
 verschwand. Den Uebergang der einen Form in die 
 andere kann man leicht bei einem und demselben Rhab- 
 dom von Opilio parietinus verfolgen. 
 
 Einen sehr ahnlichen Bau findet man bei alien 
 anderen Species. Die wichtigsten Abweichungen be- 
 treffen die Zahl der die Wandung der „Rohre“ bilden- 
 den Schichten. Gewohnlich sind es drei, wie bei Opilio. 
 Bei Leiobunum konnte ich hingegen nur zwei linden, 
 die aber sehr klar und deutlich waren; nur zuweilen 
 war hier und dort eine dritte sichtbar. Auch bei den 
 Formen, wo man fur gewohnlich drei Schichten trifft, 
 scheinen stellenweise nur zwei vorhanden zu sein. 
 
 In alien Fallen, wo neue Leisten (Strahlen) in einer 
 Region des Rhabdoms gebildet werden, z. B. im distalen 
 Abschnitt bei Leiobunum, heben sie sich durch eine 
 einfache Faltung der zwei- resp. dreischichtigen Wan- 
 dung der vermeintlichen Rohre, gerade so wie bei Opilio 
 parietinus, ab. 
 
 Abgesehen von der Bildung von 4 — 6 Langs- 
 schichten, sind die kleinen Kastchen auch noch in Quer- 
 schichten angeordnet, die durch Zwischenlamellen von 
 einander getrennt sind. Diese Lamellen, die ich als 
 Querlamellen oder Querlinien bezeichne, sind viel 
 zahlreicher, aber weniger deutlich als die zwischen den 
 Langsschichten liegenden Mittel- und Seitenlamellen. 
 
37 
 
 Von der Seite gesehen, scheint sonach das Rhab- 
 dom aus einer Anzahl von uber einander gelegenen 
 Platten von der Dicke eines Kastchens aufgebaut zu 
 sein. Die Querstreifung des Rhabdoms, die schon 
 Patten gesehen hat, beruht auf den zwischen den Platten 
 befindlichen Querlamellen. 
 
 Es muss besonders betont werden, dass, wenigstens 
 im proximalen Abschnitt des Rhabdoms, die Grenzlinien 
 zwischen den kleinen Kastchen durch die zwei- resp. 
 dreischichtige Wand der „Rohre u in ganz geraden Linien 
 von der ausseren Oberflache bis zur Mittellamelle fort- 
 laufend zu verfolgen sind. Weiter scheinen sie konti- 
 nuirlich in die Grenzlinien der gegeniiberliegenden Wan- 
 dung zu verlaufen, doch findet man zuweilen auch, dass sie 
 mit jenen alterniren. Nach dem Mitgetheilten kann, 
 meiner Ansicht nach, kein Zweifel sein, dass die kleinen 
 Maschen in Wirklichkeit sechsseitige Kastchen sind und 
 dass das Rhabdom mithin eine „ Waben struktur u 
 besitzt. 
 
 Die Waben oder Kastchen treten in alien Theilen 
 des Rhabdoms hervor, sowohl in dem centralen wie in 
 den peripheren Rhabdomeren. Bei dem verschieden- 
 artigen chemischen Verhalten der Rhabdomere ist dies 
 jedenfalls bemerkenswerth. 
 
 Die Anordnung der Waben im distalen Abschnitt 
 des Rhabdoms bei der Acantholophus-Gruppe ist sehr 
 schwer zu erkennen und scheint weniger regelmassig zu 
 sein. Die Zahl der Langsschichten kann an sehr dicken 
 Stellen jene im proximalen Abschnitt iibertreffen. 
 
 Ich habe schon gelegentlich erwahnt (S. 33), dass 
 bei verschiedenen Arten (z. B. Acantholophus) das Ver- 
 standniss des Baues durch die Art und Weise, wie sich 
 die peripheren Rhabdomere farben, erschwert wird. Auf 
 diesen Punkt muss ich hier zuruckkommen, denn die 
 Wabenstruktur des Rhabdoms wird auch durch dieselbe 
 
38 
 
 Farbungsweise theilweise ankenntlich gemacht. Wenn 
 wir nun einen solchen Schnitt von Acantholophus mit 
 einem anderen vergleichen, in dem nur das centrale 
 Rhabdomer durch gelostes Pigment gefarbt ist, so sieht 
 man, wie bei dem ersteren die mittlere von den drei 
 Schichten, welche die Wand der Rohre bilden, in den 
 peripheren Rhabdomeren so dunkel gefarbt ist, dass ihre 
 Wabenstruktur verwischt ist. Die ausseren und inneren 
 Schichten dagegen sind fast ungefarbt geblieben, wie das 
 centrale Rhabdomer. In anderen Rhabdomen derselben 
 Retina kann die aussere Schicht ebenfalls tief gefarbt 
 sein und nur die innere Schicht kann hell bleiben. 
 
 5. Struktur des Protoplasmas in den Retina-Zellen. 
 
 Der grosste Theil der Zelle ist mit Pigmentkornern 
 so dicht gefullt, dass es durchaus nothwendig ist, die- 
 selben zu entfernen, um eine Vorstellung von der 
 feineren Struktur in der pigmentirten Region zu er- 
 halten. 
 
 Das Protoplasma zeigt im optischen Schnitte einen 
 ausgepragten wabigen Bau von hell sich farbender Sub- 
 stanz. Die Maschen des Wabenwerkes sind, besonders 
 in der distalen, am dichtesten pigmentirten Region der 
 Zelle, ziemlich, wenn nicht ganz gleichmassig gross und 
 gewohnlich fiinf- bis sechsseitig. In den Knotenpunkten 
 des Wabenwerkes, in welchen in der Regel drei, seltener 
 vier Seiten zusammentreffen, sammeln sich sehr haufig 
 farbbare Substanzen an und zwar oft in betrachtlicher 
 Menge. Die Knotenpunkte erscheinen dann viel dicker 
 als die Seiten der Maschen. Solche Knotenpunkte, 
 welche man besonders im proximalen Theil der Zelle 
 trifft, geben dem Protoplasma bei minder genauer Be- 
 trachtung ein grobkorniges Aussehen. Bei eingehender 
 Untersuchung bemerkt man jedoch, dass die Korner 
 
39 
 
 durch feine, sich leicht farbende Linien zu einem schein- 
 baren Netzwerke verbunden sind.. Die Mascben oder 
 Waben ordnen sich dort in einer Reihe an, wo sie mit 
 einer festeren Wandung, z. B. dem Rhabdom oder der 
 Zellwand, in Beriihrung kommen (Bfitschli’s 
 Alveolar schicht). 
 
 Untersucht man die Retinazellen auf Langsschnitten, 
 so zeigt sich gewohnlich derselbe Bau. Nicht selten 
 freilich kommt es vor, dass die Waben eine Tendenz 
 zeigen, sich in Langsreihen anzuordnen. 
 
 Eine kleinere oder grossere Anzahl Waben — je 
 nach der betreffenden Region — enthalt je ein sphari- 
 sches Pigmentkorn, das dieselbe Grosse hat und sie 
 anscheinend vollig ausffillt. Eine solche Wabe kann 
 natiirlich, wenn man will, als ein ein Pigmentkorn ent- 
 haltender Knotenpunkt betrachtet werden. 
 
 Bekanntlich ist es sehr schwierig, iiber die wahre 
 Natur eines so gebauten Protoplasmas, wie ich es be- 
 schrieben habe, in’s Klare zu kommen, ob es ein kom- 
 plicirtes Netzwerk von feinen Faden ist, oder ob es nur 
 aus einer Menge von Blaschen oder Vakuolen besteht, 
 die im optischen Schnitt nur das Vorhandensein eines 
 Netzwerkes vortauschen. Das letztere oder die Schaum- 
 struktur, wie sie von Bfitschli (3) beschrieben worden 
 ist, scheint mir in unserem Fall die grosste Wahrschein- 
 lichkeit fur sich zu haben. Sie erklart am besten die 
 allgemeinen Erscheinungen, welche ich oben beschrieben 
 habe: die fiinf- bis sechsseitigen Maschen, die verdickten 
 Knotenpunkte, die Anordnung in Reihen langs der 
 festeren Wande etc. 
 
 Ftir mich ist folgende Beobachtung maassgebend-’ 
 Wenn man auf eine der Maschen mit dem Mikroskop 
 genau eingestellt hat und dann den Tubus etwas hebt 
 und senkt, so treten an Stelle der einen beobachteten 
 Masche sehr oft drei andere Maschen, die genau fiber 
 
40 
 
 bezw. unter ihr liegen, so dass der gemeinsame Knoten- 
 punkt der drei Maschen mit dem Centrum der ersteren 
 zusammenfallt. 
 
 Zu Gunsten der Ansicht, dass das Protoplasma ein 
 Netzwerk von feinen Faden ist, konnte angefuhrt wer- 
 den, dass alle oben erwahnten Erscheinungen durch die 
 Gegenwart einer grossen Anzahl von Pigmentkornern 
 hervorgerufen werden konnten, welche ebenso gross wie 
 die Hohlungen der Maschen sind. Indessen findet sich 
 genau derselbe Bau in jenen Regionen der Zellen, welche 
 wenig oder kein Pigment enthalten, wovon man sich 
 leicht an Schnitten iiberzeugen kann, aus denen das 
 Pigment nicht entfernt worden ist. 
 
 Wenn wir nun die Wabenstruktur des Protoplas- 
 mas mit der des Rhabdoms in Vergleich ziehen, so 
 finden wir eine auffallige Aehnlichkeit zwischen beiden. 
 Ja die dem Rhabdom anliegende Alveolarschicht ist oft 
 von der ausseren Kastchenschicht des Rhabdoms nur 
 durch das starke Lichtbrechungsvermogen des letzteren 
 zu unterscheiden. Es macht ganz den Eindruck, als 
 wenn die Kastchen des Rhabdoms erstarrte Protoplasma- 
 waben waren. Erstere sind natiirlich, im Gegensatz zu 
 den Plasmawaben, nicht unter einander beweglich. 
 
 6. Pigment der Retinazellen. 
 
 Die distale Region der Zellen ist vollstandig pig- 
 mentirt, die proximale dagegen ist mehr oder weniger 
 von Pigment frei. In der Rhabdomregion ist dasselbe 
 besonders stark vertreten und, da es vor allem die 
 Plasmastrange der centralen Zelle erfullt, so kann man 
 diese gewohnlich auf Schnitten, aus denen das Pigment 
 noch nicht entfernt ist, als intensiv schwarze oder braune 
 Flecken, die in den Rinnen des Rhabdoms liegen, er- 
 kennen. 
 
41 
 
 Es ist vollig sicher, dass echte Pigmentzellen, die 
 kein Rhabdomer tragen, ganz fehlen. 
 
 Wie Stefanowska (25) schon beobachtet hat, sind 
 die Pigmentkorner in der proximalen Region der Retina 
 oft in Langsreihen angeordnet. Jedenfalls hangt dieses 
 mit einer reihenformigen Anordnung der Protoplasma- 
 waben in dieser Region zusammen. 
 
 In frischem Zustande gepriift erscheint das Pigment 
 bei Phalangium dnnkelbraun mit einem Stick in’s Roth- 
 liche, sowohl im durchscheinenden wie auch im reflektirten 
 Licht. Bei alien anderen Gattungen ist es schwarzbraun, 
 fast schwarz im durchscheinenden, und dunkelbraun mit 
 einem Stich in’s Rothliche im reflektirten Licht. 
 
 7. Die Kerne der Retinazellen. 
 
 Die Kerne finden sich sammtlich in der proximalen 
 Halfte der Retina. Sie enthalten ein oder zwei Nucleoli 
 und sind, je nach der Art, sehr wechselnd in ihrer Ge- 
 stalt, bald spharisch (Platybunus), bald langlich-oval 
 (Opilio), am haufigsten aber ist die Gestalt eine ovale. 
 
 Zum Unterschied von den Kernen der centralen 
 Zellen verursachen die der peripheren keine Anschwellung 
 der Zellen. Bei Leiobunum liegen die Kerne der ersteren 
 mehr proximal als die der peripheren Zellen derselben 
 Retinula, und dies scheint auch bei den anderen Arten 
 der Fall zu sein. 
 
 Ausser den gewohnlichen Kernen kann man in der 
 am meisten proximal gelegenen Region der Retina immer 
 eine geringe Anzahl kleiner, mehr rundlicher Kerne 
 finden, die wahrscheinlich, wenigstens in den meisten 
 Fallen, centralen Zellen angehoren, welche eine unge- 
 wohnliche Lange erreicht haben. Freilich ist es sehr 
 schwer, dies mit volliger Sicherheit nachzuweisen. 
 
42 
 
 V. Xervenschiclit unci Nervi optici. 
 
 Wenn auch, wie die Entwicklung lehrt, die Retina 
 invers gebaut ist, so verbinden sich doch die Nerven- 
 fasern mit dem proximalen Ende der Zellen, welches 
 dem urspriinglich (vor der Inversion) freien, ausseren 
 Ende entspricht. Die Zellen werden an diesem Ende 
 hinter dem Kern sehr rasch diinner und gehen in die 
 Nervenfaser uber. Da ich diesem wichtigen Punkte be- 
 sondere Aufmerksamkeit gewidmet habe, so kann ich 
 diese Angabe mit absoluter Sicherheit machen. 
 
 Am besten studirt man die betreffenden Yerhalt- 
 nisse auf Praparaten, welche in warmen Fliissigkeiten 
 konservirt oder in HallePscher Flussigkeit macerirt sind. 
 Die Axencylinder sind oft so gut konservirt, dass die 
 Querschnitte durch einen jeden sofort erkannt werden 
 konnen, doch ist es mir niemals gelungen, eine Faser zu 
 entdecken, welche zwischen den Zellen aufwarts zieht 
 und sich irgendwo an der distalen Seite vom Kern an- 
 heftet. 
 
 Die Nervenfasern bilden eine Schicht, welche der 
 proximalen Flache des Retinazellenlagers aufliegt und 
 welche nahe der Mitte am dicksten ist, gegen die Peri- 
 pherie zu aber ausserst dunn und daher schwer zu er- 
 kennen ist. Alle Fasern nehmen ihren Weg zu dem 
 nachsten optischen Nerven und treten noch innerhalb 
 der Retinalkapsel zu kleineren Biindeln zusammen, von 
 denen ein jedes von einer cellularen Scheide einge- 
 schlossen wird. 
 
 Wahrend ihres centripetalen Yerlaufes vereinigen 
 sich kleinere Biindel zu grosseren und gehen zuletzt 
 durch die Locher, welche sich in der Retinalkapsel an 
 der ventralen Seite finden, in ebensoviele Sehnerven 
 
43 
 
 uber, wie Locher vorhanden sind. Jeder Sehnerv theilt 
 sich bei seinem Eintritt in das Auge in zwei Hauptaste, 
 die eine dorsale bezw. laterale Richtung nebmen und 
 nach den anliegenden Retinazellen auf ihrem ganzenWege 
 Fasern abgeben. 
 
 In der Regel sind acht Nerven fin* jedes Auge vor- 
 handen, doch kommen Abweichungen vor, und bei einigen 
 Individuen kann sich ihre Zahl bis auf fiinf vermindern. 
 Da sie sich auf ihrem Wege zum Gehirn untereinander 
 vereinigen, so wird ihre urspriingliche Zahl schliesslich 
 bis auf die Halfte oder auf noch weniger reducirt. Ihr 
 weiterer Verlauf im Gehirn ist von St. Remy (23) be- 
 schrieben worden. 
 
 Die Scheide der Sehnerven und diejenige der inner- 
 halb der Retinalkapsel befindlichen Biindel mit ihren 
 Kernen bleibt stets auf die Peripherie beschrankt. So- 
 mit fehlen jegliche innere Scheiden, welche den Nerven 
 oder das Biindel in sekundare Biindel theilen. Die 
 kleinen ovalen oder langlichen Kerne veranlassen in der 
 Wand der Scheide eine Verdickung und sind leicht von 
 den mehr rundlichen, viel grosseren Kernen der Retina- 
 zellen zu unterscheiden. Zuweilen trifft man auch ahn- 
 liche Kerne zwischen den Opticusnervenfasern. 
 
 Obwohl der Nachweis, dass die peripheren Zellen 
 je in eine Nervenfaser iibergehen, nicht sehr schwer ist, 
 so ist er doch nicht so einfach fiir die centrale Zelle. 
 Ich habe versucht, mir dadurch iiber diesen Punkt Ge- 
 wissheit zu schaffen, dass ich die Zahl der Nervenfasern 
 in den optischen Nerven mit der Zahl der Rhabdome in 
 demselben Auge verglich. 
 
 Wenn es auch miihsam ist, so ist es doch durchaus 
 moglich, die genaue Zahl der Rhabdome zu ermitteln, 
 indem man in der oben (S. 12) beschriebenen Weise 
 Zeichnungen einer liickenlosen Serie von Schnitten, aus 
 denen das Pigment nicht entfernt ist und welche die 
 
44 
 
 Sehachse genau quer durchschnitten haben, miteinander 
 vergleicht. DieNervenfasern lassen sich ebenfalls zahlen, 
 wenn auch nicht mit solcher Genauigkeit. 
 
 Dies geschiebt am besten anf Querschnitten von 
 Nervi optici, die in warmen Fliissigkeiten gehartet und 
 mit Hamatoxylin gefarbt sind. Auf ibnen erscheint 
 jeder Acbsencylinder wie ein heller, farbloser Raum auf 
 blauem Grunde. Die Yertbeilung der Fasern eines Auges 
 auf acht Nervi optici, deren jeder 60 — 400 Fasern ent- 
 halt, vereinfacbt die Zahlung bedeutend. 
 
 Das Resultat der Zablung ergab fur ein linkes Auge 
 von Acantbolopbus genau 588 Rhabdome, wabrend die 
 acbt Nervi optici desselben Auges bei demselben Indivi- 
 duum annahernd 2449 Fasern entbielten. Wiirden alle 
 Zellen in einer Retinula innervirt, so wiirde die Ge- 
 sammtzabl von Fasern 4 mal 588 = 2352 ergeben, 
 welcbe Zabl sebr gut mit der gefundenen (2449) iiberein- 
 stimmt. Die letztere Zabl iibertrifft zwar um 97 Fasern 
 die wirklicbe Zabl, d. b. es ist ein Irrthum von nur 4,1 
 Procent vorbanden, indessen kommt er nicht in Betracht, 
 wenn man bedenkt, dass, wenn nur die peripheren Zellen 
 innervirt wiirden, die Zabl derFasern nur 3mal 588 = 1764 
 betragen miisste, also um 685 hinter der gefundenen zu- 
 ruckbleiben wiirde. 
 
 Hier mag einer etwas iiberraschenden Ueberein- 
 stimmung in der Zahl der Rhabdome, welcbe ich bei 
 verscbiedenen Augen von Acantbolophus hispidus fand, 
 gedacht werden. Das oben erwiihnte linke Auge besass 
 588 Rhabdome, und in dem rechten Auge desselben In- 
 dividuums erhielt icb dieselbe Zabl 588. In dem Auge 
 eines anderen Individuums zahlte icb 587, also nur ein 
 Rhabdom weniger. Diese Zahlen sind iiberdies sehr 
 genau, nicht nur annahernd bestimmt, so dass ein 
 etwaiger Irrtbum hochstens ein Rhabdom mehr oder 
 weniger betragen kann. 
 
45 
 
 VI. Die Retinalkapsel und das zwiselien 
 ihr und den Scliichten der Retina und der 
 Xervercfasern liegende Gewebe. 
 
 Die Retinalkapsel, aaf welche schon zu verschiede- 
 nen Malen Bezug genommen wurde, ist eine Membran 
 von betrachtlicher Festigkeit, wie man sich leicht auf 
 Praparaten, die kurze Zeit in Haller’scher Fliissigkeit 
 macerirt warden, iiberzeugen kann. Sie schliesst alles 
 Gewebe beider Augen ein, welches proximal vom Glas- 
 korper liegt. 
 
 DerName praretinale Membranist vonGraber (5) 
 jenem Theil der Retinalkapsel gegeben worden, wel- 
 cher zwischen Glaskorper und der Retina liegt. Sie 
 trennt diese letzteren, doch halt sie dieselben gleichzeitig 
 auch zusammen, weil die Zellwande beider Schichten fest 
 mit ihr verbunden sind. Diese Membran ist jedenfalls 
 eine Folge der Verwachsung der die Zellen jederseits von 
 ihr begrenzenden Zellwande. Niemals enthalt sie Kerne. 
 
 An ihrem Rande ist die praretinale Lamelle recht- 
 winklig umgeschlagen and geht in die periretinale 
 Membran iiber, welche die periphere Flache und einen 
 Theil der basalen Flache des Augenbulbus bedeckt. 
 Der inneren Seite dieser Membran liegt eine besondere 
 zellige Matrix an. Da die Hypodermis eine betrachtliche 
 Strecke mit der peripheren Wand des Augenbulbus in 
 Beriihrung steht, so verschmilzt die periretinale Membran 
 in dieser Region mit der Basalmembran der Hypodermis. 
 Diese beiden Membranen schliessen gewohnlich eine An- 
 zahl von Tracheen zwischen sich ein und trennen sich 
 in der Nahe der basalen Flache des Auges. 
 
 Der Raum zwischen der periretinalen Membran und 
 der Sehicht der Retinazellen, sowie der Nervenfasern 
 
46 
 
 wird von der diinnen zelligen Matrix der Membran ein- 
 genommen. Der distale Theil der Matrix, welcher 
 zwischen dem doppelten Theil der Periretinalmembran 
 und den randstandigen Retinulae liegt, setzt sich aus 
 zwei bis drei Schichten von langen, etwas komprimirten, 
 prismatischen Zellen zusammen, welche mit den rand- 
 standigen Retinazellen parallel verlaufen, fast ebenso lang 
 sind und langgestreckte, abgeplattete Kerne besitzen. 
 Diese Zone ist dicker wie der iibrige Theil der Matrix 
 und bildet, da sie gewohnlich sehr intensiv pigmentirt 
 ist, eine ringformige Pigmentzone, welche alle Retinulae 
 wie ein Fassreifen umspannt. Bei Platybunus sind die 
 Zellen dieser Zone statt mit Pigment mit kleinen, glan- 
 zenden Krystallen, und bei Phalangium brevicorne zum 
 Theil mit Pigment, zum Theil mit Krystallen erfiillt. 
 
 ^er Rest der Matrix, welcher zwischen der peri- 
 retinalen Membran und der Nervenfaserschicht liegt, ist 
 ausserst dunn, mit wenigen langgestreckten Kernen ver- 
 sehen. Er kann Pigment oder Krystalle enthalten, aber 
 auch stellenweise von beiden frei sein. 
 
 Schon oben habe ich erwahnt, dass die periretinale 
 Membran des einen Auges direkt in die des anderen 
 iibergeht, ohne zwischen dieselben einzudringen. Hier- 
 durch ist in der ventralen Region zwischen der Kapsel 
 und den beiden Nervenfaserschichten ein betrachtlicher 
 Raum freigelassen, der gewohnlich von dreieckiger Ge- 
 stalt und mit einem besonderen faserig lockeren Gewebe 
 erfiillt ist. In ihm finden sich neben zahlreichen Kernen 
 gewohnlich Glanzkrystalle in grosser Menge, besonders 
 in dem Theil, welcher dem vorderen Korperende zu- 
 nachst liegt. 
 
 In der Medianebene des Korpers scheinen die 
 Nervenfaserschichten beider Augen in direkter Beruhrung 
 miteinander zu stehen; nur hier und dort trifft man ver- 
 einzelte, sehr abgeplattete Kerne. 
 
47 
 
 Die oben erwahnten Glanzkrystalle, welche man 
 in der Matrix findet, sind sehr kleine, kurze, eckige 
 Stabchen von gelblicher Farbung in dnrchscheinendem 
 Licht. Im reflektirten Lichte schillern sie prachtvoll 
 golden, griin und rotb, wie jene des Tapetums bei den 
 Spinnen. Ausserdem trifft man sie in grosser Menge in 
 den Hypodermiszellen und oft auch in anderen Geweben 
 (Fettkorper), wie es von Ley dig (IB, S. 384) des Ge- 
 naueren bescbrieben ist. 
 
 VII. Iiitteraturubersicht. 
 
 Scbon Tulk (1843 No. 27) kannte die Thatsache, 
 dass die beiden Augen der Phalangiden mit einander 
 verbunden sind. Ferner beschreibt er und bildet (Fig. 32 
 PI. V) ein Paar Muskeln ab, welche jederseits unter den 
 Augen verlaufen und eine Verschiebung des Augeninhalts 
 ermoglichen sollen. 
 
 In der That fand ich verschiedene Langsmuskeln, 
 welche auf der lateralen Seite der Nervi optici dicht 
 unterhalb der Augen vorbeilaufen. Sie verbinden sich 
 aber nicht mit den Augen, sondern setzen sich einer- 
 seits an der Cuticula unmittelbar hinter dem Augen- 
 hocker und andererseits am oberen Rande der Basen der 
 Cheliceren fest. Ohne Zweifel sind dies, wie schon 
 Grenacher erwahnt, die Muskeln, welche Tulk ge- 
 sehen hat. 
 
 Ley dig (1855 No. 13) beschreibt bei Phalangium 
 ein aus zerstreuten Flitterchen gebildetes Tapetum, 
 welches sich am Augengrund ausnimmt „wie Sterne am 
 dunkelen Firmament 44 (S. 439). In seiner Figur 21 
 bildet Leydig die Verhaltnisse ab. Das Tapetum be- 
 
48 
 
 steht aus Kiigelchen, die grosser sind als die Pigment- 
 korner. Es ist mir niemals moglich gewesen, bei irgend 
 einer Species etwas einem solchen Tapetum Ent- 
 sprechendes aufzufinden; auch Grenacher erwiihnt nichts 
 davon. 
 
 Leydig giebt ferner eine schone Figur (No. 15, 
 Taf. VIII, Fig. 2), welche die Beziehungen des Auges 
 und Gehirns zu einander zeigt. 
 
 Die sehr genaue Beschreibung Grenacher’s (1879 
 No. 6) von dem Verhalten der Linse and des Glas- 
 korpers kann ich in alien Punkten bestatigen. Dagegen 
 kann ich seiner Darstellung iiber den Bau der Retina 
 nicht ganz beistimmen. Er sagt: Die hinter dem Glas- 
 korper gelegene Retina besteht aus einer einzigen Zellen- 
 lage. Die Elemente derselben sind stark verlangert, von 
 vorn bis hinten mit Pigment erfiillt und im frischen Zu- 
 stand mehr cylindrisch als nach der Erhartung, wo sie 
 ziemlich spindelformig werden. An ikrem Vorderende 
 ist ihnen je ein Stabchen eingelagert, und an ibrem 
 hinteren Ende treten sie mit dem Nervus opticus in Zu- 
 sammenkang. Die Kerne liegen hinter der Mitte und 
 verursachen das spindelformige Aussehen der Zelle durch 
 ihre Auftreibung. Die spindelformigen Stabchen lassen 
 sich etwa mit einem Getreidekorn vergleichen. Bei 
 Langsansichten zeigen sie eine sehr feine Langslinie, 
 welcher oft leichte Einkerbungen an den abgerundeten 
 Enden entsprechen. Die Stabchen bestehen aus drei 
 gleich grossen, der Lange nach an einander gekitteten 
 Segmenten, sodass der Querschnitt ein Kleeblattahnliches 
 Aussehen zeigt. 
 
 Aus dem Mitgetheilten kann man ersehen, dass 
 meine Darstellung von derjenigen Grenacher’s in einem 
 sehr wesentlichen Punkte abweicht, obwohl ich ganz be- 
 sonders hervorheben mochte, dass die von Grenacher 
 gegebene Darstellung vom Bau des Auges im Uebrigen 
 
49 
 
 eine sehr naturgetreue ist. Der erwahnte Differenzpunkt 
 besteht darin, dass nach Grenacher jedes dreitheilige 
 Stabchen nur einer einzigen Zelle zugehort, die Zellen 
 der Retina gleichwerthig und nicht in Gruppen oder Re- 
 tinnlae angeordnet sind, dass dagegen nach meinen Beob- 
 achtungen jedes solche „Stabchen“ in Wirklichkeit ein 
 Rhabdom ist, d. b. eine Gruppe von vier Zellen, welche 
 zusammen eine Retinula bilden. 
 
 Die von Grenacher untersuchte Art, Phalangium 
 opilio, ist allerdings wegen der Kleinheit der betreffen- 
 den Elemente weit weniger giinstig fur die Unter- 
 suchung als Leiobunum und Opilio parietinus, an 
 welchen ich meine Beobachtungen zuerst gemacht habe. 
 
 Patten (1886 No. 19) fand gelegentlich seiner aus- 
 gedehnten Untersuchungen, dass die Retina der Mollusken 
 und Arthropoden sowohl bei den Stemmata, wie bei den 
 facettirten Augen sich aus kreisformig angeordneten 
 Gruppen von pigmentirten Zellen zusammensetzt, die 
 central liegende Zellen umgeben, welche letztere durch 
 konstante Merkmale im Bau ausgezeichnet sind. Diese 
 letzteren Zellen nennt er „Retinophoren“. Ihrer sind 
 stets zwei oder mehr vorhanden, die allerdings mitein- 
 ander verschmelzen konnen, wobei der Kern der einen 
 degenerirt. Die Sehstabe werden von diesen erzeugt 
 und haben somit stets die Natur eines Rhabdoms. So- 
 wohl die farblosen Zellen, als auch die Theile der Seh- 
 stabe schliessen eine axial, urspriinglich intercellular ge- 
 legene Nervenfaser zwischen sich ein. Diese Gruppen 
 von pigmentirten und farblosen Zellen, welche Patten 
 mit einem fniher etwas anders gebrauchten Namen 
 als „Ommatidia“ bezeichnet, sollen, wie er glaubt, 
 die Strukturelemente der meisten, wenn nicht aller 
 Augen sein. 
 
 Bei seinen Untersuchungen iiber das Auge von 
 Phalangium (20) findet er nun auch, dass die Retina 
 p. 4 
 
50 
 
 dieser Form nach demselben Schema gebaut ist. Sie 
 soil aus wohlentwickelten „Ommatidia“ bestehen, von 
 denen jedes sich aus wenigstens neun Zellen zusammen- 
 setzt. Das Centrum eines jeden „Ommatidiums“ wird 
 von drei Retinophoren gebildet, die je einen Sehstab 
 tragen, welcher am proximalen Ende in eine Spitze aus- 
 lauft. Die drei Sehstabe vereinigen sich mittelst ihrer 
 axialen Flachen zu einem konischen Korper, so dass 
 man auf Querschnitten das Bild einer T-formigen oder 
 dreistrahligen Figur erhalt, deren Arme eingekerbt sind. 
 Die drei Sehstabe verlangern sich proximal in drei 
 dtinne Faden, die in einer zarten Rohre eingeschlossen 
 sind, welche ihrerseits durch Verwachsuug der proxi- 
 malen Enden der farblosen Retinophoren gebildet wird. 
 Die genannten Faden stehen in Zusammenhang mit 
 einer axialen Nervenfaser, welche durch die Rohre in 
 das Centrum des Kegels (Rhabdoms) iibergeht. Die 
 Kerne der Retinophoren, d. h. der drei farblosen cen- 
 tralen Zellen sind wahrscheiulich iiber dem distalen 
 Ende der Sehstabe gelegen, wenn auch Patten diesen 
 Punkt nicht mit Bestimmtheit festgestellt bat. Die Re- 
 tinophoren sind von sechs, in zwei Kreisen angeordneten 
 pigmentirten Zellen umgeben. Der aussere und innere 
 Kreis besteht aus je drei Zellen. Diejenigen des ausseren 
 sind schmal und liegen in den Rinnen des Kegels 
 (Rhabdoms). Die Kerne haben ihre Lage gegeniiber 
 der Mitte desselbeD, aber sie sind kaum zu erkennen. 
 Die proximalen Enden der Zellen laufen in lange hyaline 
 Faden aus. Die drei Zellen des inneren Kreises sind 
 dagegen viel grosser und sind den Leisten des Rhab- 
 doms angelagert. Ihre grossen ovalen Kerne liegen 
 proximal von den inneren Enden der Sehstabe und 
 scheinen beim ersten Anblick die einzigen Kerne zu 
 sein, welche in der Retina vorhanden sind. 
 
 Henking (1888 No. 9) machte einige Beobach- 
 
51 
 
 tungen liber die Linse wahrend der Hautung. Das 
 weiche Material fiir die zukiinftige neue Linse wird be- 
 reits unter der gelockerten alten Linse abgeschieden und 
 ist anfangs nach aussen konkav, entsprechend der ge- 
 kriimmten Unterseite der alten Linse. Diese Angaben 
 Henking’s kann ich auch bestatigen. 
 
 VIII. Ueberblick fiber die Entwickelnng der 
 verschiedenen Theile des Anges. 
 
 Da ich schon in meiner vorlaufigen Mittheilung (22) 
 eine kurze Uebersicht fiber die Entwickelung des Auges 
 von Leiobunum rotundum gegeben habe und dieselbe 
 spater ausfiihrlich zu behandeln gedenke, so werde ich 
 hier nur so viel wiederholen, als zum Verstandnisse der 
 genetischen Beziehungen der Theile zu einander noth- 
 wendig ist. 
 
 Die Augen entstehen aus einem Paar ektodermaler 
 Taschen am Kopfsegment des Keimstreifens, weiche 
 durch einen komplicirten Faltungsprocess sich bilden, 
 anfangs mit einem Lumen versehen und von einander 
 vollstandig getrennt sind. Spater werden sie vollig von 
 der Hypodermis abgeschniirt, bleiben jedoch mit ihr 
 immer in Beriihrung. Bald riicken sie mehr und mehr 
 in der Medianebene zusammen. 
 
 Die aussere Wand jedes Sackes wird sehr dick 
 und gleichzeitig wird sie in das Innere des Sackes ein- 
 gestiilpt, so dass das Lumen des letzteren vollstandig 
 verschwindet. Diese verdickte Wand giebt der Retina 
 den Ursprung. Ihre Zellen ordnen sich in einer einzigen 
 Schicht an und bilden zuletzt die Rhabdomere an ihrem 
 
52 
 
 jetzt distalen Ende, welches indessen dem urspriinglich 
 (vor der Inversion) inneren basalen Ende entspricht. 
 Die Nervenfasern haben ihre definitiven Ansatzpunkte 
 an dem entgegengesetzten proximalen, dem Lumen zu- 
 gekehrten Ende, welches dem urspriinglich freien, ausseren 
 Ende der Zellen gleich ist. Indessen erzeugen nicht 
 alle Zellen der ausseren Wand des Sackes Rhabdomere, 
 sondern die randstandigen, welche eine zwei bis drei 
 Zellen dicke Zone bilden, werden sehr intensiv pigmen- 
 tirt und geben dem periretinalen Pigmentring den Ur- 
 sprung, welcher, wie oben beschrieben wurde, einen 
 Theil der Matrix der periretinalen Membran bildet. 
 
 Die proximale innere Wand des Sackes, welche 
 niemals besonders stark sich verdickt, bleibt an den 
 meisten Stellen sehr diinn und aus ihr geht erstens der 
 dunnste Theil der Matrix hervor, welcher zwischen der 
 periretinalen Membran und der Nervenfaserschicht liegt, 
 ferner das ein Dreieck bildende, faserig-lockere Ge- 
 webe auf der Yentralseite zwischen den beiden Augen, 
 und wahrscheinlich entstehen aus ihr auch die lang- 
 gestreckten Kerne, welche sich zwischen der Nerven- 
 faserschicht der beiden Augen in der Medianebene 
 finden. 
 
 Jener Theil der Hypodermis, welcher direkt iiber 
 der Retina liegt, erfahrt auch eine Yerdickung und wird 
 zum Glaskdrper, der an seiner distalen Wand die cuti- 
 culare Linse abscheidet. So ergiebt das Resultat, dass 
 die Augen der Phalangiden dreischichtige in- 
 verse Augen ektodermalen Ursprungs sind. 
 
IX. Vergleiclmng der Phalangiden augen 
 mit deneii der Aracliniden im Allgemeinen. 
 
 Wenn wir die Augen der Phalangiden mit denen 
 der iibrigen Arachniden, so weit sie bis jetzt untersucht 
 sind, vergleichen, so finden wir, dass sie mit den Augen 
 der Skorpione noch am ehesten ubereinstimmen. 
 
 Die Skorpione besitzen bekanntlich ein Paar Mittel- 
 augen und mehrere Paare von Seitenaugen. Die letz- 
 teren sind nach den iibereinstimmenden Angaben von 
 Lankester und Bourne (12) sowie G. H. Parker (18), 
 die ich bestatigen kann, einschichtig und bilden nur 
 einen besonderen Theil der Hypodermis, so dass sich 
 die Retina in direkter Kontinuitat mit der Linse befindet. 
 Sie entstehen nach Parker durch eine einfach gruben- 
 formige Einsenkung des Korperepitkels, ohne dass eine 
 Inversion der Retina erfolgt. 
 
 Diese Augen kaben kein Homologon bei den Pha- 
 langiden und kommen deshalb fur uns nicht in Betracht. 
 Die Mittelaugen dagegen, welche einen ganz anderen Bau 
 zeigen, sind ohne Zweifel den Phalangidenaugen homolog. 
 
 Die Entwickelung der Mittelaugen verliiuft nach 
 den Untersuchungen von G. H. Parker (18) in ganz 
 ahnlicher Weise wie bei den Augen der Phalangiden 
 und stimmt, in vielen Punkten sogar bis in’s Einzelne 
 mit diesen iiberein. Die Mittelaugen der Skorpione ent- 
 stehen ebenfalls aus drei Schichten, von denen die 
 aussere den Glaskorper und die mittlere die invertirte 
 Retina bildet, wahrend aus der inneren eine diinne 
 postretinale Pigmentschicht hervorgeht. Die letztere 
 fungirt als Matrix fur einen Theil der Retinalkapsel, 
 welche genau so wie bei den Phalangiden die Retinae bei- 
 der Augen in einer gemeinsamen Hiille einschliesst. 
 
54 
 
 Da es zu weit fuhren wurde, alle Ansichten, welche 
 uber den Ban der Retina der Mittelaugen bei den Skor- 
 pionen geaussert worden sind, zu diskutiren, so werde 
 ich mich nur an die genaueren Untersucbungen von 
 Lankester und Bourne (12) und von Parker (18) 
 halten, zumal ich deren Angaben fur Euscorpius be- 
 statigen kann. 
 
 Die Retina besteht aus Retinulae, von denen sich 
 jede aus fiinf Zellen, die in einem Kreise angeordnet 
 sind, zusammensetzt. Jede Zelle producirt seitlich ein 
 rinnenformiges Rhabdomer, welches aber ihr distales 
 Ende nicht erreicht. Die Rhabdomere vereinigen sich 
 in jeder Retinula zu einem mit fiinf Langsrinnen ver- 
 sehenen Rhabdom, wodurch dieses auf dem Querschnitt 
 fiinfstrahlig erscheint. Der distale Theil der Retinazelle 
 ist dicht pigmentirt, wahrend der proximale Theil den 
 Kern enthalt und in die Nervenfaser iibergeht. Bei 
 Centrurus fand Parker zwischen den Retinulae in der 
 distalen Region besondere keulenformige Pigmentzellen, 
 welche in der Rhabdomregion sehr fein und fadenformig 
 werden und in der proximalen Region, wo der Kern 
 liegt, wieder an Grosse zunehmen. Ganz ahnliche 
 Pigmentzellen habe ich auch bei Euscorpius finden 
 konnen. Lankester und Bourne beschreiben zwar 
 bei Androctonus ausserst feine Pigmentzellen zwischen 
 den Retinulae im distalen und mittleren Abschnitt der- 
 selben, indessen sind bei Buthus nach Grenacher (7), 
 bei Centrurus nach Parker (18), sowie bei Euscorpius 
 nach Lankester und Bourne (1.2) und nach mir in 
 der distalen Halfte der Retina gar keine Kerne zu finden. 
 Zwischen der Retina und der postretinalen Pigmentschicht 
 befindet sich eine Lage von Nervenfasern, die gerade 
 so wie bei den Phalangiden zu zahlreichen Biindeln 
 innerhalb der Retinalkapsel vereinigt sind. 
 
 Dieses mag geniigen, um die Homologie zwischen 
 
den Mittelaugen der Skorpione und den Phalangiden- 
 augen zu zeigen. Die Hauptunterschiede liegen in der 
 Abwesenheit einer centralen Zelle, im Baa der Retinulae 
 und in dem Vorhandensein von echten, interretinularen 
 Pigmentzellen bei den Skorpionen. Ich mochte aber die 
 Aufmerksamkeit auf die Aehnlichkeit lenken, welche 
 zwiscben den Plasmastrangen der centralen Zelle in der 
 Acantholophusgruppe und dem keulenformigen distalen 
 Abschnitt der Pigmentzellen bei Centrurus und Euscor- 
 pius besteht, ohne damit natiirlich eine Homologie 
 zwiscben ihnen bebaupten zu wollen. 
 
 Bei den echten Spinnen (Araneae), wo ein ahn- 
 licber Dimorpbismus der Augen, wie bei den Skorpionen, 
 schon langst durch Grenacber (6) bekannt geworden 
 ist, unterscheidet man die vorderen Mittelaugen oder 
 Hauptaugen 1 ) von den iibrigen sogenannten „Neben- 
 augen“ ! ) (vorderen Seitenaugen und binteren Augen). 
 
 Die Nebenaugen sollen nacb Locy (16) nach ibrer 
 Entwickelung inverse Augen sein, doch konnte Mark (17), 
 der die Praparate von Locy daraufbin weiter untersucbt 
 bat, diese Angabe nicbt mit Sicberheit bestatigen, wenn 
 er aucb den inversen Bau fur den wahrscheinlichsten 
 bielt. Erst Kisbinouye (10) konnte beweisen, was 
 schon Patten vorber behauptet hatte, dass die Neben- 
 augen durcb eine becherformige ektodermale Einsenkung 
 entsteben. Der Boden der letzteren wird zur Retina, 
 die keine Spur einer Inversion zeigt. Diese Angaben 
 Kishinouye’s babe ich fruher schon bestatigt (Kor- 
 scbet und Heider No. 11, S. 597). 
 
 Durcb anatomiscbe Untersuchungen ist ferner Bert- 
 kau (1) zu dem Schluss gekommen, dass die Neben- 
 augen inverse Augen seien, obwohl er dieses nicht mit 
 volliger Sicberheit beweisen konnte. Diese Verhaltnisse 
 
 J ) So von Bert kau (1) genannt. 
 
56 
 
 habe ich ebenfalls eingehend untersucht und kann im 
 Gegensatz zu Bertkau die alteren Angaben Grenacher’s 
 iiber die Nebenaugen in alien wichtigen Punkten vollig 
 bestatigen. 
 
 Die Hauptaugen (vordere Mittelaugen) der Spinnen 
 dagegen sind nach den iibereinstimmenden Angaben yon 
 Locy (16), Mark (17) und Kishinouye (10) inverse 
 Augen und aus drei Schichten gebaut. Es kann deshalb 
 nicht zweifelhaft sein, dass diese den Phalangidenaugen 
 homolog sind. 
 
 In der Retina der Hauptaugen kommen ausser den 
 eigentlichen Sebzellen nocb ecbte Pigmentzellen vor. 
 Die Sebzellen sind nacb Grenacber und Bertkau 
 nicbt in Retinulae angeordnet, besitzen einen im proxi- 
 malen Tbeil gelegenen Kern und gehen am proximalen 
 Ende in die Nervenfaser iiber. Letzteres Verhaltniss ist 
 ganz so wie bei den Pbalangiden und den Skorpionen. 
 Wahrend icb dies aucb fiir einige Spinnen (Attiden) be- 
 statigen kann, babe icb bei den meisten Dipneumones 
 gefunden, dass der Nerv sicb nicbt am proximalen Ende, 
 sondern in der Mitte der Retinazelle ansetzt. 
 
 Eine Verbindung zwischen den am distalen Ende 
 gelegenen Stabchen benachbarter Zellen ist bis jetzt 
 nicht beschrieben worden. Icb babe indessen bei einer 
 grossen siidamerikani scben Vogelspinne, sowie 
 bei vielen Dipneumones eine echte Rhabdombil- 
 dung gefunden, welcbe einen Typus aufweist, der, so- 
 weit mir bekannt ist, ganz vereinzelt unter den Arthro- 
 poden dasteht. Jede Zelle namlich producirt ungefahr 
 fiinf Rbabdomere, von denen jedes mit einem der Rhab- 
 domere jeder anliegenden Zelle verwachst. Auf diese 
 Weise nimmt jede Zelle Tbeil an der Bildung von 4 bis 
 6 verscbiedenen Rhabdomen. Jedes Rhabdom ist das 
 Produkt von in der Regel nur zwei Zellen und er- 
 scheint deshalb aucb zweitheilig. Eine Gruppirung der 
 
57 
 
 Zellen zu Retinulae findet sich indessen nicht. Ich 
 werde diese Verhaltnisse spater noch eingehender be- 
 schreiben, wenn ich meine Untersuchungen fiber den Bau 
 der Spinnenaugen veroffentlichen werde. 
 
 Ein ahnlicher Typus von Rhabdombildung ist von 
 Grenacher (8) in seinen schonen Untersuchungen fiir 
 das Cephalopodenauge beschrieben worden. Bei diesen 
 Thieren scheidet jede Zelle in der Regel zwei Rhabdo- 
 mere ab und nimmt an der Bildung von zwei verschie- 
 denen Rhabdomen Theil. Die letzteren werden stets von 
 3 bis 5 Zellen gebildet. 
 
 Yon dem Augen der iibrigen Arachniden sind, so- 
 weit ich weiss, nur noch die der Ps eudoskorpione 
 untersucht worden. Nach der Beschreibung von Bert- 
 kau (2) sind sie den Nebenaugen der Spinnen in vielen 
 Punkten sehr ahnlich und sehr wahrscheinlich mit diesen 
 homolog. 
 
 Die Homologie der Mittelaugen von Limulus mit 
 den Mittelaugen der Skorpione wurde zuerst von Lan- 
 k ester und Bourne behauptet. Diese Augen, welche 
 fast in der Medianlinie liegen, besitzen nach den An- 
 gaben der genannten Autoren einen Glaskorper und eine 
 darunter liegende Retina, deren Zellen hier und dort 
 eine Anordnung zu Retinulae von je 5 rhabdombilden- 
 den Zellen zeigt. 
 
 Aus den mitgetheilten Angaben geht klar hervor, 
 dass die vorderen Mittelaugen der Spinnen, die 
 Augen der Phalangiden und die Mittelaugen der 
 Skorpione, sowie jedenfalls die Mittelaugen des 
 Limulus eine Reihe von homologen Gebilden 
 darstellen, welche durch eine invertirte Retina 
 mit Retinulae oder wenigstens Rhabdomen cha- 
 rakterisirt sind. 
 
 Da wir bei den niedrigsten Arachnidenformen, wie 
 den Skorpionen, sowie bei Limulus, eine Retina mit Re- 
 
58 
 
 tinulae antreffen, so ergiebt sich ferner, dass der ur- 
 spriingliche Arachnidentypus auch eine aus Retinulae zu- 
 sammengesetzte Retina besass. Ich sebe das Hauptre- 
 sultat dieser Arbeit darin, nacbgewiesen zu haben, dass 
 eine aus Retinulae z usammen gesetzte Retina 
 bezw. eine Modifikation derselben auch bei den 
 hoheren Arachnidenordnungen, den Phalangiden 
 u nd den Spinnen, noch besteht. 
 
 Korschelt (No. 11 S. 600) ist der Ansicht, dass 
 die Mittelaugen der Skorpione und des Limulus von zu- 
 sammengesetzten Facetten- Augen abgeleitet sind, deren 
 Einzelaugen (Ommatidia) beim Skorpion und Limulus 
 durch die Retinulae reprasentirt werden. Diesen Ge- 
 danken weiter verfolgend, kam der genannte Autor zu 
 dem Schluss, dass auch bei den hoheren Arachniden- 
 ordnungen in Homologie mit den Mittelaugen der 
 Skorpione friiber eine zusammengesetzte Retina vor- 
 handen gewesen sein musste. Obgleich ihm das Vor- 
 handensein von Rhabdomen bei den Spinnen und Pha- 
 langiden unbekannt war, so fuhrten ihn die Angaben 
 Grenacher > s, dass die Stabchen der Spinnen aus zwei, 
 die der Phalangiden aus drei Stucken bestanden, zu der 
 Vermuthung, dass es sich bei der Zwei- resp. Dreitheilig- 
 keit der Stabchen vielleicht um Reste der Rhabdom- 
 und Retinulabildung handeln mochte (No. 11 S. 601). 
 Wie in der vorliegenden Arbeit nachgewiesen wurde, 
 hat diese Vermuthung fur die Augen der Phalangiden 
 und fur die Mittelaugen der Spinnen vollig das Richtige 
 getroffen. 
 
Litteratur. 
 
 1. P. Bertkau, Beitrage zur Kenntniss der Sinnesorgane der 
 
 Spinnen. I. Die Augen der Spinnen. Arch. f. mikr. Anat. 
 27. Band. 1886. Bonn. 
 
 2. — — Ueber die Chernetiden oder Pseudoskorpione. Verhand- 
 
 lungen des naturhist. Yereins der preussischen Rheinlande, 
 Sitzungsberichte. 44. Jahrg. 1887. Bonn. 
 
 3. 0. Biitschli, Untersuchungen iiber mikroskopisehe Schaume 
 
 und das Protoplasma. Leipzig 1892. 
 
 A* 4. J. Carriere, Bau und Entwicklung des Auges der 10 fiissigen 
 Crustaceen und der Arachniden. Biolog. Centralb. Band 9. 
 1889. 
 
 5. Y. Graber, Ueber das unicorneale Tracheaten- und speciell 
 
 das Arachniden- und Myriapoden-Auge. Archiv f. mikr. 
 Anat. Bd. 17. 1879. Bonn. 
 
 6. H. Grenacher, Untersuchungen iiber das Sehorgan der Arthro- 
 
 poden, insbesondere der Spinnen, Insekten und Crustaceen. 
 Gottingen 1879. 
 
 7. — — Ueber die Augen einiger Myriapoden. Arch. f. mikrosk. 
 
 Anat. 18. Bd. 1880. 
 
 8. — — Abhandlung zur vergleich. Anatomie des Auges. I. Die 
 
 Retina der Cephalopoden. Abhandl. naturw. Gesellsch. Halle. 
 16. Bd. 1884. 
 
 9. H. Henking, Biologische Beobachtungen an Phalangiden. Zool. 
 
 Jahrbiicher, Abth. f. System. 3. Bd. 1888. Jena. 
 
 ^10. Kishinouye, On the Development of Araneina. Journ. of the 
 Coll, of sci., Imperial Uni vers. Japan. Yol. IV. Parti. 1891. 
 
 ^ 11. E. Korschelt und K. Heider, Lehrbuch der vergleichenden 
 Entwicklungsgeschichte der wirbellosen Thiere. Specieller 
 Theil. 2. Heft. Jena 1892. 
 
 A 12. Ray Lankester and Bourne, The minute structure of the 
 lateral and the central eyes of Scorpio and of Limulus. 
 Quart. Journ. of micr. sci. Yol. XXIII. n. ser. 1883. 
 
60 
 
 13. F. Ley dig, Zum feineren Bau der Arthropoden. Archiv fur 
 
 Anat., Physiol, u. wiss. Med. Jahrg. 1855. Berlin. 
 
 14. — — Ueber das Nervensystem der Afterspinne (Phalangium). 
 
 Archiv f. Anat., Physiol, und wiss. Med. Jahrg. 1862. Leipzig. 
 
 15. — — Tafeln zur vergleichenden Anatomie. Tubingen 1864. 
 ^16. W. Locy, Observations on the development of Agalena naevia. 
 
 Bull, of the Mus. of Compar. Zool. at Harv. Coll. Yol. XII. 
 No. 3. Cambridge 1886. 
 
 K 17. E. L. Mark, Simple Eyes in Arthropods. Bull, of the Museum 
 of Comp. Zool. at Harv. Coll. Cambridge Vol. XII. No. 3. 
 1887. 
 
 < 18. G. H. Parker, The Eyes in Scorpions. Bull, of the Museum 
 of Comp. Zool. at Harv. Coll. Cambridge Yol. XIII. No. 6. 
 1887. 
 
 ^ 19. W. Patten, Eyes of Molluscs and Arthropods. Mitth. aus der 
 Zoolog. Station zu Neapel. Bd. 6. Berlin 1886. 
 
 A 20. — — Studies on the Eyes of Arthropods. I. Develop, of the 
 Eyes of Vfespa, with Observ. on the Ocells of some Insects. 
 Journ. of Morph. Vol. 1. Boston 1887. 
 
 ^ 21. F. Plateau, Recherches experimentales sur la vision chez les 
 Arthropodes (deuxieme partie). Vision chez les Arachnides. 
 Bulletins de l’Acad. Royale des sci. etc. de Belgique 57. 
 Ann. 3. Ser. T. XIV. 1887. 
 
 ^ 22. F. Purcell, Ueber den Bau und die Entwicklung der Phalan- 
 giden-Augen. (Vorlaufige Mitth.) Zool. Anz. Jhg. 15. 1892. 
 
 23. G. Saint Remy, Contribution a P etude du cerveaux chez les 
 
 Arthropodes Tracheates. Arch, de Zool. exp. et gen. 2. Ser. 
 T. V. bis, Suppl. 1887. 
 
 24. E. Simon, Les Arachnides de France. T. 7. Paris 1879. 8°. 
 
 25. M. Stefanowska, La Disposition Histologique du Pigment 
 
 dans les yeux des Arthropodes sous l’influence de la lumiere 
 directe et de l’obscurite complete. Recueil Zoolog. Suisse. 
 T. V. 1890. 
 
 26. G. R. und L. C. Treviranus, Vermischte Schriften anatomischen 
 
 und physiologischen Inhalts. Bd. 1. Gottingen 1816. 
 
 27. A. Tulk, Upon the Anatomy of Phalangium opilio. Ann. of 
 
 Nat. Hist. Vol. 12. 1843. 
 
 ^~\ r M 
 
T h e s e n. 
 
 i. 
 
 Die Biologie der Thiere wird nicht in geniigendem 
 Maasse getrieben. 
 
 II. 
 
 Die Mikrochemie soil dem Zoologen zuganglicher 
 gemacht werden. 
 
 III. 
 
 Die Abstammung der Wirbelthiere lasst sich zur 
 Zeit mit Sicherheit nicbt bestimmen. 
 

 Vita. 
 
 Natus sum Fredericus Purcell Londinii die XVIII Mensis 
 Sept. a. h. s. LX VI, patre Walter cuius mortem pio ammo lugeo, 
 matre Sophia, qua superstite gaudeo. 
 
 Fidei addictus sum evangelicae. 
 
 Primum in urbe capense collegium Africae australis frequen- 
 tavi. Deinde in Germaniam profectus, civibus universitatis Wilhelmae 
 Argentinensis adscriptus sum. Turn Berolini studiis per decern 
 semestria rerum naturalium operam dedi. 
 
 Docuerunt me praeceptores illustrissimi Berolini: du Bois- 
 
 Reymond, Dames, Dilthey, Heider, 0. Hertwig, Kny, Korschelt, 
 Moebius, F. E. Schulze, Seeliger, Waldeyer, 
 
 Argentorati: Fittig, Gotte, Kohlrausch, comes de Solms. 
 
 Quibus omnibus viris gratias ago quam maximas, imprimis 
 vero viro clarissimo F. E. Schulze. 
 

 
 
 
 
 


 Uri/Ht /ht Ctsrn 'TYlJZ^lX, 
 Y , /Ccn*Ces{/' 
 
 Uber den 
 
 it 
 
 Bau der Phalangidenaugen. 
 
 Yon 
 
 F. Purcell. 
 
 (Mit Tafel I u. II.) 
 
 (Separat-Abdruck aus: »Zeitschrift fur wissenschaftliche Zoologies LVIII. 1.) 
 

< 5 ^ /f/Y 
 
 Uber den Ban der Pbalangidenaugen. 
 
 Von 
 
 Fred. Purcell aus Kapstadt. 
 
 (Aus dem Zoologischen Institut zu Berlin.) 
 
 Slit Tafel I mid II. 
 
 Einleitung. 
 
 Wiederholt wurde in den letzten Jahren der Bau der Arachniden- 
 augen studirt. Zumeist betreffen diese Untersuchungen die Skorpione 
 und Spinnen. Den Phalangiden wurde nach dieser Hinsicht wenige 
 Aufmerksamkeit geschenkt, und die beiden einzigen^orscher, Grenacher 
 und Patten, welche sie mit Hilfe der neuen Methoden untersuchten, 
 gelangten bezliglich der Auffassung der Pbalangidenaugen zu vollig 
 entgegengesetzten Resultaten. 
 
 Der Erste, welcher sich, so weit mir bekannt ist, mit der Anatomie 
 der Phalangidenaugen beschaftigte, war Treviranus (I 81 6 , 26), dem dann 
 einige Zeit spater Tulk (1843, 27) folgte. Beide Arbeiten sind im alten 
 Stile geschrieben und haben nur mehr historisches Interesse. 
 
 Leydig (1855 — 1862, 13, 14) verdanken wir einige gelegentliche 
 Bemerkungen Uber verschiedene Theile des Auges; eine eingehende 
 Beschreibung wurde jedoch von ihm nicht gegeben. 
 
 Erst Grenacher (1879, 6), welcher in seiner groBen Monographic 
 auch das Auge von Phalangium opilio L. behandelte und es durch einige 
 Abbildungen erlauterte, giebt die erste eingehende und, abgesehen von 
 einem wichtigen Punkte, die genaueste Darstellung, welche wir be- 
 sitzen. 
 
 1887 veroffentlichte Patten (20) eine sehr kurze, anderthalb Seiten 
 umfassende, vorlaufige Mittheilung, ohne Abbildungen, liber eine ver- 
 muthlich amerikanische Phalangide. 
 
 Endlich habe ich selbst (22) eine kurze vorlaufige Notiz liber die 
 Anatomie und Entwicklung des Auges von Leiobunum rotundum Latr. 
 
 Zeitschrift f. wissensch. Zoologie. LYIII. Bd. 
 
 1 
 
2 
 
 Fred. Purcell. 
 
 (L. hemisphaericum Herbst) gegeben, welche sich aber nur auf einige 
 wenige der bemerkenswerthesten Ztige beschrankte. 
 
 Die vorliegende Untersuchungbeabsichtigthauptsachlick eine mog- 
 lichst genaue und vollstandige Beschreibung der Relinaelemente zu 
 geben. Die Linse und der Glaskorper werden besonders aus dem 
 Grunde, weil sie von Grenacher schon sehr genau beschrieben sind, 
 weniger eingehend behandelt werden. 
 
 Die Prdparate, welche als Beleg fiir die vorliegenden Ausftihrungen 
 dienen, wurden bei Gelegenheit der zweiten Jahresversammlung der 
 Deutschen Zoologischen Gesellschaft demonstrirt. Die Beobachtungen 
 sind an alien denjenigen Species, welche ich mir in der Umgegend 
 von Berlin in gentigender Menge verschaffen konnte, gewonnen wor- 
 den. Diese acht Species gehoren sechs Genera 1 an. Dieselben lassen 
 sich nach dem Bau ihrer Rhabdome in zwei natiirliche Gruppen unter- 
 scheiden, welche ich durch den Namen eines charakteristischen Genus 
 gekennzeichnet habe : 
 
 a) die Leiobunum-Gruppe; sie umfasst: 
 
 1) Leiobunum rotundum Latr. (L. hemisphaericum Herbst), 
 
 2) Phalangium opilio L. (Ph. cornutum L.), 
 
 3) Phalangium brevicorne C. Koch, 
 
 4) Platybunus triangularis Herbst; 
 
 b) die Acantholophus-Gruppe; die umfasst: 
 
 5) Opilio parietinus de Geer, 
 
 6) Acantholophus hispidus Herbst, 
 
 1 Die Bestimmung der Species geschah mit Hilfe des ausgezeichneten Buches 
 von Simon (24), dessen Nomenklatur ich, auBer in dem folgenden Punkte, ange- 
 wendet habe. Das Genus Phalangium, wie ich es hier verstehe, und welches in 
 meiner Arbeit durch Phalangium opilio L. und Ph. brevicorne G. K. vertreten ist, 
 stimmt iiberein mit dem Genus Cerastoma von C. L. Koch und Anderen. Es ist 
 charakterisirt durch die hornahnliche Verlangerung, welche sich an der Basis des 
 zweiten Gliedes der Cheliceren beim Mannchen findet. Simon dagegen, welcher 
 keinen Unterschied zwischen den Weibchen von Cerastoma und Opilio fand, ver- 
 einigte beide Genera unter dem Namen Phalangium. Ich habe nun aber gefunden, 
 dass im Bau der Retina die Species von Koch’s Gattung Cerastoma (Phalangium 
 opilio und Ph. brevicorne) sehr nahe Leiobunum verwandt sind, aber betrachtlich 
 von Opilio parietinus de Geer abweichen, wahrend die letztere Art wiederum sehr 
 nahe Acantholophus und Oligolophus verwandt ist. Aus diesen Grtinden bin ich 
 gezwungen, die beiden erstgenannten Species (Phalangium opilio und Ph. brevi- 
 corne) von Opilio parietinus wieder zu trennen, und ich habe statt Koch’s Namen 
 Cerastoma den Namen Phalangium L., fiir welche Gattung Phalangium opilio L. 
 typisch ist, gebraucht, wahrend Opilio Herbst fiir die Gattung bleibt, fiir welche 
 Opilio parietinus de Geer den Typus darstellt. 
 
Uber den Ban der Plialangidenangen. 
 
 3 
 
 7) Oligolophus palpinalis Herbst (Op. terricola C. Koch), 
 
 8) Oligolophus tridens C. Koch. 
 
 Jede von diesen Arten wurde von mir mit moglichster Genauigkeit 
 geprtift, und alle Angaben in der folgenden Darstellung gelten fiir alle 
 Species, falls das Gegentheil nicht besonders hervorgehoben ist. 
 
 Es ist mir eine angenehme Pflicht Herrn Geheimrath Professor 
 F. E. Schulze filr das lebhafte Interesse, das er meiner Arbeit zuwandte, 
 sowie fUr die Uberlassung der reichen Hilfsmittel des Berliner Zoo- 
 logischen Instituts meinen aufrichtigen Dank auszusprechen. Auch 
 Herrn Professor E. Korschelt bin ich fiir vielfachen freundlichen Rath 
 zu berzlichem Dank verpflichtet. 
 
 Untersuclmiigsmethoden, 
 
 Von den technischen Schwierigkeiten, mit welchen bekanntlich 
 eine Untersuchung der Retina zu kampfen hat. ist die groBte diejenige, 
 eine geeignete Konservirungsfltissigkeit zu finden. 
 
 Die gewohnlichen kalten wasserigen Lbsungen sind wenig 
 brauchbar, wahrscheinlich weil sie viel zu langsam eindringen und 
 die rasch sich verandernden Gewebe des Auges erst einige Zeit nach 
 dem Tode erreichen. 
 
 Warme wasserige Losungen zwischen 35 und 95° C. dringen 
 zwar rasch ein, aber die wichtigen Zellgrenzen werden oft undeutlich 
 wiedergegeben und die Rhabdome werden durch die Hitze stark ver- 
 langert, so dass ihre Bilder zum Studium weniger geeignet sind. 
 
 Um diese Schwierigkeiten zu iiberwinden, und um ein rascheres 
 Eindringen ohne Anwendung von Hitze zu bewirken, bin ich fast aus- 
 schlieBlich nach folgendem Princip vorgegangen. Die gewohnlichen 
 wasserigen Reagentien (Pikrinsaure, FLEMMiNG’sche Losung etc.) werden 
 mit einer gleichen Menge von absolutem Alkohol vermischt, wodurch 
 sich eine Flttssigkeit ergiebt, die 50% Alkohol enthalt und welche die 
 Fahigkeit hat, schnell in das Gewebe einzudringen. Diese Mischungen 
 habe ich gewohnlich in kaltem Zustande, nur fiir bestimmte Zwecke 
 auch warm angewandt. 
 
 Kalte alkoholische Pikrinsau relos ung, welche ich durch 
 Mischung von absolutem Alkohol und einer gesattigten wasserigen 
 Pikrinsaurelosung zu gleichen Raumtheilen erhielt, ist die bei Weitem 
 beste Konservirungsfltissigkeit, welche ich kenne, fiir die Darstellung 
 der feinsten Strukturen in der Retina. Der abgeschnittene Cephalo- 
 thorax bleibt in der Mischung drei oder mehr Stunden, und wird 
 nachher mit 63%igem Alkohol ausgewaschen. Die Kerne, Zellgrenzen, 
 die Wabenstruktur des Protoplasmas und die Rhabdome bleiben aus- 
 
 4 * 
 
4 
 
 Fred. Purcell, 
 
 gezeichnet erhalten. Das hohe Lichtbrechungsvermogen der Rhabdome 
 und die Eigenschaft der verschiedenen Theile der letzteren, sich ver- 
 schieden zu fhrben, sowie besonders die Ldsbarkeit des Pigments wer- 
 den dadurch nicht beeintrSchtigt oder gemindert. Gewisse Nachtheile 
 dieses Reagens beruhen in der schlechten Konservirung der Nerven- 
 fasern. In Folge dessen mussten neben dieser M^thode noch andere 
 angewendet werden. 
 
 Da die N ervenfasern durch kalte FlUssigkeiten iiberhaupt sehr 
 schlecht zu konserviren sind, so muss man die betreffenden Reagentien 
 warm anwenden, und zwar bei Temperaturen zwischen 35 und 95° C. 
 (z. B. alkoholische Pikrinsaurelosung bei 45° C. ; Pikrinschwefelsaure 
 bei 56° C.| 
 
 Zum Studium der Rhabdome eignet sich am besten die Hartung 
 in kalter alkoholischer Pikrinsaure und die Farbung mit Hamatoxy- 
 lin. Um bei gewissen Species (z. B. Phalangium, Acantholophusj die 
 Wabenstruktur der Rhabdome deutlich zu machen, muss man, so lange 
 das Objekt in der FItissigkeit sich befindet, dieselbe auf 35° C. er- 
 warmt halten. Ferner habe ich, um eine Differenzirung zwischen 
 dem centralen Rhabdomer und den peripheren Rhabdomeren zu be- 
 kommen, die Objekte 20 Minuten lang bei 45 — 50° C. in 50%igem 
 Alkohol, der bei derselben Temperatur mit Pikrinsaure ges&ttigt war, 
 gehartet. Das Pigment des Auges, welches durch die warme FItissig- 
 keit gelost wird, farbt die Kerne und gewisse Theile des Rhabdoms 
 (besonders das centrale Rhabdomer) tief braun oder schwarz, wahrend 
 die tibrigen Theile des Rhabdoms ungefarbt bleiben oder hochstens 
 einen hellbraunen Ton annehmen. Aus solchen Praparaten darf natiir- 
 lich das Pigment nicht entfernt werden, und eine weitere Farbung ist 
 iiberfltissig. 
 
 Ubersichtsbilder, welche die richtigen topographischen Ver- 
 haltnisse der Theile des Auges zeigen, sind sehr schwer zu gewinnen, 
 weil bei Anwendung von kalten Losungen der Glaskorper schrumpft, 
 bei Anwendung von warmen die Rhabdome und Retinazellen sich ver- 
 langern. Am besten eignet sich hierfiir vielleicht eine Mischung von 
 FLEMMiNG 7 scher Losung und Alkohol absolutus zu gleichen Theilen, 
 welche man bei gewohnlicher Temperatur anwendet; ferner auch 
 warme alkoholische Mischungen (mit FLEMMiNG’scher Losung, Pikrin- 
 saure etc.) bei einer Temperatur von 56° C. ftlr 40 Minuten. 
 
 Das Pigment wurde mittels einer Mischung von zwei Theilen 
 80°/ 0 igem Alkohol, einem Theil Glycerin und 2 bis 3% Salzsaure (nach 
 Grenacher’s [8] Angaben) entfernt. Die Fahigkeit dieser Mischung, 
 das Pigment zu losen, wird sehr durch die Methode der Hartung beein- 
 
Uber den Bau der Phalangidenaugen. 
 
 5 
 
 flusst. Sie entfernt das Pigment vollstandig in y 4 — V 2 Minute aus 
 dtinnen Schnitten (3,5 fi), welche in kalter alkoholischer Pikrinsaure 
 gehartet sind, ohne das Gewebe im geringsten zu schadigen. Dagegen 
 ist fiir solche Praparate, welche in warmen Fltissigkeiten gehartet sind, 
 eine langere Einwirkung und oft ein gelindes Erwarmen nothig, wobei 
 die Schnitte mehr ader weniger stark leiden. 
 
 Man farbt am besten mit Hamatoxylin. Schnitte von 3,5 Dicke 
 werden nach der Entfernung des Pigments 20 Minuten in Delafield- 
 sches Hamatoxylin gebracht, dann einige Stunden in gewohnlichem 
 Wasser (nicht in salzsaurem Alkohol) ausgewaschen. 
 
 Es ist in Folge der Kleinheit der Elemente der Retina unbedingt 
 nothwendig, liickenlose Serien von Schnitten zu bekommen, welche 
 nicht mehr als 3,5 dick sind. Es ware mir dieses ohne Anwendung 
 der Losung von Mastix und Kollodium (in Alkohol und Ather) unmog- 
 lich gewesen. Diese Methode verdanke ich der Freundlichkeit des 
 Herrn Professor Karl Heider 1 , der sie zuerst in seiner Hydrophilus- 
 Arbeit erwiihnt. Die Oberflache des Paraffinblockes wird jedes Mai vor 
 dem Schneiden mit dieser Lbsung tlberstrichen. Das dtinne HSutchen 
 von Mastixkollodium, welches nach der Verdunstung des Alkohols und 
 Athers zuriickbleibt, gentigt um ein ZerreiBen oder Brechen des Schnit- 
 tes, wenn das Messer durch das harte Chitin des Korpers geht, zu ver- 
 hindern. Das Schneiden wird dadurch sehr erleichtert, dass man 
 zuerst mit dem Mikrotom einen groBen Theil der Linse und der vor- 
 deren chitinosen Wand des Augenhockers wegschneidet. Das Objekt 
 wird dann noch einmal in Paraffin eingebettet, bevor man die defini- 
 tiven Schnitte, welche quer zur Sehachse liegen, anfertigt. 
 
 Die Untersuchung wurde in einem Medium vorgenommen, 
 welches einen niedrigen Brechungsindex hat, w T ie z. B. Wasser, Alkohol 
 und besonders einer Losung von essigsaurem Kali. Praparate halten 
 sich in letzterem Medium mehrere Jabre, jedoch wird die Farbung ge- 
 wohnlich nach einigen Wochen theilweise ausgezogen. Sie konnen aber 
 dann leicht, wenn es nothwendig ist, zu jeder Zeit wieder gefarbt 
 werden. Kanadabalsam ist wegen seines hohen Brechungsindex wenig 
 geeignet, w’eil die feineren Details verschwinden oder sehr undeutlich 
 werden. 
 
 In Bezug auf die oben angegebene Herstellungsmethode der 
 Schnitte (alkoholische Pikrinsaurelosung — GRENACHER’sche Fliissigkeit 
 — Hamatoxylin — essigsaures Kali) ist es wichtig zu bemerken, dass der 
 Zustand der definitiven Praparate bei verschiedenen Individuen der- 
 
 1 K. Heider, Die Embryonalentwicklung von Hydrophilus piceus L. I. Theil. 
 Jena 1889. 
 
6 
 
 Fred. Purcell, 
 
 selben Species sehr verschieden ist. Es ist desshalb zu rathen, von jeder 
 Art, die man untersucht, eine groBere Zahl von Schnittserien anzufertigen. 
 Yon meinen Schnittserien erwies sich zwar gewohnlich die groBere 
 Zahl als gut, doch fand ich immer unter ihnen eine oder zwei Serien, 
 deren Erhaltungszustand und Farbung ganz musterhaft waren, und 
 welche die ubrigen hierin weit iibertrafen. Nachdem ich mir lticken- 
 lose Schnittserien mit gut erhaltenen Zellgrenzen verschafft hatte, wur- 
 den die Zellgrenzen in jenem Theil der Retina, welcher genau quer 
 getroffen war, mit Hilfe eines Zeichenapparates entworfen. Jede Reti- 
 nula in einer der Zeichnungen wurde dann mit einer Zahl versehen. 
 Durch sorgfaltige Yergleichung mit der Zeichnung des nachsten Schnit- 
 tes lasst sich mit absoluter Sicherheit jede Retinula in letzterem 
 identificiren und jede mit der ihr zukommenden Zahl versehen. Nach- 
 dem derart mit alien Zeichnungen von sammtlichen Schnitten ver- 
 fahren ist. kann man schlieBlich den Querschnitt durch eine und 
 dieselbe Retinula in jeder Region der Retina genau feststellen und 
 studiren. 
 
 I. Orientirung iiber den groberen Bau der Phalangidenaugen. 
 
 Die Phalangiden besitzen im Gegensatz zu den meisten Arachniden 
 nur zwei Augen. Dieses Augenpaar ist bekanntlich in einem Docker 
 auf der Dorsalflache des Cephalothorax enthalten. Die Sehachsen bilden 
 einen rechten Winkel mit einander und liegen in einer Transversal- 
 ebene des Korpers, so dass sie seitwarts und aufwarts gerichtet sind. 
 Da die Basen der beiden Augen zum groBten Theil in der Medianebene 
 (in Fig. 1 1 durch den Pfeil angedeutet) sich bertihren, so sind sie hier 
 abgeplattet, wahrend der Ubrige Theil der Oberflache des Augenbulbus 
 gewolbt ist. 
 
 Jedes Auge ist durch eine Scheidewand, die praretinale Zwi- 
 schenlamelle Ghaber’s (5) (prae.m Fig. 10), welche eine direkte 
 Fortsetzung der Basalmembran [bm Fig. I 0) der Hypodermis ist, in einen 
 auBeren und inneren Abschnitt geschieden. Der erstere funktionirt 
 als dioptrischer Apparat und besteht aus dem Glaskorper [Gl) 
 mit seinem Abscheidungsprodukt, der Lins e (L). Glaskorper und Linse 
 sind von einer Zone pigmentirter Hypodermiszellen ( Pg 2 ) resp. 
 pigmentirter Cuticula [Pg 1 ) umgeben. 
 
 Der innere Abschnitt dagegen bildet den percipirenden oder ner- 
 vosen Theil des Auges, der sich hauptsachlich aus einer machtigen 
 Retina {Rt) und einer viel diinneren Nervenfaserschicht (Nv 
 Fig. I 1). die proximal von ihr liegt, zusammensetzt. Die Oberflache 
 dieses Abschnittes ist, mit Ausnahme des abgeplatteten medianen 
 
Uber den Bau der Phalangidenaugen. 
 
 7 
 
 Theiles, von einer Membran (der Periretinalmembran Mark’s, peri.m ) 
 und ihrer dtinnen zelligen Matrix (Mx) umgeben. Da diese Membran 
 nicht zwischen die Augen dringt, sondern am Rande der erwahnten 
 Beriihrungsstelle beider Augen kontinuirlich von einem zum anderen 
 Auge tibergeht, so bilden die periretinalen und die oben genannten 
 praretinalen Membranen beider Augen zusammen eine einzige Kapsel 
 (die Retinalkapsel Mark’s), welche die beiden Retinae sammt Nerven- 
 faserschichten in einer gemeinsamen Htille einschlieBt. Auf der ven- 
 tralen Seite ist die Kapsel von einer Anzahl Lochern durchbrochen, 
 welche in zwei parallelen Reihen von gewohnlich je acht angeordnet 
 sind, durch deren jedes ein Sehnerv hindurchtritt. 
 
 II. Linse und angrenzende Cuticula. 
 
 Die Linse ist bekanntlich eine verdickte durchsichtige Partie der 
 chitinosen Cuticula des Korpers. Ihre stark konvexen Flachen gehoren, 
 wie Grenacher sagt, Kugelflachen von ungleichen Radien an, und zwar 
 tibertrifft derjenige der auBeren den der inneren. 
 
 Die an die Linse angrenzende Cuticula lasst deutlich drei chitindse 
 Schichten erkennen, von denen die auBerste ( Ct 1 Fig. 10 u. 11) sehr 
 dtinn, stark lichtbrechend, homogen ist und sich nicht farbt. Die mitt- 
 lere (Ct 2 ) erscheint viel dicker und meist pigmentirt, besonders in der 
 unmittelbar der Linse benachbarten Partie, welche dadurch wie eine 
 breite dunkle Zone (Pg l ) erscheint. Die mittlere Lage zeigt nur eine 
 undeutliche Schichtung, wdhrend diese bei der innersten Lage (Cfi) sehr 
 deutlich ausgepragt ist. 
 
 Yon diesen drei Schichten geht nur die auBerste unverandert be- 
 zilglich ihrer Dicke und sonstigen Beschaffenheit in die auBere Lage der 
 Linse tiber; die mittlere und innere Schicht dagegen werden stark 
 verdickt und bilden die Masse der Linse. In der letzteren lassen sich 
 diese beiden inneren Schichten nicht mehr von einander unterschei- 
 den, zumal sie auch in ihrer Beschaffenheit jetzt sich ahnlich erweisen, 
 indem sie mit Hamatoxylin sich intensiv farben , gerade so wie die 
 weiche, neugebildete Cuticula wahrend der Hautung. Die Cuticula des 
 Korpers nimmt sonst kaum eine Farbung an. Auch die Substanz der 
 Linse ist gegen Reagentien weniger widerstandsfahig: so wird sie z. B. 
 durch heiBe Pikrinschwefel- und Pikrinsalpetersaure stark angegriffen, 
 obwohl dies bei gewohnlicher Cuticula durchaus nicht der Fall ist. 
 
 In der Cuticula finden sich zwei Arten von Porenkanalen, 
 grobere und feinere. Von diesen konnte ich die groberen, eben so 
 wenig wie Grenacher, in der Linse auffinden, obwohl sie Leydig (13, 
 p. 434) beschreibt und abbildet. Die anderen Porenkanale (por), welche 
 
8 
 
 Fred. Purcell, 
 
 auBerst fein sind und die ganze Dicke der Cuticula in groBer Anzahl 
 durchsetzen, sind auch in der ganzen distalen Partie der Linse deutlich 
 zu unterscheiden, dagegen konnte ich sie nicht im Centrum und in der 
 proximalen Partie erkennen. 
 
 111. Glaskorper. 
 
 Zu der genauen Beschreibung, welche Grenacher vom Glaskorper 
 ( Gl Fig. 10 u. 11) giebt, kann ich nichts Neues hinzuftlgen. Er ist ein 
 durchsichtiger Theil der Hypodermis (Hy ) , in welche er kontinuirlich 
 tibergeht. Die kleinen Kerne liegen am proximalen Ende ihrer Zellen. 
 Nur an der Grenze, wo der Ubergang in die Hypodermis stattfindet, 
 verschiebt sich ihre Lage nach der Mitte und dem distalen Ende ( Pg 2 ). 
 Hier sind auch die Zellen pigmentirt und ihre Kerne erscheinen lang- 
 gestreckt. Dieser Ring von Pigmentzellen ( Pg 2 ) : der direkt unter der 
 oben erwahnten Pigmentzone (Pg 1 ) der Cuticula liegt , bildet mit der 
 letzteren zusammen eine Pigmentwand, welche verhindert, dass die 
 Lichtstrahlen auf anderem Wege als durch die Linse in das Auge ein- 
 dringen. 
 
 Der Glaskorper umschlieBt die proximale Flache der Linse kapsel- 
 artig. Am starksten entwickelt ist er bei Platybunus, wo seine Dicke 
 mehr als 2 /s derjenigen der Linse betragt. Bei Phalangium ist er ein 
 halbes Mai so dick, bei den tibrigen Genera dagegen betrachtlich 
 diinner, etwa Vs so dick wie die Linse. 
 
 IV. Retina. 
 
 Die Retina (Rt Fig. 10 u. 11) liegt direkt unter dem Glaskorper, 
 indem sie von ihm nur durch die praretinale Membran ( prae.m ) getrennt 
 ist. Ihre distale Flache ist also konkav und mit der inneren Wolbung 
 der Linse koncentrisch. 
 
 Wie schon Grenacher (6) beschreibt, setzt sich die Retina aus einer 
 einzigen Lage sehr langer Nervenendzellen (Fig. 23) zusammen, von 
 denen eine jede an ihrem distalen Ende (bei x) einen stark licht- 
 brechenden Korper, das Rhabdomer, abscheidet, an ihrem proximalen 
 Ende dagegen, in dessen Nahe der Kern (k) liegt, in eine Nervenfaser 
 (nf) tibergeht. Wahrend der distale den Sehstab tragende Theil gerade 
 und mehr oder weniger senkrecht zur proximalen Flache des Glas- 
 korpers steht, biegt der proximale Abschnitt der meisten Zellen all- 
 mahlich nach der Stelle um, wo der zunachst befindliche Sehnerv (vgl. 
 Fig. 11) durch die Retinakapsel dringt. Da diese Stellen nicht in der 
 Sehachse liegen, sondern ventral von ihr, so erscheint die Retina in den 
 meisten Fallen bilateral symmetrisch gebaut (vgl. Fig. 10 u. 11), was 
 
Uber den Bau der Phalangidenaugen. 
 
 9 
 
 ferner auch durch den Umstand bewirkt wird, dass der Breitendurch- 
 messer zwischen der hinteren und vorderen Flache (Fig. 1 0) groBer ist 
 als zwischen der medialen (dorsalen) und lateralen Flache (Fig. 11). 
 Ein zur Sehachse vertikal gefilhrter Schnitt durch die Retina zeigt sie 
 desshalb von ovalem Umriss. Es dtirfte nicht tiberfliissig sein zu be- 
 merken, dass die bilaterale Symmetric auf die Retina beschrankt ist 
 und nicht ftir die Linse und den Glaskorper gilt, da diese letzteren und 
 also auch die distale Flache der Retina selbst vollig kreisformig sind. 
 
 Leiobunum rotundum unterscheidet sich von den anderen Arten 
 dadurch, dass bei der groBten Zahl der Retinazellen ihr proximaler 
 Theil nicht nur ventralwarts, sondern auch nach vorn gekrttmmt ist, 
 w r odurch die Retina asymmetrisch und nicht bilateral-symmetrisch ge- 
 baut erscheint. 
 
 Als eine der wichtigsten und bem erkens werthesten 
 E igenscha ften der Retina ist die konstante Anordnung 
 ihrer Elemente zu Gruppen (Retinulae) von je vier Zellen 
 hervorzuheben, sowie di e Ye rei n igung der Sehstabe die- 
 ser vier Zellen zu einem einzigen Stilck, dem Rhabdom, 
 welches mithin aus vier Theilen oder Rhabdomeren ge- 
 bildet ist. 
 
 Die ganze Retina setzt sich aus diesen Retinulae zusammen, welche 
 in direkter Bertihrung mit einander stehen, da keine Pigment- oder 
 andere Zellen zwischen ihnen vorkommen. 
 
 1. Die Form der Retinulazellen. 
 
 Die Zellen einer Retinula (Fig. 1) sind immer so zu einander an- 
 geordnet, dass eine in der Achse liegt und von den anderen drei um- 
 geben wird. Ich bezeichne die erstere als die centrale (c), und die 
 letzteren als die peripheren ( p ) Zellen. Diese beiden Zellarten unter- 
 scheiden sich sowohl durch ihre Gestalt als auch durch den Bau ihrer 
 Rhabdomere betrachtlich von einander. 
 
 Die peripheren Zellen 1 (p 1 , p 2 , p 3 Fig. 1 — 8) sind die groBten 
 und erscheinen in alien Regionen, das distale Ende ausgenommen, fast 
 gleichmaBig dick; nur am distalen Ende warden sie bei den meisten 
 Arten etwas dtinner oder (bei der Acantholophus-Gruppe) sogar zuge- 
 spitzt. Sie sind prismatisch, fiinf- bis sechsseitig oder etwas abgerundet 
 
 1 In den Fig. \ — 8 sind die peripheren Zellen dunkler schattirt als die cen- 
 trale, um dem Leser die Unterscheidung beider Zellarten leichter zu machen. Es 
 
 muss indessen bemerkt werden, dass diese Verschiedenheit nur bisweilen, nicht 
 
 aber fur gewobnlich in den Praparaten zu erkennen ist. 
 
10 Fred. Purcell, 
 
 und sind auch an ihrem proximalen Ende nicht durch den Kern 
 (k Fig. I B) aufgetrieben. 
 
 Die centrale Zelle (c, c l , c 2 , c 3 Fig. 1 — 8) dagegen variirt in den 
 verschiedenen Abschnitten betrachtlich in ihrer Form (c Fig. I). Da sie 
 stets ihre Lage zwischen den drei peripheren Zellen beibehalt, so nimint 
 sie im groBten Theil ihrer Lange auBer an den Enden eine dreiseitige 
 Gestalt an. Der groBte Theil des nicht stabchentragenden Abschnittes 
 der Zelle ist (auBer bei Opilio) schlank, dreiseitig und nur halb so dick 
 im Durchmesser wie die peripheren Zellen oder noch dtinner (Fig. \ C 
 und 1 D). Am proximalen Ende aber ist sie durch den hier liegenden 
 Kern (k Fig. 1 A) stark aufgetrieben und fttnf- bis sechsseitig. Auch 
 gerade proximal vom Rhabdom verdickt sich die centrale Zelle be- 
 trachtlich (Fig. I E und 6 A) und wird ungefahr eben so dick wie die 
 peripheren, behalt aber ihre dreiseitige Form bei. In der Rhabdom- 
 region werden die drei flachen Seiten konkav und bilden dann drei 
 Langsrinnen, welche um so tiefer werden, je mehr man sich dem dista- 
 len Ende nahert. In jeder Rinne liegt eine periphere Zelle. Bisweilen 
 erstrecken sich diese Rinnen auch weiter proximalwarts vom Rhab- 
 dom Fig. 1 E). 
 
 Durch gelungene Maceration in HALLER J scher Fliissigkeit kann man 
 eine Anzahl von isolirten Retinulae erhalten , deren vier Zellen noch 
 durch ihre Rhabdomere zusammenhangen. Ein solches PrSparat, wel- 
 ches Fig. 22 wiedergiebt, zeigt die charakteristische centrale Zelle (c) 
 sehr gut. In Fig. 23 ist eine vollig isolirte periphere Zelle dargestellt, 
 von welcher der Sehstab bei x abgetrennt ist. 
 
 Bei Opilio parietinus weicht die Gestalt der centralen Zelle von 
 der typischen nur in so weit ab, als der Abschnitt zwischen Rhabdom 
 und Kern nur ein wenig dtinner ist als die peripheren Zellen und oft 
 die Langsrinnen sich auf der ganzen Lange linden. Ein Querschnitt 
 (Fig. 5 A u. 51?) durch diese Zelle bietet ein sehr charakteristisches Bild 
 wegen der scharfen Kanten der Zelle, welche das Aussehen hervorrufen, 
 als ob die letztere in den Raum zwischen die peripheren Zellen wie 
 hineingegossen ware. Die Querschnitte durch die letzteren erscheinen 
 dagegen mehr abgerundet, da die Kanten dieser Zellen stumpfere sind. 
 
 2. Der sehstabtragende Theil der Zellen und seine Beziehung zum 
 
 Rhabdom. 
 
 Die centrale Zelle erzeugt ein axiales Rhabdomer (hellblau in 
 Fig. I F bis I M ) , welches die seitliche Grenzflache der Zelle an drei 
 Stellen erreicht, namlich am Boden jeder der Langsrinnen, um hier 
 mit den drei peripheren Rhabdomeren zu verschmelzen. Die Rhabdo- 
 
Uber den Bau der Phalangidenaugen. 
 
 11 
 
 mere (dunkelblau in Fig. 1 F bis I M) der peripheren Zellen dagegen 
 liegen excentrisch, der centralen Zelle genahert, und erreichen die 
 Zellenoberflache nur an einer Stelle, namlich da, wo sie mit dem Rhab- 
 domer der centralen Zelle in Beriihrung kommen. 
 
 Durch diese Art von Rhabdombildung wird das Protoplasma der 
 centralen Zelle in der Rhabdomregion in drei Plasmastrange (c 1 , c 2 , c 3 
 Fig. I G bis \ M und 6 B bis 6 G) getheilt, welche nur proximal vom 
 Rhabdom zusammenhangen. Ein Querschnitt muss demnach genau ein 
 Bild geben, als ob das Rhabdom von sechs Zellen umgeben ware, wie 
 es Patten (20) wirklich angenommen hat. In der That, wenn man 
 solche Schnitte wie die von Opilio (Fig. 5 C bis 5 F) oder die von 
 Acantholophus (Fig. 6 D bis 6 F ) betrachtet, so ist es beim ersten An- 
 blick wirklich schwer zu glauben, dass nur vier und nicht sechs Zellen 
 vorhanden sind. 
 
 Durch zahlreiche Schnitte, welche ich durch das proximale Ende 
 des Rhabdoms bei jeder der acht Arten ftihrte, habe ich mich vollig 
 (iberzeugt, dass es wirklich so ist, wie ich oben angegeben habe, dass 
 namlich drei von den scheinbaren Zellen (c 1 , c 2 , c 3 ) nur die distalen 
 Theile der centralen Zelle der Retinula sind. Besonders klar ist dies 
 auf solchen Praparaten zu'erkennen, auf welchen die Zellgrenzen sehr 
 scharf hervortreten, wie es in dieser Region gewohnlich der Fall ist 
 (vgl. Fig. 1 E mit 1 F und 5 A mit 5 B). 
 
 Die drei Plasmastrange der centralen Zelle spielen jeden- 
 falls eine nicht unwichtige Rolle in der Physiologie des Auges, da ihnen, 
 wie weiter unten (p. 28) beschrieben wird, bei der Acantholophus- 
 gruppe hauptsachlich die Aufgabe zufallt, den Zugang des Lichtes zu 
 den Rhabdomen zu reguliren. Ich werde sie desshalb ausfiihrlich be- 
 schreiben. 
 
 Da jeder der Plasmastrange, wie ein Blick auf Fig. 5 D zeigt, zwi- 
 schen dem Rhabdom, zwei peripheren Zellen und einer angrenzenden 
 Retinula liegt, so konnen wir vier korrespondirende BegrenzungsflSchen 
 unterscheiden, von denen drei stets vorhanden sind, die vierte da- 
 gegen mitunter fehlen kann. 
 
 Diese letztere ist diejenige, welche an das Rhabdom grenzt und 
 deren Breite, wie spater gezeigt wird, mit der Breite des centralen 
 Rhabdomers an derselben Stelle tibereinstimmt (vgl. Fig. I F bis I M). 
 Folglich ist bei solchen Arten, welche ein gut entwickeltes centrales 
 Rhabdomer besitzen(Leiobunum, Fig. \ G bis 1 il/und Phalangium, Fig. 8), 
 diese Grenzflache immer zu sehen,’ die Strange sind mindestens vier- 
 seitig (niemals dreiseitig) und verhindern vollkommen, dass die drei 
 peripheren Zellen einander bertthren. 
 
12 
 
 Fred. Purcell, 
 
 Bei der Acantholophusgruppe finden wir andere Verhaltnisse. 
 Hier ist das centrale Rhabdomer in der Mittelregion des Rhabdoms sehr 
 reducirt und kann stellenweise sogar ganz fehlen (z. B. Acantholophus, 
 Fig. 6 D und 6 E). Ebenfalls verliert auf dieser gleichen Region jene 
 Grenzflache der Plasmastrange an Breite und verschmalert sich bis zu 
 auBerster Feinheit (Opilio parietinus, Fig. 5 C) oder sie verschwindet 
 ganz (Acantholophus, Fig. 6 D). Im letzteren Falle stehen die Plasma- 
 strange der centralen Zelle sogar nicht mehr in Bertthrung mit dem 
 Rhabdom an der betrefFenden Stelle, werden sehr fein und sind daher 
 manchmal schwer zu erkennen (Fig. 6 D). In beiden Fallen nehmen 
 die Strange an der betrefFenden Stelle eine charakteristische oft drei- 
 seitige Form an, indem sie mit scharfen Kanten versehen sind. Da- 
 durch erinnern sie in der Gestalt sehr stark an den nicht rhabdom- 
 tragenden Abschnitt der centralen Zelle selbst (vgl. Fig. 5 A mit 5 C). 
 
 Es verdient bemerkt zu werden, dass die Strange der centralen 
 Zelle, wie diinn sie auch sein mogen, in der Rhabdomregion bei alien 
 Arten ohne Ausnahme an der Bildung der seitlichen Grenzflache des 
 ganzen rhabdomtragenden Abschnittes der Retinula Antheil nehmen. 
 Dieses Verhalten ist besonders gut in Fig. 6 D und 6 2? zu erkennen. 
 
 Auf Querschnitten durch den proximalen Abschnitt 1 des Rhabdoms 
 (Fig. \ F bis \ /; 5 C bis 5.Z); 6 B bis 6 E) konnen die Strange der 
 Centralzelle leicht daran erkannt werden, dass sie konstant in den 
 Winkeln liegen, welche von den Strahlen des Rhabdoms gebildet wer- 
 den, und hier viel dttnner sind als eine der peripheren Zellen, Bei 
 Platybunus ist dieses Letztere auch im distalen Abschnitt des Rhab- 
 doms der Fall, indem die Plasmastrange bei dieser Gattung tiberall fast 
 dieselbe geringe Dicke behalten. Bei alien anderen Formen indessen 
 erscheint der distale Abschnitt der Strange, welcher ungefshr ein 
 Drittel des ganzen Stranges ausmacht, keulenformig verdickt. Dadurch 
 ist eine entsprechende GroBenabnahme der distalen Enden der peri- 
 pheren Zellen bedingt. Dieses Verhalten ist weniger ausgepragt bei 
 Leiobunum (Fig. 1 M) und Phalangium (Fig. 8) bei denen alle sechs 
 Enden der Zellen fast gleich groB sind. Bei der Acantholophusgruppe 
 ist dieser Process so ins Extreme gesteigert, dass die GroBenverhalt- 
 nisse zwischen den beiden Zellarten genau entgegengesetzt sind denen, 
 welche man in dem proximalen Abschnitt des Rhabdoms findet. Man 
 kann am besten eine Vorstellung von diesen GroBenverhaltnissen ge- 
 winnen, wenn man die Schnittserie in Fig. 6 B bis 6 G, oder Fig. 5 C 
 bis 5 //, Retinula 4, vergleicht. Das Anwachsen der Strange der Cen- 
 
 1 Wie vveiter unten (p. 14) erklSrt wird ; zerfallt das Rhabdom in einen proxi- 
 malen ( prox , Fig. 1, 5 u. 6) und einen distalen (dist, Fig. 1, 5 u. 6) Abschnitt. 
 
Uber den Bail der Phalangidenaugen. 
 
 13 
 
 tralzelle an ihrem distalen Ende kann man leicht an einem und dem- 
 selben Schnitt durch verschiedene Einstellung beobachten; Fig. 6 F' 
 und 6 F" stellen z. B. die proximale und die distale Flache desselben 
 Schnittes dar. 
 
 Da die distale Region aller Zellen intensiv pigmentirt ist, so folgt 
 daraus, dass das auBere Ende des Rhabdoms, welches dem Lichte aus- 
 gesetzt ist, in einen Pigmentmantel eingehtillt ist. Derselbe gehdrt bei 
 Platybunus hauptsachlich den peripheren Zellen an, bei Leiobunum 
 und Phalangium findet er sich zur Halfte in der centralen Zelle, zur 
 Halfte in den peripheren Zellen, wahrend er in der Acantholophus- 
 gruppe fast vollig der centralen Zelle zukommt. 
 
 3. Bau des Rhahdoms. 
 
 Wie oben schon erwiihnt wurde, tragen alle Zellen einer Retinula 
 zur Bildung des Rhabdoms bei, indem eine jede ein Rhabdomer ab- 
 scheidet; das der centralen Zelle aber unterscheidet sich betrachtlich 
 im Bau und auch in der Beschaffenheit von denen der peripheren Zellen. 
 Das centrale Rhabdomer farbt sich stets hellblau mit Hamatoxylin, die 
 peripheren dagegen entweder ganz, oder doch in ihrem groBten Theil, 
 dunkelblau. Diese Unterschiede in der Farbung habe ich in alien kolo- 
 rirten Figuren so getreu als moglich wiederzugeben versucht. 
 
 Frisch im Blute des Thieres untersucht erscheinen die Rhabdome 
 farblos. Ihre Lange variirt sehr in derselben Retina. Die randstandigen 
 sind gewohnlich ktirzer, oft nur halb so lang als die mehr central 
 liegenden. Dieser Unterschied stimmt im Allgemeinen mit der Lange 
 der Zellen tiberein und ist auf Grenacher’s Fig. 15 (6) sehr gut zu er- 
 kennen L Die Rhabdome konnen cylindrisch, abgeplattet oder mit drei 
 mehr oder weniger hohen Lbngsleisten versehen sein. Das letztere 
 Verhalten ist das haufigste und in diesem Falle erscheint der Quer- 
 schnitt dreistrahlig. 
 
 In den vollkommeneren Augen (Platybunus, Phalangium), in denen 
 der Glaskorper am starksten entwickelt und sehr dick ist (vgl. Gre- 
 nacher’s Fig. 15), sind die Rhabdome klein, kompakter und sehr zahl- 
 reich, wahrend sie bei den Arten mit weniger entwickeltem, relativ 
 dtinnem Glaskorper (Leiobunum, Acantholophus Fig. 10 u. 11, Opilio, 
 Oligolophus) sowohl relativ wie absolut groBer und weit weniger zahlreich 
 
 1 Auf meinen Fig. 10 u. 4 1, die genau nach den Praparaten gezeichnet sind, 
 kommt dieser Unterschied zufalligerweise nicht zum Ausdruck, da ich bei ihrer 
 Herstellung vor alien Dingen darauf ausging, die besten Praparate zu verwenden 
 und desshalb auf die betreffende Thatsache nicht besonders achten konnte. Der 
 erwahnte Unterschied ist nicht in alien Fallen vorhanden. 
 
14 
 
 Fred. Purcell, 
 
 sind und vielmehr die Tendenz zeigen mittels Leisten sich seitlich 
 auszubreiten. Diese Verhaltnisse sind in Fig. 25 veranschaulicht, indem 
 die mit stark entwickeltera Glaskorper versehenen Arten unten, die 
 mit schwack entwickeltem Glaskorper oben angebracht sind. 
 
 Bei alien Arten konnen wir einen distalen (dist, Fig. 1, 5, 6) von 
 einem proximalen Abschnitt (proa?, Fig. 1, 5, 6) unterscheiden. 
 
 Der distale Abschnitt differirt in seiner Gestalt und in seinem 
 inneren Bau sehr auffallig von dem proximalen , weit groBeren Ab- 
 schnitt. Er betragt gewohnlich ein Drittel, bei Platybunus sogar nur 
 ein Sechstel des ganzen Rhabdoms. Besonders charakteristisch ist ftir 
 ihn die sehr unregelmaBige und wechselnde Gestalt und Struktur, 
 welche er nicht nur bei den verschiedenen Species zeigt, sondern sogar 
 bei der Retina eines und desselben Thieres erkennen lasst. Er ist in 
 der That der am meisten der Variation unterworfene Theil des ganzen 
 Rhabdoms. 
 
 Wie schon oben angeftthrt, lassen sich die von mir untersuchten 
 Arten nach der Struktur des Rhabdoms in zwei nattirliche Gruppen 
 eintheilen, deren Angehorige ich jetzt zunachst charakterisiren und 
 beschreiben werde. 
 
 A. Die Leiobunumgruppe. 
 
 Bei den Arten dieser Gruppe ist der proximale Abschnitt des 
 Rhabdoms cylindrisch oder mehr oder weniger dreistraklig. In der- 
 selben Retina zeigt er sich stets von sehr gleichmaBiger Gestalt. Da die 
 peripheren Rhabdomere sich iiberall in der ganzen Lange mit Hama- 
 toxylin dunkelblau fiirben, so konnen sie leicht selbst im distalen 
 Abschnitt des Rhabdoms erkannt werden. Die Rhabdome bleiben 
 normalerweise getrennt und treten nicht mit den benachbarten in Ver- 
 bindung; nur hier und dort findet man bei alien Arten ein oder zwei 
 Paare mit einander vereinigt. 
 
 Leiobunum rotundum Latr. 
 
 Diese Species ist zum Studium bei Weitem am besten geeignet, 
 weil die beiden Arten von Rhabdomeren in besonders deutlicher Aus- 
 bildung vorhanden sind und sich sehr leicht in alien Regionen unter- 
 scheiden lassen. Es ist mir desshalb besonders bei dieser Art moglich 
 gewesen, einen tieferen Einblick in den Bau zu gewinnen und damit 
 einen Schltissel zum Verstandnis^'der weniger giinstigen Formen zu 
 erhalten. Die Schnittserie Fig. 1 F bis 1 M stellt ein typisches Rhab- 
 dom dar. 
 
 Der p roxim ale Abschnitt des Rhabdoms (Fig. 1 F bis 1 7) ist 
 / 
 
Uber den Bail der Phalangidenaugen. 
 
 15 
 
 mit drei hohen Langsleisten versehen, so dass er im Querschnitt deut- 
 lich dreistrahlig erscheint. Die Strahlen (oder Leisten) sind am hoch- 
 sten nahe dem proximalen Ende (Fig. \ G) und werden nach dem 
 distalen Abschnitt zu niedriger (Fig. \ I). Sie sind hauptsSchlich von 
 den peripheren Rhabdomeren gebildet, und nur ein kleiner Theil von 
 jedem kann auch vom centralen Rhabdomer herriihren (Fig. 1 G). 
 
 Das centrale Rhabdomer ist durch die ganze Lange des Rhabdoms gut 
 entwickelt. Nahe an seinem proximalen Ende ist es ungefahr einem 
 der peripheren Rhabdomere an GrcjBe gleich (Fig. 1 G), wird in der Mitte 
 am dtinnsten (Fig. 4) und nimmt dann gegen das distale Ende (Fig. 1 K 
 bis 1 M) wieder an Umfang zu. Hierdurch unterscheidet es sich von 
 den peripheren Rhabdomeren, welche proximal am dicksten, distal 
 dagegen am dilnnsten sind (Fig. 1). 
 
 Der di stale Abschni tt (Fig. 1 A^bis 1 M), welcher gewohnlich 
 seitlich komprimirt ist, verdankt seinen Unterschied im Bau hauptsach- 
 lich dem zunehmenden Umfang des centralen Rhabdomers unter ent- 
 sprechender Abnahme der drei peripheren Rhabdomere. Dabei ver- 
 schwinden einige der Leisten, welche von den letzteren gebildet werden. 
 Diese Veranderung beginnt an der Grenze zwischen dem distalen und 
 dem proximalen Abschnitt. 
 
 Ein sehr haufiger Fall ist in den Schnitten Fig. 1 1 bis 1 M darge- 
 stellt. Hier drangen sich zwei von den peripheren Rhabdomeren 
 ( p 2 und p 3 ) in die Substanz des centralen Rhabdomers ein, bis sie sich 
 treffen und vereinigen (Fig. \ I). Zu gleicher Zeit nehmen die zwei 
 Strahlen, welche sie bildeten, an Hohe ab und verschwinden schlieB- 
 lich (Fig. 1 M). Jener Theil des centralen Rhabdomers, welcher zwischen 
 den zwei peripheren Rhabdomeren (p 2 , p 3 ) lag, wachst in den angren- 
 zenden Plasmastrang (c 1 Fig. \ K ) der Centralzelle vor und bildet einen 
 neuen Strahl, der nattirlich auf demselben Schnitt nicht langer in Be- 
 rUhrung mit dem anderen Theil des centralen Rhabdomers sein kann. 
 Hierdurch wird das centrale Rhabdomer an seiriem distalen Ende ge- 
 spalten, und der Spalt wird durch zwei periphere Rhabdomere aus- 
 geftlllt. Dadurch erhalt der Querschnitt (Fig. 1 M) des Rhabdoms ein 
 gebandertes Aussehen, indem ein dunkelblauer mit einem hellblauen 
 Theil alternirt. 
 
 Wie schon bemerkt, ist der distale Abschnitt sehr der Variation 
 unterworfen. So begegnet man Modifikationen des oben beschriebenen 
 Baues immer bei einer kleineren odbr groBeren Anzahl von Rhabdomen. 
 Dieselben konnen auf eine der folg'enden Weisen zu Stande kommen: 
 
 Erstens, das centrale Rhabdomer kann in einen oder auch in die 
 beiden anderen Plasmastrange (c 2 und c 3 ) der centralen Zelle eindringen 
 
16 
 
 Fred. Purcell. 
 
 und so neue Strahlen bilden, wodurch auf dem Querschnitt (Fig. 2) die 
 Gestalt kreuzformig und nicht nur seitlich komprimirt erscheint. Wttr- 
 den diese neuen Strahlen [c 11 und c 111 ) des centralen Rhabdomers auf- 
 treten, bevor diejenigen der peripheren ( p 11 und p 111 ) verschwunden 
 sind, wie es oft in dem Ubergangstheil des Rhabdoms vorkommt, so 
 wtirde der Querschnitt durch diese Region vier-, fiinf- oder sechs- 
 strahlig erscheinen (Fig. 3). 
 
 Zweitens, das dritte periphere Rhabdomer (p^ann genau in der- 
 selben Weise, wie die beiden anderen (p 2 undp 3 ), in das centrale 
 Rhabdomer eindringen, so weit, dass alle drei sich in der Achse des 
 Rhabdoms treffen und vereinigen (Fig. 4). Das centrale Rhabdomer er- 
 scheint alsdann an seinem distalen Ende in drei Stiicke gespalten, und 
 da es aus seiner axialen Lage verdrangt wird, wachst es oft in die drei 
 Plasmastrange der centralen Zelle hinein. Auf dem Querschnitt kann 
 ein so gebautes Rhabdom in diesem Ende genau die dreistrahlige Ge- 
 stalt (Fig. 4) zeigen, wie im proximalen Abschnitt ; aber die Rhabdomere 
 der centralen und peripheren Zellen haben gewissermaBen ihre Rolle 
 vertauscht, indem die Strahlen am distalen Ende vom centralen Rhab- 
 domer gebildet sind und mit jenen des proximalen Abschnittes alter- 
 niren, wie ein Yergleich von Fig. 4 und Fig. \ G deutlich zeigt. 
 
 Diese drei Formen konnen dazu dienen, den Weg zu zeigen, wie 
 das distale Ende des Rhabdoms modificirt ist. Auch andere Unregel- 
 maBigkeiten, denen man noch begegnet, lassen sich leicht als kleine 
 Abweichungen oder Kombinationen von einer der schon beschriebenen 
 Formen auffassen. 
 
 Obgleich die Rhabdome normal ganz getrennt sind, so kommt es 
 doch hin und wieder vor, dass zwei benachbarte durch eine Brvicke 
 von Rhabdomsubstanz verbunden sind. Diese Yerbindung ist fast stets 
 auf den distalen Abschnitt beschrankt und entsteht durch das Zu- 
 sammentreffen und Yerschmelzen zweier Strahlen (Leisten), die nattir- 
 lich dem centralen Bhabdomer angehoren, da ja die der peripheren 
 Rhabdomere in diesem Abschnitt sehr an GroBe reducirt sind. Ich 
 habe in derselben Retina niemals mehr als ftinf Paare von Rhabdomen 
 gefunden, welche in dieser Weise vereinigt waren. Im proximalen 
 Abschnitt sind solche Verbindungen viel seltener, und nur zweimal 
 konnte ich einen derartigen Fall beobachten. In dem einen Fall wurde 
 die Verbindung durch centrale, im anderen durch periphere Rhabdo- 
 mere hergestellt. 
 
 Zuweilen findet man Retinulae mit einer ilberzahligen peri- 
 pheren Zelle, so dass sie aus einer centralen und vier peripheren 
 
Uber den Bau der Phalangidenaugen. 
 
 17 
 
 Zellen bestehen. Dieses Vorkommnis lieB sich in ein und derselben 
 Retina selten an meht* als zwei oder drei Retinulae nachweisen. 
 
 Ich habe schon betont, wie leicht sich das centrale und die peri- 
 pheren Rhabdomere bei dieser Species von einander unterscheiden 
 lassen. Die Grenzlinie zwischen einer hell und dunkel gefarbten Partie 
 des Rhabdoms ist scharf und deutlich, und oft, besonders iin proximalen 
 Abschnitt, durch eine noch dunklere Farbung als sie die peripheren 
 Rhabdomere annehmen, ausgezeichnet. Sie stimmt in Bezug auf ihre 
 Lage stets genau tlberein mit einer der Grenzen zwischen der centra- 
 len und den peripheren Zellen und erscheint als direkte Fortsetzung 
 dieser Zellgrenze in das Rhabdom hinein. Dass dieses wirklich der 
 Fall ist, lehrt ein Yergleich von zwei auf einander folgenden Schnitten 
 (wie Fig. 1 ^und 1 F), von denen der eine kein Rhabdom enthalt, 
 wahrend der andere durch sein auBerstes proximales Ende geht. Hier 
 ist es vollig klar, dass die Grenzen zwischen den hell und dunkel ge- 
 farbten Theilen des Rhabdoms zusammenfallen mit denen zwischen 
 der centralen Zelle und den peripheren Zellen. Und wenn man weiter 
 bedenkt, dass jene Flache der Plasmastrange der Centralzelle, die dem 
 Rhabdom anliegt, stets und in alien Regionen genau einer freien seit- 
 lichen Grenzflache des hell gefarbten Theiles des letzteren entspricht, 
 so kann kein Zw T eifel darilber obwalten, dass die hell und dunkel ge- 
 farbten Theile des Rhabdoms von Leiobunum identisch sind mit den 
 Rhabdomeren der centralen, bezw. peripheren Zellen. 
 
 Ich habe noch eines anderen Unterschiedes zwischen den Rhabdo- 
 meren zu gedenken, welcher sich in solcheti Praparaten findet, die bei 
 45° G. in einer gesattigten alkoholischen Pikrinsaurelosung gehartet 
 sind (siehe p. 4). Ein Theil des Augenpigmentes wird durch die warme 
 Fliissigkeit gelost und farbt das ganze centrale Rhabdomer tief braun 
 oder fast schwarz, wahrend die peripheren Rhabdomere fast farblos 
 bleiben oder doch nur hell braun werden. 
 
 Phalangium opilio L. und P. brevicorne C. K. 
 schlieBen sich in Betreff des Baues ihrer Rhabdome ganz an Leio- 
 bunum an. Die Rhabdome sind aber nicht mit so hohen Leisten ver- 
 sehen wie bei letzterer Art. Sie sind mehr dreiseitig und kompakter, 
 ein Umstand, der Hand in Hand geht mit ihrer geringeren GroBe und 
 groBeren Zahl. Yon den beiden in Fig. 25 ( Phal.op . und Phal.brev.) 
 abgebildeten Formen ist die dreiseitige die gewohnlichste, beide 
 Formen finden sich aber nicht zu gleicher Zeit in derselben Retina. 
 Der distale Abschnitt ist ganz ahnlich gestaltet wie bei Leiobunum und 
 zeigt dasselbe Anwachsen des centralen Rhabdomers und entsprechend 
 
 Zeitschrift f. wissensch. Zoologie. LVIII. Bd. 9 , 
 
18 
 
 Fred. Purcell, 
 
 dieselbe Abnahme der peripheren Rhabdomere, indem die letzteren 
 tief in die Substanz des centralen Rhabdomers eindringen. Die Modi- 
 fikation, welche Fig. 8 zeigt, ist die hSufigste und iihnelt ganz der 
 dritten, welche ich fiir Leiobunum beschrieben habe (Fig. 4). 
 
 Platybunus triangularis Herbst. 
 
 Hier sind die Rhabdome manchmal etwas dreiseitig, gewohnlich 
 indessen vollig cylindrisch (Fig. 25, Plat, trian.), und der distale Ab- 
 schnitt ist sehr kurz, indem er weniger als den sechsten Theil des 
 ganzen Rhabdoms ausmacht. Das centrale Rhabdomer erscheint fast 
 ganz unterdriickt und ist gewohnlich nur am distalen Ende sicht- 
 bar, wo es in die Plasmastrange der centralen Zelle auswachst, wo- 
 durch dieses Ende etwas dicker erscheint und sich von dem tibrigen 
 Theil des cylindrischen Rhabdoms unterscheidet. Der Bau dieses dista- 
 len Abschnittes lasst sich ableiten von dem Bau, welchen Fig. 4 von 
 Leiobunum zeigt, wenn wir annehmen, dass hier das centrale Rhabdo- 
 mer und die centrale Zelle sehr klein geworden sind, ohne aber ganz 
 zu verschwinden. Die Plasmastrange der centralen Zelle sind sehr fein 
 und werden an ihrem auBeren Ende nicht verdickt. Das das Rhabdom 
 umhtlllende Protoplasma gehort somit auch im distalen Abschnitt fast 
 ausschlieBlich den peripheren Zellen an , wodurch sich diese Species 
 von alien tibrigen unterscheidet. 
 
 B. Die Acantkolopliusgruppe. 
 
 Das charakteristischste Merkmal dieser Gruppe besteht darin, dass 
 der distale Abschnitt des Rhabdoms mit alien oder den meisten der 
 benachbarten Rhabdome durch Briicken von Rhabdomsubstanz ver- 
 bunden ist, so dass ein unregelmaBiges Netzwerk auf der ganzen dista- 
 len Flaehe der Retina entsteht. Der distale Abschnitt setzt sich aus 
 dem centralen und den peripheren Rhabdomeren zusammen, doch 
 farben sich die letzteren in diesem Abschnitt hellblau wie das erstere, 
 wesshalb eine scharfe Unterscheidung der beiden Arten von einander 
 nicht moglich ist. Das Protoplasma sammt Pigment, welches dieses 
 Ende umhtlllt, gehort fast ganz der centralen Zelle an. 
 
 Den proximalen Abschnitt ( prox Fig. 5 u. 6) des Rhabdoms bilden 
 fast ausschlieBlich die sich dunkelblau farbenden peripheren Rhabdo- 
 mere, da das centrale Rhabdomer hier nur sehr klein ist. Die Vielge- 
 staltigkeit dieses Abschnittes ist charakteristisch (Fig. 25), indem man 
 in derselben Retina sowohl die dreistrahlige, als auch die platte Form 
 stets finden kann, wenn auch das Verhaltnis, in welchem beide Formen 
 vorkommen, bei verschiedenen Individuen derselben Species in weiten 
 
Uber den Bau der Phalangidenaugen. 
 
 19 
 
 Grenzen variirt. Wenn auch beide Formen in jedem Theil der Retina 
 zu treffen sind, so findet man doch die abgeplatteten zum grftBten Theil 
 in der vorderen lateralen Region des Auges, die dreistrahligen dagegen 
 am haufigsten in der gegentlberliegenden medialen hinteren Region. 
 Ferner sind die meisten der abgeplatteten Rhabdome so gestellt, dass 
 ihre breiten Flachen parallel mit der Transversalebene des Khrpers 
 liegen (Fig. 12). Daher zeigen Schnitte, die rechtwinkelig zu dieser 
 Ebene geftlhrt sind, von den Rhabdomen in der vorderen Region nur 
 die schmalen Kanten derselben (Fig. 10). 
 
 Opilio parietinus de Geer 
 
 besitzt groBere Rhabdome als irgend eine von den anderen von mir 
 untersuchten Arten (Fig. 25, Op. par.). Die Fig. 5 A bis 5 H stellen 
 Querschnitte durch verschiedene Regionen derselben vier Retinulae 
 dar und konnen zur Erlauterung der charakteristischsten Eigenschaften 
 dienen. 
 
 Der pro xi male Abschnitt (Fig. 5 C bis 5 E) ist dimorph, bezw. 
 trimorph. Er erscheint abgeplattet, von oblongem Querschnitt [ret. / in 
 Fig. 5 C), oder er zeigt drei Langsleisten und erscheint dann im Quer- 
 schnitt dreistrahlig (ret. 2 ) ; endlich konnen auch nur zwei Leisten vor- 
 handen sein, so dass dann der Querschnitt zweistrahlig erscheint (ret. 3 
 in Fig. 5 E). Die letztere Form leitet sich von der dreistrahligen durch 
 Reduktion eines Strahles ab, kommt aber im Ganzen seltener vor. 
 SchlieBlich kann dasselbe Rhabdom in verschiedenen Regionen des 
 proximalen Abschnittes verschieden geformte Querschnitte zeigen; so 
 z. B. ist das der Retinula 1 proximalwarts abgeplattet (Fig. 5 C), in der 
 Mitte dreistrahlig (Fig. 5 D) und distalwarts wieder abgeplattet (Fig. 52?). 
 
 Diese verschiedenen Formen sind in der Retina nicht in gleicher 
 Zahl vorhanden, vielmehr schwankt dieselbe sehr. Bei einigen Augen 
 sind bei Weitem die meisten Rhabdome abgeplattet und nur wenige 
 dreistrahlige werden im hinteren medialen Theil der Retina gefunden. 
 Bei anderen Augen wieder ist das Gegentheil c|er Fall, indem die drei- 
 strahlige Form stark iiberwiegt und die abgeplatteten nur in kleiner 
 Anzahl im vorderen lateralen Theil zu finden sind. Ferner khnnen 
 beide Formen in ungefahr gleicher Zahl vorhanden sein und mehr oder 
 weniger durch einander gemischt liegen oder das Vorkommen der ab- 
 geplatteten Formen beschrankt sich auf den vorderen lateralen Theil, 
 und das der dreistrahligen auf die hintere mediane Region der Retina 
 (Fig. 42). 
 
 Die Dicke der Rhabdome im proximalen Abschnitt ist tlberall fast 
 
 2 * 
 
20 
 
 Fred. Purcell, 
 
 dieselbe, und niemals sind sie in der Mitte , wie bei den anderen 
 Species dieser Gruppe, eingeschniirt. 
 
 Sehr selten irifft man in diesem Abschnitt eine Yerbindung zwi- 
 schen benachbarten Rhabdomen. In einem ganz besonderen Aus- 
 nahmefall allerdings war ungefiihr die Halfte aller Rhabdome in der 
 Retina zu Gruppen vereinigt; dieses ist indessen ganz abnormal. In alien 
 Fallen wurde die Verbindung durch die peripheren, niemals durch das 
 centrale Rhabdomer hergestellt. 
 
 Der distale Abschnitt des Rhabdoms farbt sich ganz hellblau, 
 ist vollig unregelmaBig gestaltet und steht mit den anliegenden Rhab- 
 domen mittels Brticken von Rhabdomsubstanz, die sowohl von der 
 centralen als auch von den peripheren Zellen erzeugt wird, in Yer- 
 bindung. Um einige Beispiele anzuftihren, so kann die Bildung der 
 Brtlcke durch zwei centrale Zellen veranlasst worden sein, und dann 
 sind die beiden Rhabdome durch ihre centralen Rhabdomere verbun- 
 den. Im anderen Falle wird die Brticke durch zwei periphere Zellen 
 erzeugt, und dann erfolgt die Yerbindung durch periphere Rhabdomere 
 (z. B. zwischen ret. 2 und 3 , vgl. Fig. 5 E und 5 F). Endlich kann das 
 centrale Rhabdomer eines Rhabdoms vereinigt sein mit einem peri- 
 pheren eines anderen , wie es am haufigsten zu finden ist (z. B. zwi- 
 schen ret. / und 2 sowie 2 und 4, vgl. Fig. 5 E und 5 F). Auf diese Weise 
 entsteht ein unregelmaBiges Netzwerk von Rhabdomsubstanz, an deren 
 Bildung alle Rhabdome in der Retina Antheil nehmen. 
 
 Die Zusammensetzung des Rhabdoms aus centralem und peripheren 
 Rhabdomeren und die Lagenbeziehung derselben zu einander sind 
 keineswegs so klar wie bei Leiobunum. Der proximate Abschnitt farbt 
 sich zum groBten Theil dunkelblau und scheint fast ganz aus peripheren 
 Rhabdomeren zu bestehen. Indessen bemerkt man gewohnlich bei 
 sorgfaltiger Priifung auf Querschnitten (Fig. 5 C bis 5 E) einige wenige 
 schmale hellblaue Querstreifen, welche von der anliegenden Flache 
 eines Plasmastranges der Centralzelle ausgehen und stets genau mit 
 derselben in der Breite tibereinstimmen. Yon diesen hellblauen Strei- 
 fen sind niemals mehr als drei vorhanden. Sie konnen sich quer 
 durch das Rhabdom erstrecken und mit zwei gegentiberliegenden 
 Striingen der centralen Zelle in Verbindung treten, oder sie reichen 
 nur halb so weit, nur bis zur Mittellinie (ml Fig. 5 C). Schnitte durch 
 das auBerste proximale Ende (Fig. 5 B) zeigen, dass das centrale Rhab- 
 domer wirklich vorhanden ist und sich hellblau farbt. 
 
 Erinnern wir uns nun an die Verhaltnisse bei Leiobunum, wie sie 
 besonders an solchen Schnitten (wie Fig. 1 M und Fig. 4) dargestellt 
 sind, wo das centrale Bhabdomer durch die sich eindrangenden peri- 
 
Uber den Bau der Phalangidenaugen. 
 
 21 
 
 pheren Rhabdomere in zwei oder drei getrennte Stticke gespalten ist, 
 wodurch die peripheren in direkte Bertihrung mit einander treten, und 
 denken wir ferner daran, dass die Breite des centralen Rhabdomers 
 bei Leiobunum stets genau mit der anliegenden Flache der Gentralzelle 
 tlbereinstimmt — so kann es keinem Zweifel unterliegen, dass die 
 schmalen Streifen von hellblauer Substanz bei Opilio nichts Anderes 
 vorstellen als den Rest des stark reducirten centralen Rhabdomers. 
 
 Dies wird ferner auch dadurch bewiesen, dass bei Praparaten, die 
 mit warmer gesattigter alkoholischer Pikrinsaurelosung behandelt wor- 
 den sind, die abgeplatteten Rhabdome im Querschnitt drei schmale 
 Streifen (cr Fig. 14 B) zeigen, welche mit jenen oben erwahnten hell- 
 blauen genau in der Lage iibereinstimmen und durch das geloste Pig- 
 ment dunkelbraun oder schwarz gefarbt sind. Letztere Eigenthtimlich- 
 keit erkannten wir schon als eines der Kennzeichen ftir das centrale 
 Rhabdomer bei Leiobunum. 
 
 Im distalen Abschnitt (bei Opilio) ist es ganz unmoglich, in Folge 
 der ganz gleichmaBigen Farbung, die relative GroBe der beiden Arten 
 von Rhabdomeren zu bestimmen. Dass aber beide sicher vorhanden 
 sind, lehrt die Bildung der Brtlcken, wie oben gezeigt wurde. Aller 
 Wahrscheinlichkeit nach wird der groBte Theil des distalen Abschnittes 
 vom centralen Rhabdomer gebildet, wie wir vielleicht aus der sehr 
 starken Entwicklung der Gentralzelle in diesem Abschnitt schlieBen 
 dtirfen (vgl. ret. 4 in Fig. 5 G und 5 H, wo zwei von den sehr reducirten 
 peripheren Zellen sogar nicht mehr in Bertihrung mit dem Rhabdom 
 stehen). 
 
 Acantholophus hispidus Herbst. 
 
 Bei dieser Art treffen wir wieder einen Bau, der dem von Opilio 
 beschriebenen sehr ahnlich ist und nur in wenigen Punkten, die hier 
 erwahnt zu warden verdienen, davon abweicht. 
 
 Eine neue Erscheinung bei dieser und den folgenden Arten ist die 
 kegelformige Gestalt des Rhabdoms, welche durch die allmahliche Ver- 
 jtingung des proximalen Abschnittes gegen seine distale Region zu ent- 
 steht (Fig. 11). Diese letztere Region (Fig. 6 E), d. h. also jene, welche 
 der Grenze zwischen dem proximalen und dem distalen Abschnitt 
 nahe liegt, ist der diinnste Theil des ganzen Rhabdoms. Allerdings 
 kommen betrachtliche Abweichungen vor, indem sie manchmal auBerst 
 dtinn (Fig. 7), manchmal (im Durchmesser) mehr als halb so groB als 
 das proximale Ende ist (Fig. 6 E). 
 
 Der proximale Abschnitt kann jede der oben ftir Opilio be- 
 schriebenen Gestalten besitzen. AuBer der dreistrahligen (Fig. 16) 
 
22 
 
 Fred. Purcell, 
 
 kommt durch Zurticktreten der Leisten eine sich oft findende drei- 
 seitige Form zu Stande (Fig. 6 C) (ygl. Fig. 25, Acan. hisp.). Wenn 
 beide in demselben Auge zusammen vorkommen, so ist die letztere 
 Form immer im hinteren Theil der Retina zu finden, wahrend die drei- 
 strahlige sowie die abgeplattete Form vorzugsweise dem vorderen 
 Theil zukommen. Das centrale Rhabdomer ist ziemlich gut entwickelt 
 am auBersten proximalen Ende, wo es leicht auf Querschnitten (Fig. 6 B 
 und 6 C; cr Fig. 16, 17 und 18) erkannt werden kann. Nach der Mitte 
 zu nimmt es an GroBe sehr rasch ab, bis es zuletzt ganz verschwindet. 
 In derselben Region kommen die auBerst feinen Plasmastrange der 
 centralen Zelle in einiger Entfernung vom Rhabdom zu liegen (Fig. 6 D 
 und 6Z?), das hier nattlrlich ganz aus den peripheren Rhabdomeren be- 
 steht. Die peripheren Rhabdomere farben sich nicht iiberall dunkel- 
 blau in ihrem proximalen Abschnitt, sondern ein Theil (g 1 , q 1 , q 3 Fig. 6 B) 
 im Centrum eines jeden Strahles erscheint gewohnlich hellblau, fast wie 
 das centrale Rhabdomer, ein Umstand, welcher zuerst bei der genauen 
 Bestimmung der Grenzen des letzteren etwas Schwierigkeit bereitet. 
 Indessen eine Vergleichung solcher Praparate (Fig. 6 B) mit anderen, 
 welche mit gesattigter warmer alkoholischer Pikrinsaurelosung (Fig. 1 6) 
 behandelt sind, lasst leicht erkennen, dass nur jene hellblauen Streifen 
 zum centralen Rhabdomer gehoren, die den Strangen der centralen 
 Zelle anliegen, wahrend die anderen (g 1 , q 2 , g 3 Fig. 6.6) mit der feinen 
 Mittellinie den peripheren Rhabdomeren zuzurechnen sind. Diese 
 Yerhaltnisse werden spater in dem Kapitel tiber die feinere Histologie 
 des Rhabdoms genauer erortert werden. 
 
 Oligolophus palpinalis Herbst 
 
 schlieBt sich hinsichtlich des Baues seiner Rhabdome an Acantholo- 
 phus eng an (ygl. Fig. 25, Olig. palp .) . Das centrale Rhabdomer erscheint 
 sogar noch mehr durch die peripheren im proximalen Abschnitt ver- 
 drangt, wo kaum eine Spur von ihm zu entdecken ist. In der hin- 
 teren Region der Retina sind die Rhabdome dreiseitig oder mehr oder 
 weniger abgerundet, so dass der proximale Abschnitt hier eine 
 konische Gestalt erhalten kann. 
 
 Oligolophus tridensC.K. 
 
 Bei dieser Art treffen wir wieder ganz dieselben Verhaltnisse wie 
 bei Opilio parietinus, nur mit dem Unterschiede, dass die Rhabdome 
 kegelformig sind (vgl. Fig. 25, Olig. trid.). Bei einigen Individuen ist das 
 Netzwerk von Rhabdomsubstanz nicht so vollkommen ausgebildet, wie 
 
Uber den Ban der Phalangidenaugen. 
 
 23 
 
 bei den drei vorigen Arten, indem eine kleine Anzahl von Rhabdomen 
 getrennt bleiben kann. 
 
 Aus den vorhergehenden Beschreibungen lasst sich unter Anderen 
 folgender Schluss ziehen : Das Rhabdom bestehtbei alien Spe- 
 cies aus zwei chemisch verschieden beschaffenen Theilen. 
 Zu dem einen gehort das ganze centrale Rhabdomer und, 
 bei derAcantholophusgruppe, auch der distale Abschnitt 
 der p eripheren Rhabdomere, wahrend der andere Theil 
 bei der Leiobunumgruppe von den ganze n peripheren 
 Rhabdomer en, bei derAcantholophusgruppe aber nur von 
 ihrem proximalen Abschnitt gebildet wird. 
 
 Ferner ist der sich dunkelblau farbende Theil stets stark ent- 
 wickelt und bildet den bei Weitem groBeren Theil des proximalen 
 Abschnittes des Rhabdoms. Dagegen variirt der sich hellblau farbende 
 Theil des Rhabdoms sowohl in der Form wie in der Masse sehr, und er 
 kann sogar stark reducirt sein (Platybunus). Er ist stets am starksten 
 entwickelt in dem distalen Abschnitt des Rhabdoms. 
 
 Bei dieser Gelegenheit mochte ich noch auf eine bemerkenswerthe 
 Analogie der Phalangidenaugen mit den Augen der Cephalopoden hin- 
 weisen, welche die auffallende Yerbindung der Rhabdome zu einem 
 unregelmaBigen Netzwerk betrifft, wie sie in der Acantholophusgruppe 
 vorhanden ist. Diese letztere Erscheinung steht im Bereiche der 
 Arthropoden, so weit bekannt, ganz vereinzelt da. Dagegen findet sich 
 merkwiirdigerweise bei den Cephalopoden eine ahnliche Erscheinung, 
 wie von Grenacher (8) nachgewiesen wurde. Bei Octopus, Eledone 
 und Sepia verschmelzen namlich die gewohnlich aus vier Rhabdomeren 
 gebildeten Rhabdome in der Weise mit einander, dass in verschiedenen 
 Regionen der Retina ein unregelmaBiges Netzwerk von Rhabdomsub- 
 stanz zu Stande kommt, in dessen Maschen noch Protoplasma der zu- 
 gehOrigen Zellen liegt. 
 
 Yon einer anderen merkwiirdigen Analogie dieser Cephalopoden- 
 augen mit den Spinnenaugen wird weiter unten noch die Rede sein. 
 
 4. Feinere Struktur des Rhabdoms. 
 
 Wenn wir einen Querschnitt durch einen der abgeplatteten Rhab- 
 dome (z. B. Fig. 14 A) von Opilio parietinus genauer prttfen und ihn mit 
 einem ahnlichen Rhabdom, das man von der schmalen Seite sieht (Fig. 1 5), 
 vergleichen , so sehen wir, dass es aus parallelen Langsschichten, in 
 diesem Falle aus sechs, aufgebaut ist. Diese Schichten sind ihrerseits 
 
24 
 
 Fred. Purcell, 
 
 wieder dadurch, dass sie von feinen Linien oder Lamellen (ql) quer 
 durchzogen werden, in Maschen oder Kastchen getheilt. Die auBersten 
 Schichten stehen an dem Seitenrande des Rhabdoms in kontinuirlicher 
 Yerbindung mit einander und konnen desshalb auch als eine einzige 
 Schicht, welche die inneren Lagen umgiebt, angesehen werden. Die 
 vier inneren Schichten verhalten sich in ganz gleicher Weise, d. h. sie 
 gehen ebenfalls an den Seitenrandern in einander iiber. Ein so ge- 
 bautes Rhabdom (Fig. 1 4 A) kann als eine Rohre mit einer dreischichtigen 
 Wandung betrachtet werden, die seitlich so stark zusammengedrtickt 
 ist, dass ihr Lumen verschwindet 1 . Dieser Vergleich passt dadurch 
 noch besser, dass oft in Folge des Einwirkens der KonservirungsflUssig- 
 keiten ein ktinstlicher Spalt (sp Fig. HA) gebildet wird, welcher die 
 Wande der zusammengedrtickten Rohre trennt. Die einzige Zwischen- 
 lamelle, in der dieser Spalt auftritt, und der das verschwundene Lumen 
 der Rohre reprdsentirt, werde ich die Mittellamelle oderMittel- 
 linie (ml) nennen, zum Unterschiede yon den Seitenlamellen oder 
 Sei ten linien ( 5 /) zwischen den drei Schichten, welche die Wandung 
 der vermeintlichen Rohre bilden 2 . 
 
 Wenn wir uns vorstellen, dass eine der abgeplatteten Wande nach 
 auBen hin eine Langsfaltung erfuhre (etwa bei x Fig. 1 4 A ), so wiirden 
 wir eine dreistrahlige Figur erhalten, welche genau in ihrem Baue mit 
 einem dreistrahligen Rhabdom tlbereinstimmt. Man kann desshalb das 
 letztere betrachten als eine Rohre, welche eine dreischichtige Wand 
 hat und welche seitlich yon drei verschiedenen Stellen her so weit 
 zusammengedriickt wurde bis ihr Lumen yerschwand (vgl. Fig. 1 9, Leio- 
 bunum). Den Ubergang der einen Form in die andere kann man leicht 
 bei einem und demselben Rhabdom von Opilio parietinus verfolgen 
 (z. B. bei solchen wie das von ret. 1 in Fig. 5 C und 5 D). 
 
 Einen sehr ahnlichen Bau findet man bei alien anderen Species. 
 Die wichtigsten Abweichungen betreffen die Zahi der die Wandung der 
 » Rohre « bildenden Schichten. Gewohnlich sind es drei wie bei Opilio. 
 Bei Leiobunum konnte ich hingegen nur zwei linden, die aber sehr klar 
 und deutlich waren (Fig. 19); nur zuweilen war hier und dort eine 
 dritte sichtbar. Auch bei den Formen, wo man fiir gewohnlich drei 
 
 1 Dieser Vergleich dient lediglich zur Erleichterung der Beschreibung. Es soli 
 keineswegs damit gemeinl sein, dass das Rhabdom etwa einmal in seiner Entwick- 
 lung eine wirkliche Rohre darstellte. 
 
 2 Es diirfte nicht uberfliissig sein zu erwahnen , dass in alien kolorirten 
 Figuren nur die Mittellinie eingezeichnet ist. Im distalen Abschnitt bei der Acantho- 
 lophusgruppe tritt sie weit weniger hervor als im proximalen Abschnitt des Rhab- 
 doms (Fig. 5 F bis 5 H). 
 
Uber den Bau der Phalangidenaugen. 
 
 25 
 
 Schichlen trifft, scheinen stellenweise nur zwei vorhanden zu sein (z. B. 
 bei Acantholophus). In alien Fallen, wo neue Leisten (Strahlen) in einer 
 Region des Rhabdoms gebildet werden, z. B. im distalen Abschnitt bei 
 Leiobunum, heben sie sich durch eine einfache Faltung der zwei- 
 resp. dreischichtigen Wandung der vermeintlichen Rohre, gerade so 
 wie bei Opilio parietinus, ab. 
 
 Abgesehen von der Bildung von vier bis sechs Langsschichten, 
 sind die kleinen Kastchen auch noch in Querschichten angeordnet, die 
 durch Zwischenlamellen von einander getrennt sind. Diese Lamellen, 
 die ich als Que rlamellen oder Querlinien (ql Fig. 15) bezeichne, 
 sind viel zahlreicher aber weniger deutlich als die zwischen den Langs- 
 schichten liegenden Mittel- und Seitenlamellen (m/, s/ Fig. 15). Yon 
 der Seite gesehen scheint sonach das Rhabdom aus einer Anzahl von 
 liber einander gelegenen Platten von der Dicke eines Kastchens auf- 
 gebaut zu sein (Fig. 1 5). Die Querstreifung des Rhabdoms, die schon 
 Patten gesehen hat, beruht auf den zwischen den Platten befindlichen 
 Querlamellen (ql Fig. 15). 
 
 Es muss besonders betont werden, dass, wenigstens im proximalen 
 Abschnitt des Rhabdoms, die Grenzlinien zwischen den kleinen Kast- 
 chen durch die zwei- resp. dreischichtige Wand der » Rohre « in ganz 
 geraden Linien von der auBeren Oberflache bis zur Mittellamelle (ml) 
 fortlaufend zu verfolgen sind (Fig. 15, 16, 17 u. 19). Weiter scheinen 
 sie kontinuirlich in die Grenzlinien der gegentiberliegenden Wandung 
 zu verlaufen (Fig. 15, 16, 17 u. 19), doch findet man zuweilen auch, 
 dass sie mit jenen alterniren. 
 
 Nach dem Mitgetheilten kann, meiner Ansicht nach, kein Zweifel 
 sein, dass die kleinen Maschen in Wirklichkeit sechsseitige Kastchen 
 sind, und dass das Rhabdom mithin eine wabenartige Struktur be- 
 sitzt. Ob diese »Waben«, wie mir nicht unwahrscheinlich ist, eine 
 Fltissigkeit enthalten, und somit vacuolenartig sind, vermag ich nicht 
 mit Sicherheit zu sagen. Die Wande der Waben stellen wirkliche 
 Lamellen dar, die sich tief farben und oft von betrachtlicher Starke 
 sind, was besonders bei der Mittellamelle der Fall ist. Die Waben oder 
 Kastchen treten in alien Theilen des Rhabdoms hervor, sowohl in dem 
 centralen wie in den peripheren Rhabdomeren, wie es die instruktive 
 Fig. 1 9 von Leiobunum zeigt. Bei dem verschiedenartigen chemischen 
 Verhalten der Rhabdomere ist dies jedenfalls bemerkenswerth. 
 
 Die Anordnung der Waben im distalen Abschnitt des Rhabdoms 
 bei der Acantholophusgruppe ist sehr schwer zu erkennen und scheint 
 weniger regelmSBig zu sein. Die Zahl der Langsschichten kann an sehr 
 dicken Stellen jene im proximalen Abschnitt tlbertreffen. 
 
26 
 
 Fred. Purcell, 
 
 Ich habe schon gelegentlich erwahnt (p. 221), dass bei verschiedenen 
 Arten (z. B. Phalangium, Acantholophus) das Verstandnis des Baues 
 durch die Art und Weise, wie sich die peripheren Rhabdomere farben, 
 erschwert wird. Auf diesen Punkt muss ich hier zurtickkommen, denn 
 die Wabenstruktur des Rhabdoms wird auch durch dieselbe Farbungs- 
 weise theilweise unkenntlich gemacht 1 . Wenn man einen solchen 
 Schnitt von Acantholophus (Fig. 18) mit einem anderen vergleicht, in 
 dem nur das centrale Rhabdomer durch gelostes Pigment gefarbt ist 
 (Fig. 1 6), so sieht man, dass bei dem ersteren die mittlere von den drei 
 Schichten, welche die Wand der Rohre bilden, in den peripheren 
 Rhabdomeren so dunkel gefarbt ist, dass ihre Wabenstruktur verwischt 
 ist. Die auBeren und inneren Schichten dagegen sind fast ungefarbt 
 geblieben, wie das centrale Rhabdomer. In anderen Rhabdomen der- 
 selben Retina kann die auBere Schicht ebenfalls tief gefarbt sein, und 
 nur die innere Schicht kann hell bleiben (q* } q 2 , g 3 Fig. 6 B u. 6 C). 
 
 5. Struktur des Protoplasmas in den Retinazellen. 
 
 Der grbBte Theil der Zelle ist mit Pigmentkornerri so dicht gefiillt, 
 dass es durchaus nothwendig ist. dieselben zu entfernen, um eine Vor- 
 stellung von der feineren Struktur in der pigmentirten Region zu er- 
 halten. 
 
 Das Protoplasma zeigt im optischen Schnitte einen ausgepragten 
 wabigen Bau von hell sich farbender Substanz, den ich in den Fig. 1 — 8 
 und 13 wiederzugeben versucht habe. Die Maschen des Wabenwerkes 
 sind, besonders in der distalen, am dichtesten pigmentirten Region der 
 Zelle, ziemlich, wenn nicht ganz gleichmaBig groB und gewohnlich ftinf- 
 bis sechsseitig. In den Knotenpunkten des Netzwerkes, in welchen in 
 der Regel drei, seltener vier, Seiten zusammentreffen, sammeln sich sehr 
 haufig farbbare Substanzen an, und zwar oft in betrachtlicher Menge. 
 Die Knotenpunkte erscheinen dann viel dicker als die Seiten der 
 Maschen. Solche Knotenpunkte, welche man besonders im proximalen 
 Theil der Zelle (Fig. 1 C) trifft, geben dem Protoplasma bei minder ge- 
 nauer Betrachtung ein grobkorniges Aussehen. Bei eingehender Unter- 
 suchung bemerkt man jedoch, dass die Korner durch feine, sich leicht 
 fiirbende Linien zu einem scheinbaren Netzwerke verbunden sind. Die 
 Maschen oder Waben ordnen sich dort in einer Reihe an, wo sie mit 
 einer festeren Wandung, z. B. dem Rhabdom oder der Zellwand, in Be- 
 rilhrung kommen (Butschli’s Alveolarschicht; z. B. Fig. IX, 1 M> 
 4, 13 etc.). 
 
 1 Diesem Pbelstand kann cladurch abgeholfen werden, dass man die Konser- 
 virungslliissigkeit warm anwendet. Vgl. oben p. 4. 
 
Uber den Bau der Phalangidenaugen. 
 
 27 
 
 Untersucht man die Retinazellen auf Langsschnitten, so zeigt sich 
 gewOhnlich derselbe Bau. Nicht selten freilich kommt es vor, dass die 
 Waben eine Tendenz zeigen, sich in Langsreihen anzuordnen. Einen 
 solchen besonders ausgezeichneten Fall stellt z. B. Fig. 13 dar, wo die 
 Waben mehr oder weniger langlich viefseitig sind, und die Zelle hier- 
 durch ein langsstreifiges Aussehen bekommt. 
 
 Eine kleinere oder groBere Anzahl Waben — je nach der betreffen- 
 den Region — enthalt je ein spharisches Pigmentkorn, das dieselbe 
 GroBe hat und sie anscheinend vollig ausfiillt. Eine solche Wabe kann 
 nattirlich, wenn man will, als ein ein Pigmentkorn enthaltender Knoten- 
 punkt betrachtet werden. 
 
 Bekanntlich ist es sehr schwierig tlber die wahre Natur eines so 
 gebauten Protoplasmas, wie ich es beschrieben habe, ins Klare zu 
 kommen, ob es ein komplicirtes Netzwerk von feinen Faden ist, oder 
 ob es nur aus einer Menge von Blaschen oder Yacuolen besteht, die im 
 optischen Schnitt nur das Vorhandensein eines Netzwerkes vortauschen. 
 Das letztere oder die Schaumstruktur, wie sie von Butschu (3) 
 beschrieben worden ist, scheint mir in unserem Falle die grhBte Wahr- 
 scheinlichkeit fiir sich zu haben. Sie erklart am besten die allgemeinen 
 Erscheinungen, welche ich oben beschrieben habe : die ftinf- bis sechs- 
 seitigen Maschen , die verdickten Knotenpunkte , die Anordnung in 
 Reihen Rings der festeren Wande, etc. Ftir mich ist folgende Beobachtung 
 maBgebend: wenn man eine der Maschen bei genauer Einstellung unter 
 dem Mikroskop betrachtet und dann den Tubus etwas hebt und senkt, 
 so treten an Stelle der einen beobachteten Masche sehr oft drei andere 
 Maschen, die genau tlber, bezw. unter ihr liegen, so dass der gemein- 
 same Knotenpunkt der drei Maschen mit dem Centrum der ersteren 
 zusammenfallt. Es lasst sich dieses am besten durch das Schema Fig. 20 
 klar machen, wo die in einer optischen Ebene liegenden Maschen durch 
 dunkle Linien, die unmittelbar darunter liegenden durch helle dargestellt 
 sind. Dieses Schema entspricht der Form einer Honigwabe, und zwar 
 der Gegend, an welcher zwei Lagen von'Zellen in Berilhrung stehen. 
 
 Zu Gunsten der Ansicht, dass das Protoplasma ein Netzwerk von 
 feinen Faden ist, konnte angeftihrt werden, dass alle oben erwahnten 
 Erscheinungen durch die Gegenwart einer groBen Anzahl von Pigment- 
 kcjrnern hervorgerufen werden konnte, welche eben so groB wie die 
 Hohlungen der Maschen sind. Indessen findet sich genau derselbe Bau 
 in jenen Regionen der Zellen, welche wenig oder kein Pigment enthal- 
 ten, wo von man sich leicht an Schnitten tlberzeugen kann, aus denen 
 das Pigment nicht entfernt worden ist. 
 
 Wenn wir nun die Wabenstruktur des Protoplasmas mit der des 
 
28 
 
 Fred. Purcell, 
 
 Rhabdoms in Yergleich ziehen, so finden wir eine auffallige Ahnlichkeit 
 zwischen beiden. Ja die dem Rhabdom anliegende Alveolarschicht ist 
 oft von der auBeren Kastchenschicht des Rhabdoms nur durch das 
 starke Lichtbrechungsvermcigen des letzteren zu unterscheiden. Es 
 macht ganz den Eindruck al^ wenn die Kastchen des Rhabdoms er- 
 starrte Protoplasmawaben waren. Erstere sind natiirlich, im Gegensatz 
 zu den Plasmawaben, nicht unter einander beweglich. 
 
 6. Pigment der Retinazellen. 
 
 Die distale Region der Zellen ist vollstSndig pigmentirt, die proxi- 
 male dagegen ist mehr oder weniger von Pigment frei. In der Rhab- 
 domregion ist dasselbe besonders stark vertreten, und, da es vor Allern 
 die Plasmastrange der centralen Zelle erfilllt, so kann man diese ge- 
 wohnlich auf Schnitten, aus denen das Pigment noch nicht entfernt ist, 
 als intensiv schwarze oder braune Flecken, die in den Rinnen des 
 Rhabdoms liegen, erkennen. 
 
 Es ist vcjllig sicher, dass echte Pigmentzellen, die kein Rhabdomer 
 tragen, ganz fehlen. 
 
 Wie Stefanowska (25) schon beobachtet hat, sind die Pigmentkorner 
 in der proximalen Region der Retina oft in Langsreihen angeordnet. 
 Jedenfalls hangt dieses mit einer reihenformigen Anordnung der Proto- 
 plasmawaben in dieser Region zusammen (Fig. 13). 
 
 Im frischen Zustand geprtlft erscheint das Pigment bei Phalangium 
 dunkelbraun mit einem Stich ins Rothliche, sowohl im durchscheinen- 
 den wie auch im reflektirten Licht. Bei alien anderen Gattungen ist es 
 schwarzbraun , fast schwarz im durchscheinenden, und dunkelbraun 
 mit einem Stich ins Rothliche im reflektirten Licht. 
 
 Einige Yersuche habe ich angestellt liber den Einfluss, welcher 
 von dem Licht, das in wechselnder Starke in das Auge eintritt, auf die 
 Pigment wande rung ausgetibt wird. P]s wurden Praparate mit ein- 
 ander verglichen, einmal von Individuen, die bei gewohnlichem Tages- 
 licht getodtet waren, und andererseits von solchen, die vor dem 
 Abtodten in vollkommener Dunkelheit sieben Stunden oder langer ge- 
 blieben waren. Die Fixirung wurde in beiden Fallen auf die gleiche 
 Weise vorgenommen (alkoh. Pikrinsaure). 
 
 Wiihrend bei gewohnlichem Tageslicht das Pigment bis zu den 
 iiuBersten distalen Enden der Zellen reicht, so hatte sich dasselbe nach 
 dem volligen Abschluss des Lichtes eine Strecke von dem distalen Ende 
 zurtickgezogen , und zwar in den peripheren Zellen weiter als in der 
 centralen. In den peripheren Zellen wurde auf diese Weise eine Strecke 
 von y 2 bis 2 / 3 der Lange des Rhabdoms vollig frei von Pigment. 
 
 
Uber den Bail der Phalangidenaugen. 
 
 29 
 
 Bezttglich der centralen Zelle war das Verhalten bei den beiden 
 Gruppen ein wenig abweichend : 
 
 Bei Opilio parietinus (als Vertreter der Acantholophusgruppe) zog 
 sich das Pigment vollig bis zu etwa y 3 der Lange des Bhabdoms zuriick, 
 d. h. bis etwa zu dem Schnitt SE in Fig. 5. Der ganze distale Abschnitt 
 des Rhabdoms [dist Fig. 5) hatte somit seinen Pigmentmantel verloren. 
 Da nun, wie ich oben bereits gezeigt habe (p. 4 2), der groBere Theil 
 des Pigments in diesem Mantel der centralen Zelle angehort (ygl. Fig. 5 G, 
 5 H u. 6 G), so folgt daraus, dass bei der Acantholophusgruppe 
 die centrale Zelle den wesentlichsten Einfluss auf die 
 Regulirung des Lichtes austlbt, das zu dem Rhabdom ge- 
 langen soil. Erst wenn das Pigment sich eine Strecke von dem 
 distalen Ende zurtickgezogen hat (bis etwa Fig. 5Fu. 6F), spielen auch 
 die peripheren Zellen eine wichtige Rolle, indem sich nun ihr Pigment 
 noch weiter zurtickzieht als in der centralen Zelle. 
 
 Auch bei Phalangium opilio (als Vertreter derLeiobunumgruppe) zieht 
 sich das Pigment der centralen Zelle in einigen Fallen auf 1 / 3 der Lange 
 des Rhabdoms zuriick, in zahlreichen anderen Fallen dagegen nur weit 
 weniger, ja oft gar nicht. Diese letztere Eigenthilmlichkeit hat mich zu 
 Zweifeln veranlasst, ob die centrale Zelle bei dieser Gruppe in alien 
 Fallen innervirt wird, eine Frage, auf welche spater zurtickgekommen 
 werden soli. Da an den distalen Enden die drei peripheren Zellen 
 zusammen wenigstens denselben Umfang haben, wie die centrale Zelle 
 (vgl. Fig. 4 M u. 8), und da ferner das Pigment bei den ersteren sich 
 auf 2 / 3 der Lange des Rhabdoms zurtickzieht, so geht hieraus hervor, 
 dass bei der Leiobunum gruppe die Hauptrolle hinsichtlich 
 der Regulirung desLichtes den pe ripheren Zellen zuge- 
 schrieben werden muss. 
 
 Bei beiden Gruppen wird das Zuriickziehen des Pigments auf eine 
 groBere Strecke in den peripheren, als in der centralen Zelle, jeden- 
 falls theilweise dadurch veranlasst, dass die letztere Zelle im Bereiche 
 der Mitte des Rhabdoms sich haufig sehr betrachtlich verdiinnt, wahrend 
 eine solche Verdiinnung bei den peripheren Zellen ilberhaupt kaum 
 eintritt (vgl. Fig. 6 C, 6 D, 5 C, 1 H ). 
 
 Stefanowska (25), welche die Pigmentwanderung in den Augen von 
 Phalangium opilio studirte, erwahnt nichts von den hier besprochenen 
 Vorgangen. 
 
 7. Die Kerne der Retinazellen. 
 
 Die Kerne linden sich sammtlich in der proximalen Halfte der 
 Retina (Fig. 4 0 u. 4 4). Sie enthalten ein oder zwei Nucleoli und sind 
 
30 
 
 Fred. Purcell, 
 
 je nach der Art sehr wechselnd in ihrer Gestalt, bald spharisch (Platy- 
 bunus), bald langlich-oval (Opilio), am haufigsten aber ist die Gestalt 
 eine ovale. Zum Unterschied von den Kernen der centralen Zellen ver- 
 ursachen die der peripheren keine Anschwellung der Zellen. Bei Leio- 
 bunum (Fig. 1) liegen die Kerne der ersteren mehr proximal als die 
 der peripheren Zellen derselben Retinula, und dieses scheint auch bei 
 den anderen Arten der Fall zu sein. AuBer den gewohnlichen Kernen 
 kann man in der am meisten proximal gelegenen Region der Retina 
 immer eine geringe Anzahl kleiner, mehr rundlicher Kerne [k 1 Fig. 11) 
 finden, die wahrscheinlich, so wenigstens in den meisten Fallen, cen- 
 tralen Zellen angehoren, welche eine ungewohnliche Lange erreicht 
 haben. Freilich ist es sehr schwer, dies mit volliger Sicherheit nachzu- 
 weisen. 
 
 8. Phaosphaeren von Acantholophus und von den Skorpionen. 
 
 In den Retinulazellen von Acantholophus fallen nicht selten gewisse 
 stark lichtbrechende Korper auf, die ohne Zweifel den ahnlichen Gebil- 
 den entsprechen, welche in den Augen der Skorpione vorkommen und 
 unter dem Namen Phaosphaeren bekannt sind. 
 
 Die Gestalt und GroBe dieser starker lichtbrechenden Korper 
 (Ph Fig. 13) bei Acantholophus ist sehr verschieden; bald sind sie rund, 
 bald langlich-oval, und haben bald die GrfiBe eines Nucleolus, bald die 
 eines Kernes. Sie liegen ganz nahe beim Kern und meist auf dessen 
 distaler Seite, doch findet man stets einige auch auf der proximalen 
 Seite. Wenn Phaosphaeren vorhanden sind, so findet sich gewohnlich 
 nur eine, zuweilen auch zwei in einer Zelle (Fig. 13), doch trifft man 
 stets eine Anzahl von Zellen, die keine einzige enthalten. Manchmal 
 sind sie in betrfichtlicher Zahl in der Retina vorhanden, das andere Mai 
 nur sehr sparlich und noch haufiger finden sie sich tlberhaupt nicht. 
 Unter 19 Individuen z. B. besaBen sie nur 8, und sie fehlten ganz bei 
 den ilbrigen 1 1 . 
 
 Jede Phaosphaere liegt anscheinend in einer Vacuole (vc Fig. 13), 
 welche nicht ganz von ihr erftlllt wird, und in welcher sie durch feine 
 radiare Fasern 1 gehalten zu werden scheint. Beim ersten Anblick er- 
 scheinen die Phaosphaeren zwar homogen, mittels starker VergroBe- 
 rung lasst sich aber ein deutliches Netzwerk erkennen. Durch die matt 
 graublaue Farbung mit Hamatoxylin unterscheiden sie sich sehr scharf 
 
 1 Diese Fasern stellen moglicherweise nur die Wande zwischen Waben dar. 
 Diese Waben wiirden aber ganz verschieden von den gewohnlichen Protoplasma- 
 waben sein. 
 
 
Uber den Bau der Phalangidenaugen. 
 
 31 
 
 von dem tiefen Blau der Kerne, mit welchen sie auch nicht die ent- 
 fernteste Ahnlichkeit haben, auBer zuweilen in der GroBe. 
 
 In der Ordnung der Skorpione wurden die fraglichen Korper zu- 
 erst von Grenacher (7) bei Buthus, Ischnurus und Lycas erkannt. Die, 
 welche bei Euscorpius carpathicus und Euscorpius italicus vorkommen, 
 wurden zuerst von Lankester und Bourne (12) gefunden und erhielten 
 von ihnen den Namen » Phaosphaeren «. 
 
 Ich selbst habe sie bei den beiden zuletzt genannten Arten unter- 
 sucht, und kann kaum einen Unterschied von denen von Acantholophus 
 linden, auBer darin, dass sie bei Euscorpius carpathicus (aber nicht bei 
 Euscorpius italicus) einen etwas hoheren Brechungsindex haben. Man 
 trifft sie meist in der Nahe des Nucleus, oft ihm mittels einer abge- 
 platteten Flache direkt anliegend, gerade so wie bei Acantholophus. 
 Wenn ich sie auch wie Lankester und Bourne zum groBten Theil proxi- 
 mal vom Kern liegend fand, so habe ich in einigen meiner Praparate 
 sie doch fast eben so haufig auf der distalen wie auf der proximalen 
 Seite der Kerne, oft je eine auf beiden Seiten in derselben Zelle, an- 
 getroffen. Sonst aber stimmen sie in jeder Hinsicht mit denen von 
 Acantholophus tiberein, indem sie in Bezug auf GroBe und Gestalt eben 
 so sehr einem Wechsel unterliegen, sich ebenfalls matt graublau farben, 
 eine netzartige Struktur erkennen lassen, und in einer Vacuole von 
 feinen Fasern suspendirt zu liegen scheinen. 
 
 Wie Lankester und Bourne (12) richtig bemerken, finden sie sich 
 auch in den Seitenaugen der Skorpione. 
 
 Das Vorkommen der Phaosphaeren ist aber nicht auf die Augen 
 beschrankt. Sowohl bei den Phalangiden als auch bei Euscorpius ent- 
 hielten die Leberzellen im Cephalothorax eine sehr groBe Zahl rund- 
 licher, stark lichtbrechender Korper, die den Phaosphaeren, welche 
 man in den Augen derselben Species findet, in jeder Beziehung 
 gleichen. Es scheint diese letztere Thatsache den friiheren Autoren 
 entgangen zu sein. Ferner habe ich sie auch in den Hypodermiszellen 
 gewisser Arten gefunden, z. B. bei Acantholophus [ph Fig. 11). 
 
 Hier wie in der Leber sind sie an GroBe sehr verschieden und oft, 
 entsprechend dem grcjBeren Durchmesser der Zellen, viel groBer als 
 die der Retina. Jene, die ich in den Leberzellen von Euscorpius car- 
 pathicus fand, haben einen hbheren Brechungsindex als bei anderen 
 Arten, ein Verhalten das ganz tibereinstimmt mit dem der Phaosphaeren 
 der Retina bei diesem Thier. 
 
 Uber die chemische Natur dieser Korper weiB ich nichts zu sagen. 
 Die Phaosphaeren in der Leber und im Auge sowohl von Euscorpius 
 carpathicus wie auch von Acantholophus hispidus werden allem Anschein 
 
32 
 
 Fred. Purcell, 
 
 nach durch maBig koncentrirte Sauren (Salzsaure, SalpetersSure, 
 Schwefelsaure, Essigsaure) und Alkalien (Kalilauge 35 %. Ammoniak) 
 nicht angegriffen. Bei Anwendung von Doppelfarbung (Eosin-Hama- 
 toxylin) farben sie sich roth, die Kerne dagegen blau. 
 
 Alle diese stark lichtbrechenden Korper der Leber, Retina und 
 Hypodermis sind jedenfalls gleichwerthig und wahrscheinlich auch in 
 ihrer Konstitution ganz oder fast identisch. 
 
 Es verdient erwahnt zu werden, dass Lankester und Bourne (12) 
 bei einigen Skorpionen (Androctonus) keine Phaosphaeren, ferner Gre- 
 nacher (7) bei Buthus eine »schwankende Anzahl von Vacuolen« und 
 G. H. Parker (1 8) bei Centrums eine kleine Zahl von stark lichtbrechen- 
 den Punkten in den Phaosphaeren fand, die niemals bei Euscorpius 
 und Acantholophus vorkommen. 
 
 Die Frage nach der Natur der Phaosphaeren in den Augen wird 
 von den verschiedenen Autoren sehr verschieden beantwortet. Lan- 
 kester und Bourne (12) glauben, dass sie aus derselben Substanz wie 
 das Rhabdom bestehen. Mark (17) vermuthet in ihnen rudimentare 
 Rhabdome. Patten (19) und Parker (18) sind geneigt sie als Gebilde 
 nuclearen Ursprungs zu betrachten. 
 
 Meiner Ansicht nach mtissen die Phaosphaeren als Stoffwechsel- 
 produkte, die in den verschiedensten Geweben auftreten konnen und 
 wahrscheinlich in einem Zusammenhang mit der Ernahrung stehen, 
 angesehen werden. Im Ganzen mochte ich beztiglich der Natur der 
 Phaosphaeren mit CarriEre (4) iibereinstimmen, welcher sagt: »Da so- 
 wohl die Stelle ihres Vorkommens bei einer Species als dieses selbst 
 bei verschiedenen Gattungen schwankt und unabhangig von ihnen 
 immer Rhabdome vorhanden sind, dilrfte diesen Gebilden tlberhaupt 
 kein besonderer Werth beizulegen sein.« 
 
 V. Nervenfaserschicht und Sehnerven. 
 
 Wenn auch, wie die Entwicklung lehrt, die Retina invers gebaut 
 ist, so verbinden sich doch die Nervenfasern mit dem proximalen Ende 
 der Zellen, welches dem urspriinglich (vor der Inversion) freien, auBe- 
 ren Ende entspricht. Die Zellen werden an diesem Ende hinter dem 
 Kern sehr rasch diinner und gehen in die Nervenfaser liber. Da ich 
 diesem wichtigen Punkte besondere Aufmerksamkeit gewidmet habe, 
 so kann ich diese Angabe mit absoluter Sicherheit machen. Am besten 
 studirt man die betreffenden Verhaltnisse auf Praparaten, welche in 
 warmen Fltissigkeiten konservirt oder in HALLER’scher Fltissigkeit 
 macerirt sind (Fig. 23 u. 24). Die Achsencylinder sind oft so gut kon- 
 servirt, dass die Querschnitte durch einen Jeden sofort erkannt werden 
 
Uber den Bail der Phalangidenaugen. 
 
 33 
 
 konnen, doch ist es mir niemals gelungen eine Faser zu entdecken, 
 welche zwischen den Zellen aufwarts zieht und sich irgendwo an der 
 distalen Seite vom Kern anheftet. 
 
 Die Nervenfasern bilden eine Schicht [Nv Fig. I I), welche der 
 proximalen Flache des Retinazellenlagers aufliegt, and welche nahe der 
 Mitte am dicksten ist, gegen die Peripherie zu aber auBerst diinn und 
 daher schwer zu erkennen ist. Alle Fasern nehmen ihren Weg zu dem 
 nachsten optischen Nerven und treten noch innerhalb der Retinalkapsel 
 zu kleinen Biindeln zusammen, von denen ein jedes von einer cellula- 
 ren Scheide eingeschlossen wird. Wahrend ihres centripetalen Yer- 
 laufes vereinigen sich kleinere Btindel zu groBeren und gehen zuletzt 
 durch die Locher, welche sich in der Retinalkapsel an der ventralen 
 Seite linden, in eben so viele Sehnerven Uber, wie Locher vorhanden 
 sind. Jeder Sehnerv theilt sich bei seinem Eintritt in das Auge in zwei 
 Hauptaste, die eine dorsale, bezw. laterale Richtung nehmen und nach 
 den anliegenden Retinazellen auf ihrem ganzen Wege Fasern abgeben 
 (Fig. 10 u. 11). 
 
 In der Regel sind acht Nerven ftir jedes Auge vorhanden, doch 
 kommen Abweichungen vor, und bei einigen Individuen kann sich ihre 
 Zahl bis auf fUnf vermindern (Fig. 10). Da sie sich auf ihrem Wege 
 zum Gehirn unter einander vereinigen, so wird ihre ursprUngliche Zahl 
 schlieBlich bis auf die Halfte oder auf noch weniger reducirt. Ihr wei- 
 terer Yerlauf im Gehirn ist von St. Remy (23) beschrieben worden. 
 
 Die Scheide der Sehnerven und diejenige der innerhalb der Reti- 
 nalkapsel befindlichen Btindel mit ihren Kernen bleiben stets auf die 
 Peripherie beschrankt. Somit fehlen jegliche innere Scheiden, welche 
 den Nerven oder das Btindel in sekundare Btindel theilen. Die kleinen 
 ovalen oder langlichen Kerne (ft 4 Fig. 11) veranlassen in der Wand der 
 Scheide eine Verdickung und sind leicht von den rundlichen viel 
 groBeren Kernen (ft) der Retinazellen zu unterscheiden. Zuweilen trifft 
 man auch ahnliche Kerne zwischen den Sehnervenfasern (ft 3 Fig. 1 0). 
 
 Obwohl der Nachweis, dass die peripheren Zellen je in eine Nerven- 
 faser tibergehen, nicht sehr schwer ist, so ist er doch nicht so einfach 
 ftir die centrale Zelle. Ich habe versucht mir dadurch ttber diesen 
 Punkt Gewissheit zu schaffen, dass ich die Zahl der Nervenfasern in 
 den optischen Nerven mit der Zahl der Rhabdome in demselben Auge 
 verglich. Wenn es auch mtihsam ist, so ist es doch durchaus moglich, 
 die genaue Zahl der Rhabdome zu ermitteln, indem man in der oben 
 p. 6) beschriebenen Weise Zeichnungen einer ltickenlosen Serie von 
 Schnitten, aus denen das Pigment nicht entfernt ist und welche die 
 Sehachse genau quer durch schnitten haben, mit einander vergleicht. 
 
 Zeitschrift f. wissensch. Zoologie. LVI1I. Bd. 3 
 
34 
 
 Fred. Purcell, 
 
 Die Nervenfasern lassen sich ebenfalls zahlen, wenn auch nicht mit 
 solcher Genauigkeit. Dies geschieht am besten auf Querschnitten von 
 Sehnerven, die in warmen Fliissigkeiten gehartet und mit Hamatoxylin 
 gefarbt sind. Auf ihnen erscheint jeder Achsencylinder wie ein heller, 
 farbloser Raum auf blauem Grunde. Die Yertheilung der Fasern eines 
 Auges auf acht Sehnerven, deren jeder 60 bis 400 Fasern enthalt, 
 vereinfacht die Zahlung bedeutend. 
 
 Das Resultat der Zahlung ergab ftlr ein linkes Auge von Acantho- 
 lophus genau 588 Rhabdome, wahrend die acht Sehnerven dessel- 
 ben Auges bei demselben Individuum annahernd 2449 Fasern enthiel- 
 ten. Wtlrden alle Zellen in einer Retinula innervirt, so wtirde die 
 Gesammtzahl von Fasern 4mal 588 = 2352 ergeben, welche Zahl sehr 
 gut mit der gefundenen (2449) tibereinstimmt. Die letztere Zahl tiber- 
 trifft zwar um 97 Fasern die wirkliche Zahl, d. h. es ist ein Irrthum 
 von nur 4,1 °/ 0 vorhanden, indessen kommt er nicht in Betracht, wenn 
 man bedenkt, dass, wenn nur die peripheren Zellen innervirt wtirden, 
 die Zahl der Fasern nur 3 X 588 = 1764 betragen mtisste, also um 
 685 hinter der gefundenen zurttckbleiben wilrde. 
 
 Hier mag einer etwas iiberraschenden Ubereinstimmung in der 
 Zahl der Rhabdome, w 7 elche ich bei verschiedenen Augen von Acantho- 
 loplius hispidus fand, gedacht werden. Das oben erwahnte linke Auge 
 besaB 588 Rhabdome, und in dem rechten Auge desselben Individuums 
 erhielt ich dieselbe Zahl 588. In dem Auge eines anderen Individuums 
 zahlte ich 587, also nur ein Rhabdom weniger. Diese Zahlen sind iiber- 
 dies sehr genau, nicht nur annahernd, bestimmt, so dass ein etwaiger 
 Irrthum hochstens ein Rhabdom mehr oder weniger betragen kann. 
 
 VI. Die Retinalkapsel und das zwischen ihr und den Schichten der Retina 
 und der Nervenfasern liegende Gewebe. 
 
 Die Retinalkapsel, auf welche schon zu verschiedenen Malen Bezug 
 genommen wurde, ist eine Membran von betrachtlicher Festigkeit, wie 
 man sich leicht auf Praparaten, die kurze Zeit in HALLER’scher Fliissig- 
 keit macerirt wurden, tiberzeugen kann. Sie schlieBt alles Gewebe 
 beider Augen ein, welches proximal vom Glaskorper liegt. 
 
 Der Name praretinale Membran ( prae.rn Fig. 10) ist von Gra- 
 ber (5) jenem Theil der Retinalkapsel gegeben worden, welcher zwi- 
 schen dem Glaskorper und der Retina liegt. Sie trennt diese letzteren, 
 doch halt sie dieselben gleichzeitig auch zusammen, weil die ZellwSnde 
 beider Schichten fest mit ihr verbunden sind. Diese Membran ist 
 jedenfalls eine Folge der Verw 7 achsung der die Zellen jederseits von ihr 
 begrcnzenden Zellwande. Niemals enthalt sie Kerne. 
 
Uber den Bail der Phalangidenaugen. 
 
 35 
 
 An ihrern Rande ist die praretinale Lamelle rechtwinkelig umge- 
 schlagen und geht in die periretinale Membran ( peri.m ) liber, 
 welche die periphere Flache und einen Theil der basalen Flache des 
 Augenbulbus bedeckt. Der inneren Seite dieser Membran liegt eine 
 besondere zellige Matrix (Mx Fig. 14) an. Da die Hypodermis (. Hy Fig. 1 0 
 u. 14) eine betrachtliche Strecke mit der peripheren Wand des Augen- 
 bulbus in Beriihrung steht, so verschmilzt die periretinale Membran in 
 dieser Region mit der Rasalmembran (bm) der Hypodermis. Diese bei- 
 den Membranen schlieBen gewohnlich eine Anzahl von Tracheen zwi- 
 schen sich ein (Tr Fig. 41) und trennen sich in der Nahe der basalen 
 Flache des Auges (bei x Fig. 10 u. 14). 
 
 Der Raum zwischen der periretinalen Membran und der Schicht 
 der Retinazellen sowie der Nervenfasern wird von der dUnnen zelligen 
 Matrix der Membran eingenommen. Der distale Theil der Matrix, 
 welcher zwischen dem doppelten Theil der Periretinalmembran und 
 den randstandigen Retinulae liegt, setzt sich aus zwei bis drei Schichten 
 von langen, etwas komprimirten, prismatischen Zellen ( Pg 3 ) zusammen, 
 welche mit den randstandigen Retinazellen parallel verlaufen, fast eben 
 so lang sind und langgestreckte abgeplattete Kerne besitzen. Diese 
 Zone ist dicker als der tlbrige Theil der Matrix und bildet, da sie ge- 
 wohnlich sehr intensiv pigmentirt ist, eine ringformige Pigmentzone 
 ( Pg 3 Fig. 10 u. 11), welche alle Retinulae wie ein Fassreifen umspannt. 
 Bei Platybunus sind die Zellen dieser Zone statt mit Pigment, mit kleinen 
 glanzenden Krystallen, und bei Phalangium brevicorne zum Theil mit 
 Pigment, zum Theil mit Krystallen erftillt. 
 
 Der Rest der Matrix (Mx Fig. 11), welcher zwischen der perireti- 
 nalen Membran und der Nervenfaserschicht liegt, ist auBerst dtinn mit 
 wenigen langgestreckten Kernen versehen. Er kann Pigment oder 
 Krystalle enthalten, aber auch stellenweise von beiden frei sein. 
 
 Schon oben habe ich erwahnt, dass die periretinale Membran des 
 einen Auges direkt in die des anderen iibergeht, ohne zwischen die- 
 selben einzudringen. Hierdurch ist in der ventralen Region zwischen 
 der Kapsel und den beiden Nervenfaserschichten ein betrSchtlicher 
 Raum freigelassen, der gewohnlich von dreieckiger Gestalt und mit 
 einem besonderen faserig-lockeren Gewebe erftillt ist (zw.g Fig. 14). 
 In ihm linden sich neben zahlreichen Kernen gewohnlich Glanzkrystalle 
 in groBer Menge, besonders in dem Theil, welcher dem vorderen 
 Korperende zunachst liegt. 
 
 In der Medianebene des Korpers scheinen die Nervenfaserschichten 
 beider Augen in direkter Beriihrung mit einander zu stehen; nur hier 
 und dort trifft man vereinzelte, sehr abgeplattete Kerne ( k 2 Fig. 1 1). 
 
 3 * 
 
36 
 
 Fred. Purcell, 
 
 Die oben erwahnten Glanzkrystalle, welche man in der Matrix 
 findet, sind sehr kleine, kurze, eckige Stabchen (Fig. 9) von gelblicher 
 Farbung in durchscheinendem Licht. In reflektirtem Licht schillern sie 
 prachtvoll golden, grtln und roth, wie jene des Tapetums bei den 
 Spinnen. AuBerdem trifft man sie in groBer Menge in den Hypodermis- 
 zellen (Hy Fig. 10 u. 11) und oft auch in anderen Geweben, wie von 
 Leydig (13, p. 384) des Genaueren beschrieben wurde. 
 
 VII. Litteratur. 
 
 Schon Tulk(1 843, 27) kannte die Thatsache, dass die beiden Augen 
 der Phalangiden mit einander verbunden sind. Ferner beschreibt er 
 und bildet (Taf. Y, Fig. 32) ein Paar Muskeln ab, welche jederseits unter 
 den Augen verlaufen und eine Verschiebung des Augeninhaltes ermcjg- 
 lichen sollen. 
 
 In der That fand ich verschiedene Langsmuskeln [Mk Fig. 11), 
 welche auf der lateralen Seite der Sehnerven dicht unterhalb der 
 Augen vorbeilaufen. Sie verbinden sich aber nicht mit den Augen, 
 sondern setzen sich einerseits an der Cuticula unmittelbar hinter dem 
 Augenhocker, und andererseits am oberen Rande der Basen der Cheli- 
 ceren fest. Ohne Zweifel sind dies, wie schon Grenacher erwahnt, die 
 Muskeln, welche Tulk gesehen hat. 
 
 Leydig (1855, 13) beschreibt bei Phalangium ein aus zerstreuten 
 Flitterchen gebildetes Tapetum, welches sich am Augengrund ausnimmt 
 «wie Sterne am dunkeln Firmament « (p. 439). In seiner Fig. 21 bildet 
 Leydig die Verhaltnisse ab. Das Tapetum besteht aus Ktigelchen, die 
 groBer sind als die Pigmentkorner. 
 
 Es ist mir niemals moglich gewesen, bei irgend einer Species etwas 
 einem solchen Tapetum Entsprechendes aufzufinden; auch Grenacher 
 erwahnt nichts davon. 
 
 Leydig giebt ferner eine schone Figur (15, Taf. VIII, Fig. 2), welche 
 die Beziehungen des Auges und Gehirns zu einander zeigt. 
 
 Die sehr genaue Beschreibung Ghenacher’s (1879, 6) von dem Ver- 
 halten der Linse und des Glaskorpers kann ich in alien Punkten besta- 
 tigen. Dagegen kann ich seiner Darstellung ttber den Bau der Retina 
 nicht ganz beistimmen. Er sagt: Die hinter dem Glaskorper gelegene 
 Retina besteht aus einer einzigen Zellenlage. Die Elemente derselben 
 sind stark verliingert, von vorn bis hinten mit Pigment erftillt, und im 
 frischen Zustand mehr cylindrisch als nach der Erhiirtung, wo sie 
 ziemlich spindelformig werden. An ihrem Yorderende ist ihnen je ein 
 Stabchen eingelagert, und an ihrem hinteren Ende treten sie mit dem 
 Opticus in Zusammenhang. Die Kerne liegen hinter der Mitte und 
 
Uber den Ban der Phalangidenaugen. 
 
 37 
 
 verursachen das spindelformige Aussehen der Zelle durch ihre Auf- 
 treibung. Die spindelformigen Stabchen lassen sich etwa mit einem 
 Getreidekorn vergleichen. Bei Langsansichten zeigen sie eine sehr feine 
 Langslinie , welcher oft leichte Einkerbungen an den abgerundeten 
 Enden entsprechen. Die Stabchen bestehen aus drei gleichgroBen, der 
 Lange nach an einander gekitteten Segmenten, so dass der Querschnitt 
 ein kleeblattahnliches Aussehen zeigt. 
 
 Aus dem Mitgetheilten kann man ersehen, dass meine Darstellung 
 von derjenigen Grenacher’s in einem sehr wesentlichen Punkte ab- 
 weicht, obwohl ich ganz besonders hervorheben mochte, dass die von 
 Grenacher gegebene Darstellung vom Bau des Auges im Ubrigen eine 
 sehr naturgetreue ist. Der erwahnte Differenzpunkt besteht darin, dass 
 nach Grenacher jedes dreitheilige Stabchen nur einer einzigen Zelle 
 zugehOrt, die Zellen der Retina gleichwerthig und nicht in Gru/pen 
 oder Retinulae angeordnet sind , dass dagegen nach meinen Beobach- 
 tungen jedes solche » Stabchen « in Wirklichkeit ein Rhabdom ist, d. h. 
 ein Produkt einer Gruppe von vier Zellen, welche zusammen eine 
 Retinula bilden. 
 
 Die von Grenacher untersuchte Art, Phalangium opilio, ist aller- 
 dings wegen der Kleinheit der betreffenden Elemente weit weniger 
 giinstig ftir die Untersuchung als Leiobunum und Opilio parietinus, an 
 welchen ich meine Beobachtungen zuerst gemacht habe. 
 
 Patten (1886, 19) fand gelegentlich seiner ausgedehnten Unter- 
 suchungen, dass die Retina der Mollusken und Arthropoden sowohl bei 
 den Stemmata wie bei den facettirten Augen sich aus kreisformig an- 
 geordneten Gruppen von pigmentirten Zellen zusammensetzt, die 
 central liegende Zellen umgeben, welche letztere durch konstante 
 Merkmale im Bau ausgezeichnet sind. Diese letzteren Zellen nennt er 
 »Retinophoren«. Ihrer sind stets zwei oder mehr vorhanden, die aller- 
 dings mit einander verschmelzen konnen, wobei der Kern der einen 
 degenerirt. Die Sehstabe werden von diesen Zellen erzeugt (und haben 
 spruit stets die Natur eines Rhabdoms). Sowohl die farblosen Zellen als 
 auch die Theile der Sehstabe schlieBen eine axial, ursprtinglich inter- 
 cellular gelegene Nervenfaser zwischen sich ein. Diese Gruppen von 
 pigmentirten und farblosen Zellen, welche Patten mit einem frtiher 
 etwas anders gebrauchten Namen als » Ommatidia« bezeichnet, sollen, 
 wie er glaubt, die Strukturelemente der meisten, wenn nicht aller 
 Augen sein. 
 
 Bei seinen Untersuchungen tiber das Auge von Phalangium (20) 
 findet er nun auch, dass die Retina dieser Form nach demselben Schema 
 gebaut ist. Sie soli aus wohl entwickelten »Ommatidia« bestehen, von 
 
38 
 
 Fred. Purcell, 
 
 denen jedes sich aus wenigstens neun Zellen zusammensetzt. Das 
 Centrum eines jeden » Ommatidiums cc wird von drei Retinophoren ge- 
 bildet, die je einen Sehstab tragen, welcher am proximalen Ende in 
 eine Spitze auslauft. Die drei Sehstabe vereinigen sich mittels ihrer 
 axialen Flachen zu einem konischen Korper, so dass man auf Quer- 
 schnitten das Bild einer T-formigen oder dreistrahligen Figur erhalt, 
 deren Arme eingekerbt sind (vgl. meine Fig. 5 C). Die drei Sehstabe ver- 
 langern sich proximal in drei dtinne Faden, die in einer zarten Rohre ein- 
 geschlossen sind, welche ihrerseits durch Yerwachsung der proximalen 
 Enden der farblosen Retinophoren gebildet wird. Die genannten Faden 
 stehen in Zusammenhang mit einer axialen Nervenfaser, welche durch 
 die Rohre in das Centrum des Kegels (Rhabdoms) tibergeht. Die Kerne der 
 Retinophoren, d. h. der drei farblosen centralen Zellen, sind wahrschein- 
 lich tiber dem distalen Ende der Sehstabe gelegen, wenn auch Patten 
 diesen Punkt nicht mit Bestimmtheit festgestellt hat. Die Retinophoren 
 sind von sechs in zwei Kreisen angeordneten pigmentirten Zellen um- 
 geben. Der auBere und innere Kreis besteht aus je drei Zellen. Die- 
 jenigen des auBeren sind schmal und liegen in den Rinnen des Kegels 
 (Rhabdoms). Die Kerne haben ihre Lage gegentiber der Mitte derselben, 
 aber sie sind kaum zu erkennen. Die proximalen Enden der Zellen 
 laufen in lange hyaline Faden aus. Die drei Zellen des inneren Kreises 
 sind dagegen viel groBer und sind den Leisten des Rhabdoms ange- 
 lagert. Ihre groBen ovalen Kerne liegen proximal von den inneren 
 Enden der Sehstabe und scheinen beim ersten Anblick die einzigen 
 Kerne zu sein, welche in der Retina vorhanden sind. 
 
 Wenn wir einen meiner Querschnitte durch die Rhabdomregion 
 von einer Form wie Opilio parietinus (Fig. 5 C) mit der Beschreibung 
 Patten’s vergleichen, so finden wir eine unverkennbare Ahnlichkeit 
 zwischen ihnen. Patten’s groBere Pigmentzellen mit den groBen proxi- 
 mal liegenden Kernen sind augenscheinlich meinen drei peripheren 
 Zellen gleichwerthig, wahrend seine kleineren Pigmentzellen, welche 
 in den Winkeln des Rhabdoms liegen, den drei Plasmastrangen meiner 
 centralen Zelle gleichzusetzen sind. In jeder anderen Hinsicht weicht 
 die Darstellung Patten’s so stark von der meinigen ab, dass ein weiterer 
 Vergleich ftir mich ausgeschlossen ist. 
 
 Henking (1888, 9) machte einige Beobachtungen tiber die Linse 
 wahrend der Ilautung. Das weiche Material ftir die zukiinftige neue 
 Linse wird bereits unter der gelockerten alten Linse abgeschieden und 
 ist Anfangs nach auBen konkav, entsprechend der gekrtimmten Unter- 
 seite der alten Linse. Diese Angaben Henking’s kann ich auch be- 
 stiitigen. 
 
Uber den Bau der Phalangidenaugen. 
 
 39 
 
 VIII. Uberblick iiber die Entwicklung der verschiedenen Theile 
 des Auges. 
 
 Da ich schon in meiner vorlaufigen Mittheilung (22) eine kurze 
 Ubersicht iiber die Entwicklung des Auges von Leiobunum rotundum 
 gegeben habe und dieselbe spater ausfiihrlich zu behandeln gedenke, 
 so werde ich hier nur so viel wiederholen als zum Verstandnis der 
 genetischen Beziehungen der Theile zu einander nothwendig ist. Die 
 Augen entstehen aus einem Paar ektodermaler Taschen am Kopfsegment 
 des Keimstreifens , welche durch einen komplicirten Faltungsprocess 
 sich bilden, Anfangs mit einem Lumen versehen und von einander voll- 
 standig getrennt sind. Spater werden sie vollig von der Hypodermis 
 abgeschniirt, bleiben jedoch mit ihr immer in Beriihrung. Bald riicken 
 sie mehr und mehr in der Medianebene zusammen. 
 
 Die auBere Wand jedes Sackes wird sehr dick und gleichzeitig 
 wird sie in das Innere des Sackes eingestiilpt, so dass das Lumen des 
 letzteren vollstandig verschwindet. Diese verdickte Wand giebt der 
 Retina den Ursprung. Ihre Zellen ordnen sich in einer einzigen Schicht 
 an und bilden zuletzt die Rhabdomere an ihrem jetzt distalen Ende, 
 welches indessen dem urspriinglich (vor der Inversion) inneren, basa- 
 len Ende entspricht. Die Nervenfasern haben ihre definitiven Ansatz- 
 punkte an dem entgegengesetzten proximalen, dem Lumen zugekehrten 
 Ende, Welches dem urspriinglich freien, auBeren Ende der Zellen 
 gleich ist. Indessen erzeugen nicht alle Zellen der auBeren Wand des 
 Sackes Rhabdomere, sondern die randstandigen, welche eine zwei bis 
 drei Zellen dicke Zone bilden, werden sehr intensiv pigmentirt und 
 geben dem periretinalen Pigmentring ( Pg 3 Fig. I I) den Ursprung, wel- 
 cher, wie oben beschrieben wurde, einen Theil der Matrix der peri- 
 retinalen Membran bildet. 
 
 Die proximale innere Wand des Sackes, welche niemals besonders 
 stark sich verdickt, bleibt an den meisten Stellen sehr diinn und aus ihr 
 geht erstens der diinnste Theil der Matrix (Mx Fig. 1 I) hervor, welcher 
 zwischen der periretinalen Membran und der Nervenfaserschicht liegt, 
 ferner das ein Dreieck bildende faserig-lockere Gewebe auf der Ven- 
 tralseite zwischen den beiden Augen und wahrscheinlich entstehen aus 
 ihr auch die langgestreckten Kerne ( k 2 ), welche sich zwischen den 
 Nervenfaserschichten der beiden Augen in der Medianebene finden. 
 
 Jener Theil der Hypodermis, welcher direkt iiber der Retina liegt, 
 erfahrt auch eine Yerdickung und wird zum Glaskcirper, der an seiner 
 distalen Wand die cuticulare Linse abscheidet. 
 
 Es ergiebt sich also das Resultat, dass die Augen der Phalan- 
 
40 
 
 Fred. Purcell, 
 
 giden dreischichtige inverse Augen ektodermalen Ursprungs 
 sind. 
 
 IX. Theoretische Erorterungen iiber die mogliche Funktion der 
 Retinaelemente. 
 
 Bekanntlich ist es eine ganz allgemeine Annahme, dass die Empfin- 
 dung der Lichtstrahlen in den Stabchen bezw. Rhabdomen stattfindet. 
 Dieses muss gerade bei den Phalangiden der Fall sein, wo der ganze 
 distale Theil der Retinazellen mit Pigmentkornern erfiillt ist, die sich 
 tiberall finden, so weit das Protoplasma sich distalwarts erstreckt 
 (Grenacher) . Alle Lichtstrahlen miissen desshalb sofort absorbirt 
 w T erden, auBer denjenigen, welche direkt auf die Rhabdome fallen. 
 
 Es giebt nun verschiedene Weisen, wie man den Sehstab mit 
 Bezug auf seine Lichtempfindlichkeit auffassen kann. 
 
 Man kann zum Beispiel annehmen, dass der Sehstab ganz aus 
 lichtempfindlicher Substanz besteht und so gebaut ist, dass ein Reiz, 
 welcher durch einen auf irgend eine Stelle fallenden Lichtstrahl her- 
 vorgerufen wird, sich durch die ganze Substanz des Sehstabes fort- 
 pflanzt und dann auf die nervosen Leiter tlbertragen wird. 
 
 Stellen wir uns nun die Rhabdome bei den Phalangiden derartig 
 gebaut vor und prtifen wir die Konsequenzen einer solchen Annahme. 
 Bei solchen Augen, wie sie der Acantholophusgruppe zukommen, bei 
 denen jedes Rhabdom mit den benachbarten verschmilzt, mtlsste der 
 Reiz, welcher durch einen Lichtstrahl in irgend einem Rhabdom her- 
 vorgerufen wird, sich fast alien anderen Rhabdomen in demselben 
 Auge mittheilen. Ferner w T ttrde dieses Rhabdom auch den Reiz von 
 Lichtstrahlen empfangen, welche andere Rhabdome treffen. Ein Bild 
 kann in einem solchen Auge unmoglich entstehen, nur eine gleichmaBige 
 Lichtempfindung, die aus der Mischung aller in das Auge eintretenden 
 Strahlen resultirt, konnte hervorgebracht werden. Dass dieses die ganze 
 Funktion eines Auges von einer der Acantholophusgruppe angehorigen 
 Arten, mit seinen komplicirt gebauten Retinulae und Rhabdomen, mit 
 seinem Pigmentapparat zum Reguliren der Lichtstarke, mit seiner 
 groBen Zahl von Rhabdomen und noeh groBeren Menge von Nerven- 
 fasern, sein soil, ist mir hochst unwahrscheinlich und ich mochte daher 
 der folgenden Betrachtungsweise den Vorzug geben. 
 
 Wir dtirfen nicht den Sehstab in seiner Totalitat, wie es oben 
 geschah, als einheitliches lichtempfindliches Endorgan betrachten, 
 sondern wir mtissen annehmen, dass im Rhabdom lokalisirte Stellen 
 enthalten sind, welche die Uberlieferung eines durch Lichtstrahlen 
 hervorgerufenen Reizes an den nervosen Leiter vermitteln. Jede dieser 
 
Uber den Bau der Phalangidenaugen. 
 
 41 
 
 Stellen, die man als lichtempfindliches Endorgan bezeichnen kann, 
 muss desshalb lokalisirt und isolirt sein, damit ein in ihr erzeugter 
 Reiz sich nicht auf den tibrigen Theil des Rhabdoms verbreiten kann 
 und damit sie nur yon Lichtstrahlen gereizt werden kann, welche sie 
 direkt treffen. Ein lokaler Reiz kann sich alsdann nicht ilber den 
 ganzen Sehstab verbreiten und das Vorhandensein eines Netzwerkes 
 von Rhabdomsubstanz, wie in der Acantholophusgruppe, wtirde kein 
 Hindernis fiir die Isolirung der percipirenden Organe sein. 
 
 Es ist leicht begreiflich, dass diese empfindlichen Endorgane in 
 einem Sehstab nicht nothwendig liber seine ganze Masse vertheilt zu 
 sein brauchen, dass vielleicht ein Theil desselben keine solchen Organe 
 besitzt und einfach wie ein dioptrisches Medium sich verhalt. Hierin 
 kSnnten wir vielleicht auch eine Erklarung linden fiir die bemerkens- 
 werthe Verschiedenheit zwischen beiden Arten von Substanzen des 
 Rhabdoms, wie sie in der differenten Farbung desselben zum Ausdruck 
 kommt. Wir konnten namlich annehmen, dass die sensitiven Endorgane 
 sich entweder auf den hell oder auf den dunkel farbenden Theil der 
 Rhabdome beschranken und dass sie eben die Verschiedenheit in der 
 Farbung bedingen. 
 
 Bei solcher Betrachtungsweise ist es leicht einzusehen, dass nur 
 jener Theil des Rhabdoms, welcher sich dunkelblau farbt, als Sitz der 
 sensitiven Endorgane in Frage kommen kann, wahrend der hellblaue 
 Theil nur dioptrischer Natur ist. 
 
 Bei dieser Annahme wiirden wir linden, dass der percipirende 
 Theil des Rhabdoms stets stark entwickelt ist und den bei Weitem 
 groBten Theil des proximalen Abschnittes bildet, d. h. jenen Theil, 
 welcher am weitesten von der Linse entfernt ist. Dagegen variirt der 
 dioptrische Theil des Rhabdoms sowohl in der Form wie in der Masse 
 sehr und er kann sogar fast verschwinden (Platybunus). Er ist am 
 starksten entwickelt im distalen Abschnitt, d. h. in dem dem Lichte 
 zugekehrten Theil. Darin dass dieser Abschnitt bei der Acantholophus- 
 gruppe ausschlieBlich dioptrischer Natur ist, kann ich nur einen be- 
 trachtlichen Vortheil fiir den Sehvorgang erblicken, weil hierdurch die in 
 Folge einer zu geringen Dicke des Glaskorpers herbeigefiihrten Nach- 
 theile zum Theil wieder ausgeglichen wiirden, indem die percipirende 
 Region weiter von der Linse entfernt zu liegen kommt (vgl. Fig. 1 1). 
 
 Die in Rede stehenden Verhaltnisse wiirden eine gewisse Ana- 
 logic zu ahnlichen Verhaltnissen im zusammengesetzten Auge der 
 Crustaceen und Insekten bieten. Der distale besonders abgesetzte 
 Abschnitt des Rhabdoms bei Oligolophus und Acantholophus (Fig. \ \ ) 
 lieBe sich (funktionell) mit dem Krystallkegel jener zusammengesetzten 
 
42 
 
 Fred. Purcell, 
 
 Augen vergleichen, welcher sicher dem dioptrischen Apparat zugehort. 
 Allerdings stellt er dort eine vom Rhabdom unabhangige Bildung dar, 
 kann jedoch einen Fortsatz in dasselbe hineinschicken. 
 
 Mit dem Zustandekommen eines deutlicheren Bildes dtirfte jeden- 
 falls aucb die Verschmalerung des dem Lichte zugekehrten Endes vom 
 proximalen (percipirenden) Abschnitt des Rhabdoms bei Acantholophus 
 und Oligolophus zusammenhangen. Ftir solche Formen dagegen, wo 
 wie bei Platybunus der Glaskorper maehtig entwickelt ist, kame es 
 darauf an, eine groBe Zahl von sehr kleinen Rhabdomen zu besitzen, 
 um ein gutes Sehen zu ermoglichen. Dem entsprechend finden wir 
 hier den dioptrischen Theil bis auf einen kleinen Rest am distalen Ende 
 des Rhabdoms reducirt. 
 
 Eine weitere Konsequenz der hier vorgetragenen Anschauung ist 
 die, dass die Lichtempfindlichkeit nicht der centralen, sondern aus- 
 schlieBlich den peripheren Zellen zukommen wtirde. 
 
 Man konnte die Thatsache, dass die centrale Zelle, wenigstens bei 
 Acantholophus, mit einer Nervenfaser versehen ist, als wichtigen Ein- 
 wand betrachten, indessen lasst sich meiner Ansicht nach hierftir leicht 
 eine andere Erklarung geben. Wie oben (p. 12) schon betont wurde, 
 ist bei der Acantholophusgruppe der distale Abschnitt des Rhabdoms 
 von einem Pigmentmantel eingehtillt, welcher fast ganz aus den kolbig 
 verdickten distalen Enden der centralen Zelle besteht. Und ich habe 
 ferner gezeigt (p. 29), dass das Pigment im Stande ist, von und nach 
 dem distalen Ende der centralen Zelle unter dem Einfluss eines 
 Wechsels der Lichtstarke zu wandern, wesshalb wir der centralen 
 Zelle in der Acantholophusgruppe die Funktion zuschreiben konnen, 
 den Zutritt des Lichtes zum Rhabdom zu reguliren. Dadurch wird die 
 Innervirung der centralen Zelle verstSndlich. 
 
 Bei solchen Formen wie Platybunus, wo sowohl das centrale 
 Rhabdomer wie die distale Region der centralen Zelle sehr schwach 
 entwickelt ist, halte ich es ftir nicht unwahrscheinlich, dass die centrale 
 Zelle nicht mehr mit einer Nervenfaser versehen ist (vgl. hiertiber 
 auch das auf p. 29 Gesagte). 
 
 Kurz zusammengefasst wtirde die oben erorterte theoretische An- 
 schauung folgende sein : Von vier urspriinglich gleichwerthigen Sehzellen 
 haben nur die drei peripheren ihre Lichtempfindlichkeit beibehalten, 
 das Rhabdomer der centralen Zelle dagegen hat diese letztere Eigen- 
 schaft verloren. Es funktionirt jetzt als dioptrisches Medium, wahrend 
 die centrale Zelle selbst bei der Acantholophusgruppe die Funktion 
 einer Pigmentzelle tibernommen hat, welche den Zutritt des Lichtes 
 zum Rhabdom regulirt, da ja echte Pigmentzellen in der Retina fehlen. 
 
Uber den Bau der Phalangidenaugen. 
 
 43 
 
 X. Vergleichung der Phalangidenaugen mit denen der Arachniden 
 im Allgemeinen. 
 
 Wenn wir die Augen der Phalangiden mit denen der tibrigen 
 Arachniden, so weit sie bis jetzt untersucht sind, vergleichen, so linden 
 wir, dass sie mit den Augen der Skorpione noch am ehesten tiberein- 
 stimmen. 
 
 Die Skorpione besitzen bekanntlich ein Paar Mittelaugen und 
 mehrere Paare von Seitenaugen. Die letzteren sind nach den tlberein- 
 stimmenden Angaben von Lankester und Bourne (1 2) sowie G. H. Parker 
 (18), die ich bestatigen kann, einschichtig und bilden nur einen beson- 
 deren Theil der Hypodermis, so dass sich die Retina in direkter Konti- 
 nuitat mit der Linse belindet. Sie entstehen nach Parker durch eine 
 einfache grubenformige Einsenkung des Korperepithels, ohne dass eine 
 Inversion der Retina erfolgt. Diese Augen haben kein Homologon bei 
 den Phalangiden und kommen desshalb ftir uns nicht in Betracht. 
 
 Die Entwicklung der Mittelaugen verlauft nach den Untersuchun- 
 gen von G. H. Parker (18) in ganz ahnlicher Weise wie bei den Augen 
 der Phalangiden und stimmt, in vielen Punkten sogar bis ins Einzelne, 
 mit diesen tiberein. Die Mittelaugen der Skorpione entstehen ebenfalls 
 aus drei Schichten, von denen die auBere den Glaskorper und die 
 mittlere die invertirte Retina bildet, wShrend aus der inneren eine 
 dtinne postretinale Pigmentschicht hervorgeht. Die letztere fungirt als 
 Matrix ftir einen Theil der Retinalkapsel, welche genau so wie bei den 
 Phalangiden die Retinae beider Augen in einer gemeinsamen Hiille ein- 
 schlieBt. 
 
 Da es zu weit ftihren wiirde, alle Ansichten, welche tiber den 
 Bau der Retina der Mittelaugen bei den Skorpionen geauBert worden 
 sind, zu diskutiren, so werde ich mich nur an die genaueren Unter- 
 suchungen von Lankester und Bourne (12) und yon Parker (18) halten, 
 zumal ich deren Angaben ftir Euscorpius bestatigen kann. Die Retina 
 besteht aus Retinulae, von denen sich jede aus fiinf Zellen, die in einem 
 Kreise angeordnet sind, zusammensetzt. Jede Zelle producirt seitlich 
 ein rinnenformiges Rhabdomer, welches aber ihr distales Ende nicht 
 erreicht. Die Rhabdomere vereinigen sich in jeder Retinula zu einem 
 mit fiinf Langsrinnen versehenen Rhabdom, wodurch dieses auf dem 
 Querschnitt ftinfstrahlig erscheint. Der distale Theil der Retinazelle 
 ist dicht pigmentirt, wahrend der proximale Theil den Kern enthalt 
 und in die Nervenfaser iibergeht. Bei Centrurus fand Parker zwischen 
 den Retinulae in der distalen Region besondere keulenformige Pigment- 
 zellen, welche in der Rhabdomregion sehr fein und fadenformig werden 
 
44 
 
 Fred. Purcell, 
 
 und in der proximalen Region, wo der Kern liegt, wieder an GroBe 
 zunehmen. Ganz ahnliche Pigmentzellen habe ich auch bei Euscorpius 
 finden konnen. Lankester und Bourne beschreiben zwar bei Andro- 
 ctonus auBerst feme Pigmentzellen zwischen den Retinulae im distalen 
 und mittleren Abschnitt derselben, indessen sind bei Buthus nach 
 Grenacher (7), bei Centrums nach Parker (18), sowie bei Euscorpius 
 nach Lankester und Bourne (12) und nach mir in der distalen Halfte 
 der Retina gar keine Kerne zu finden. Zwischen der Retina und der 
 postretinalen Pigmentschicht befindet sich eine Lage von Nervenfasern, 
 die gerade so wie bei den Phalangiden zu zahlreichen Btindeln inner- 
 halb der Retinalkapsel vereinigt sind. 
 
 Dieses mag gentigen, um die Homologie zwischen den Mittelaugen 
 der Skorpione und den Phalangidenaugen zu zeigen. Die Hauptunter- 
 schiede liegen in der Abwesenheit einer centralen Zelle, im Bau der 
 Retinulae und in dem Yorhandensein von echten interretinulUren 
 Pigmentzellen bei den Skorpionen. Ich mochte aber die Aufmerksamkeit 
 auf die Ahnlichkeit lenken, welche zwischen den Plasmastrangen der 
 centralen Zelle in der Acantholophusgruppe und dem keulenformigen 
 distalen Abschnitt der Pigmentzellen bei Centrums und Euscorpius 
 besteht, ohne damit natiirlich eine Homologie zwischen ihnen behaupten 
 zu wollen. 
 
 Bei den echten Spinnen (Araneae), wo ein ahnlicher Dimor- 
 phismus der Augen wie bei den Skorpionen schon langst durch Gre- 
 nacher (6) bekannt geworden ist, unterscheidet man die »Hauptaugen« 1 
 (vorderen Mittelaugen) von den tibrigen sogenannten » Nebenaugen « 1 
 (vorderen Seitenaugen und hinteren Augen). 
 
 Die Nebenaugen sollen nach Locy (16) nach ihrer Entwicklung 
 inverse Augen sein, doch konnte Mark (17), der die Praparate von Locy 
 darauf hin weiter untersucht hat, diese Angabe nicht mit Sicherheit 
 bestatigen, wenn er auch den inversen Bau ftir den wahrscheinlichsten 
 hielt. Erst Patten (20 a) gelangte zu der Ansicht, welche Kishinouye (1 0) 
 spiiter ausftlhrlicher begrUnden konnte, dass die Nebenaugen durch 
 eine becherformige ektodermale Einsenkung entstehen. Der Boden der 
 Einsenkung wird zur Retina, die keine Spur einer Inversion zeigt. 
 Diese Angaben Patten’s und Kishinouye’s habe ich frtther schon bestatigt 
 (Korschelt und Heider [1 1 p. 597]). 
 
 Durch anatomische Untersuchungen ist ferner Bertkau (1) zu dem 
 Schluss gekommen, dass die Nebenaugen inverse Augen seien, obwohl 
 er dieses nicht mit volliger Sicherheit beweisen konnte. Diese Ver- 
 
 1 So von Bertkau (1) genannt. 
 
Uber den Bau der Phalangidenaugen. 
 
 45 
 
 haltnisse habe ich ebenfalls eingehend untersucht und kann im Gegen- 
 satz zu Bertkau die alteren Angaben Grenacher’s in alien wichtigen 
 Punkten vollig bestatigen. 
 
 Die Hauptaugen (vordere Mittelaugen) der Spinnen dagegen sind, 
 nach den tlbereinstimmenden Angaben von Locy (16), Mark (17), Pat- 
 ten (20 a), Kishinouye (10) und mir (Korschelt und Heider [11, p. 602]) 
 echte inverse Augen und aus drei Schichten gebaut. Es kann desshalb 
 nicht zweifelhaft sein, dass diese den Phalangidenaugen homolog sind. 
 In der Retina der Hauptaugen kommen auBer den eigentlichen Seh- 
 zellen noch echte Pigmentzellen vor. Die Sehzellen sind nach Grenacher 
 und Bertkau, wie ich ebenfalls bestatigen kann, nicht in Retinulae an- 
 geordnet und besitzen einen im proximalen Theil gelegenen Kern ; sie 
 gehen am proximalen Ende in die Nervenfaser fiber, letzteres Yerhaltnis 
 ganz so wie bei den Phalangiden und den Skorpionen. Wahrend ich 
 dies auch ftir einige Spinnen (Attiden) bestatigen kann, habe ich bei 
 den meisten Dipneumones gefunden, dass derNerv sich nicht am 
 proximalen Ende, sondern in der Mitte der Retinazelle 
 ansetzt. 
 
 Eine Yerbindung zwischen den am distalen Ende gelegenen Stab- 
 chen benachbarter Zellen ist bis jetzt nicht beschrieben worden, ich 
 habe indessen bei einer groBen sfidamerikanischen Vogel- 
 spinne, sowie bei vielen Dipneumones eine echte Rhab- 
 dombildung gefunden, welche aber einen Typus aufweist, der 
 so weit bekannt ist, ganz vereinzelt unter den Arthropoden dasteht. 
 Jede Zelle namlich producirt zwei bis sechs Rhabdomere, von denen 
 jedes mit einem der Rhabdomere jeder anliegenden Zelle verwachst. 
 Auf diese Weise nimmt jede Zelle Theil an der Bildung von zwei bis 
 sechs verschiedenen Rhabdomen. Jedes Rhabdom ist das Produkt von 
 in der Regel nur zwei Zellen und erscheint desshalb auch zweitheilig. 
 Eine Gruppirung der Zellen zu Retinulae findet sich indessen nicht. 
 Ich werde diese Verhaltnisse spater noch eingehender beschreiben, wenn 
 ich meine Untersuchungen liber den Bau der Spinnenaugen veroffent- 
 lichen werde. 
 
 Ein ahnlicher Typus von Rhabdombildung ist von Grenacher (8) 
 in seinen Untersuchungen fiir das Gephalopodenauge beschrieben 
 worden. Bei diesen Thieren scheidet jede Zelle in der Regel zwei 
 Rhabdomere ab und nimmt an der Bildung von zwei verschiedenen 
 Rhabdomen Theil. Die letzteren werden stets von drei bis ftinf Zellen 
 gebildet. 
 
 Yon den Augen der tibrigen Arachniden sind, so weit ich weiB, 
 nur noch die der Pseud os corpione untersucht worden. Nach der 
 
46 
 
 Fred. Purcell. 
 
 Beschreibung von Bertkau (2) sind sie den Nebenaugen der Spinnen 
 in vielen Punkten sehr ahnlich und wahrscheinlich mit diesen homolog. 
 
 Die Homologie der Mittelaugen von Limulus mit den Mittelaugen 
 der Skorpione wurde zuerst von Lankester und Bourne behauptet. 
 Diese Augen, welche fast zusammen in der Medianlinie liegen, besitzen 
 nach den Angaben der genannten Autoren einen Glaskorper und eine 
 darunter liegende Retina, deren Zellen hier und dort eine Anordnung 
 zu Retinulae von je fiinf rhabdombildenden Zellen zeigt. 
 
 Aus den mitgetheilten Angaben geht klar hervor, dass die vorde- 
 ren Mittelaugen der Spinnen, die Augen der Phalangiden 
 und die Mittelaugen der Skorpione sowie jedenfalls die 
 Mittelaugen des Limulus eine Reihe von homologen Ge- 
 bilden darstellen, welche durch eine invertirte Retina 
 mit Retinulae oder we nigs t ens Rhabdomen charakterisirt 
 sind. 
 
 Da wir bei den niedrigsten Arachnidenformen wie den Skorpionen 
 sowie bei Limulus eine Retina mit Retinulae antreffen, so ergiebt sich 
 ferner, dass der ursprtingliche Arachnidentypus auch eine aus Retinulae 
 zusammengesetzte Retina besaB. Ich sehe ein Hauptresultat dieser 
 Arbeit darin, nachgewiesen zu haben, dass eine aus Retinulae 
 zusammengesetzte Retina bezw. eine Modifikation der- 
 selbenauch bei den hoheren Ar a chnidenord nu n gen, den 
 Phalangiden und den Spinnen, nochbesteht. 
 
 Korschelt (11, p. 600) ist der Ansicht, dass die Mittelaugen der 
 Skorpione und des Limulus von zusammengesetzten Facettenaugen 
 abgeleitet sind, deren Einzelaugen (Ommatidia) beim Skorpion und 
 Limulus durch die Retinulae reprasentirt werden. Diesen Gedanken 
 weiter verfolgend, kam der genannte Autor zu dem Schluss, dass 
 auch bei den hoheren Arachnidenordnungen in Homologie mit den 
 Mittelaugen der Skorpione frtiher eine zusammengesetzte Retina vor- 
 handen gewesen sein mtisse. Obgleich ihm das Yorhandensein von 
 Rhabdomen bei den Spinnen und Phalangiden unbekannt war, so 
 ftihrten ihn die Angaben Grenacher’s, dass die Stabchen der Spinnen 
 aus zwei, die der Phalangiden aus drei Stiicken bestanden, zu der 
 Vermuthung: » dass es sich bei der Zwei- resp. Dreitheiligkeit der 
 Stabchen vielleicht um Reste der Rhabdom- und Retinulabildung han- 
 deln mochte« (11, p. 601). Wie in der vorliegenden Arbeit nach- 
 gewiesen wurde, hat diese Vermuthung ftir die Augen der Phalangiden 
 und ftir die Mittelaugen der Spinnen vollig das Richtige getroffen. 
 
 Berlin, Januar 1894. 
 
Uber den Bau der Phalangidenaugen. 
 
 47 
 
 Litteraturverzeichnis. 
 
 -1. P. Bertkau, Beitrage zur Kenntnis der Sinnesorgane der Spinnen. I. Die Augen 
 der Spinnen. Archiv f. mikr. Anat. Bd. XXVII. 1886. Bonn. 
 
 2. Uber die Chernetiden oder Pseudoskorpione. Verhandl. des naturhist. 
 
 Vereins der preuB. Rheinlande etc. Sitzungsberichte. 44. Jahrg. 1887. 
 Bonn. 
 
 3. 0. Butschli, Untersuchungen uber mikroskopische Schaume und das Proto- 
 
 plasma. Leipzig 1892. 
 
 * 4. J. Carriere, Bau und Entwicklung des Auges der zehnfiiBigen Grustaceen und 
 der Arachniden. Biolog. Centralbl. Bd. IX. 1889. 
 
 5. V. Graber, Uber das unicorneale Tracheaten- und speciell das Arachniden- und 
 
 Myriapodenauge. Archiv f. mikr. Anat. Bd. XVII. 1879. Bonn. 
 
 6. H. Grenacher, Untersuchungen uber das Sehorgan der Arthropoden, insbeson- 
 
 dere der Spinnen, Insekten und Crustaceen. Gottingen 1879. 
 
 7. Uber die Augen einiger Myriapoden. Archiv f. mikr. Anat. Bd. XVIII. 
 
 1880. Bonn. 
 
 8. Abhandlungen zur vergleichenden Anatomie des Auges. I. Die Retina der 
 
 Cephalopoden. AbhandL Natur. Gesellschaft Halle. Bd. XVI. 1884. 
 
 9. H. Henking, Biologische Beobachtungen an Phalangiden. Zoolog. Jahrbiicher. 
 
 Abth. fur System. Bd. III. 1888. Jena. 
 
 10. Kishinouye, On the Development of Araneina. Journ. of the Coll, of Sci. Imperial 
 
 Univers. Japan. Vol. IV. Part. 1. 1891. 
 
 11. E. Korschelt u. K. Heider, Lehrbuch der vergleichenden Entwicklungsgesch. 
 
 der wirbellosen Thiere. Specieller Theil. 2. Heft. Jena 1892. 
 
 12. Ray Lankester and Bourne, The minute structure of the lateral and the central 
 
 Eyes of Scorpio and of Limulus. Quart. Journ. of Micr. Sci. Vol. XXIII. 
 N. ser. 1 883. 
 
 13. F. Leydig, Zum feineren Bau der Arthropoden. Archiv f. Anat., Physiol, u. wiss. 
 
 Med. Jahrg. 1855. Berlin. 
 
 14. fiber das Nervensystem der Afterspinne (Phalangium). Archiv f. Anat., 
 
 Physiol, u. wiss. Med. Jahrg. 1862. Leipzig. 
 
 15. Tafeln zur vergleichenden Anatomie. Tubingen 1 864. 
 
 16. W. Locy, Observations on the development of Agelena naevia. Bull, of the Mus. 
 
 of Gompar. Zool. at Harv. Coll. Vol. XII. No. 3. Cambridge 1886. 
 
 17. E. L. Mark, Simple Eyes in Arthropods. Bull, of the Museum of Comp. Zool. at 
 
 Harv. Coll. Cambridge. Vol. XII. No. 3. 1887. 
 
 18. G. H. Parker, The Eyes in Scorpions. Bull, of the Museum of Comp. Zool. at 
 
 Harv. Coll. Cambridge. Vol. XIII. No. 6. 1887. 
 
 1 9. W. Patten, Eyes of Molluscs and Arthropods. Mitth. aus der Zoolog. Station zu 
 
 Neapel. Bd. VI. Berlin 1886. 
 
 20. Studies on the Eyes of Arthropods. 1 . Develop, of the Eyes of Vespa, with 
 
 Observ. on the Ocelli of some Insects. Journal of Morph. Vol. I. Boston 
 1887. 
 
 20a. Segmental Sense-Organs of Arthropods. Journ. of Morph. Vol. II. Boston 
 
 1887. 
 
48 
 
 Fred. Purcell, 
 
 21. F. Plateau, Recherches experimentales sur la vision chez les Arthropodes 
 
 (deuxieme partie). Vision chez les Arachnides. Bulletins de l’Acad. Royale 
 des Sci. etc. de Belgique. 57. Ann. 3. S6r. T. XIV. 1887. 
 
 22. F. Purcell, Uber den Bau und die Entwicklung der Phalangidenaugen (Vor- 
 
 laufige Mittheilung). Zool. Anz. 15. Jahrg. 1892. 
 
 22a. Uber den Bau der Phalangidenaugen. Inaug.-Diss. Berlin 1894. (Diese 
 
 Dissertation bildet nur einen Theil der vorliegenden Abhandlung.) 
 
 23. G. Saint-Remy, Contribution a l’etude du cerveau chez les Arthropodes Tra- 
 
 chytes. Arch, de Zool. exp. et g6n. 2. s6rie. T. V bis Suppl. 1887. 
 
 24. E. Simon, Les Arachnides de France. T. VII. Paris 1879. 8°. 
 
 25. M. Stefanowska, La Disposition Histologique du Pigment dans les yeux des 
 
 Arthropodes sous l’influence de la lumiere directe et de l’obscurite 
 complete. Recueil zoolog. Suisse. T. V. 1890. 
 
 26. G. R. u. L. C. Treviranus, Vermischte Schriften anatomischen u. physiologi- 
 
 schen Inhalts. Bd. I. Gottingen 1816. 
 
 27. A. Tulk, Upon the Anatomy of Phalangium opilio. Ann. of Nat. Hist. Vol. XII. 
 
 1843. 
 
 Erklarung der Abbildungen. 
 
 Bedeutung einiger wiederholt vorkommender Buchstaben : 
 c, centrale Zelle ; 
 
 c 1 , c 2 , c 3 , cl, cii, c in , die drei Plasmastrange, in welche die centrale Zelle 
 in der Rhabdomgegend getheilt ist; 
 cr, Rhabdomer der centralen Zelle ; 
 dist, distaler Abschnitt des Rhabdoms; 
 hin, Hinterseite des Auges; 
 k, k 1 etc., Kerne ; 
 lat, laterale Seite des Auges ; 
 med, mediale Seite des Auges ; 
 ml, Mittellamelle (Mittellinie) des Rhabdoms; 
 nf, Nervenfaser; 
 p i p 2 p3 1 
 
 ’ ’ ’ TTT } die drei peripheren Zellen einer Retinula ; 
 
 P 1 , P 11 , P 111 ,) 
 
 pg, Pigmentkorner ; 
 
 pr, Rhabdomer einer peripheren Zelle; 
 
 prox, proximaler Abschnitt des Rhabdoms ; 
 
 si, Seitenlamelle (Seitenlinie) des Rhabdoms; 
 
 vor, Vorderseite des Auges. 
 
 NB. Die eingeklammerten Buchstaben und romischen Zahlen bedeuten die 
 Kombination der ZEiss’schen Objektive und Oculare, mit denen die den Figuren bei- 
 gefugte Zahl der VergroBerungerzieltwurde. Alle Figuren sind mit dem WiNKEL’schen 
 Zeichenapparat entworfen. In den Fig. 1 — 8 sind die Rhabdome blau kolorirt und 
 es ist damit moglichst genau die H&matoxylinfarbung in den Praparaten wieder- 
 gegeben. 
 
Uber den Bau der Phalangidenaugen. 
 
 49 
 
 Tafel I. 
 
 Fig. Leiobunum rotundum Latr. Schema einer ganzen Retinula, 
 kombinirt aus einer liickenlosen Serie von 24 Schnitten durch eine und dieselbe 
 Retinula. Die wirkliche Dicke der Schnitte (3,5 /x) wurde dabei beriicksichtigt, so 
 dass die Figur ein ziemlich genaues Bild einer Retinula giebt. Die parallelen Quer- 
 linien auf der linken Seite zeigen die Grenzen zwischen zwei einander folgenden 
 Schnitten an. 4 2 von den letzteren, deren Lage als \A bis \M links von Fig. 1 be- 
 zeichnet ist, sind in doppelt so groGem MaGstabe in den Fig. \A bis \M dargestellt; 
 nur die centrale Zelle (c) und zwei periphere Zellen (p 2 und p 3 ) sind zu sehen, die 
 dritte periphere ist hinter den iibrigen verborgen. 
 
 Fig. \A bis \M. Leiobunum rotundum (Immers. 1/18, IV). Alkoh. Pikrin- 
 saure, Hamatoxylin. 12 Querschnitte durch eine und dieselbe Retinula (vgl. Fig. 1). 
 Die Buchstaben p 1 , p 2 , p 3 , c 1 , c 2 , c 3 gelten fur dieselben Zellen in jedem Schnitt; die 
 Rhabdomere der peripheren Zellen sind dunkelblau, das der centralen Zelle hellblau. 
 
 Fig. \A geht durch den Kern (k) der centralen Zelle. 
 
 Fig. 1 B, durch die Kerne ( k ) der peripheren Zellen. 
 
 Fig. 1 C u. ID, durch den verdiinnten Theil der centralen Zelle. 
 
 Fig. 1 E liegt unmittelbar proximal vom Rhabdom ; die centrale Zelle erscheint 
 in diesem Theil voluminoser und dreiseitig. 
 
 Fig. IF, durch das SuGerste proximale Ende des Rhabdoms; eines der peri- 
 pheren Rhabdomere (der Zelle p 1 ) ist noch von den iibrigen getrennt. 
 
 Fig. 1G u. ID sind typisch fiir den proximalen Abschnitt des Rhabdoms. 
 
 Fig. 1 1 liegt an der Grenze zwischen dem proximalen und distalen Abschnitte ; 
 die peripheren Rhabdomere, von p 2 und p 3 , senken sich in das centrale Rhabdomer 
 ein und treffen sich, so dass letzteres in zwei Theile gespalten wird. 
 
 Fig. 1 K. DieStrahlen der peripheren Rhabdomere von p 2 und p 3 werden kiirzer 
 und das centrale wolbt sich in den angrenzenden Plasmastrang c 1 der centralen 
 Zelle vor. 
 
 Fig. 1 L u. 1 M. Die beiden am weitesten distal liegenden Schnitte. In 1 M 
 sind die Strahlen von p 2 und p 3 verschwunden, und das Rhabdom ist seitlich kom- 
 primirt. 
 
 Fig. 2. Leiobunum rotundum Latr. (Immers. 1/18, IV). Alkohol. Pikrin- 
 saure, H&matoxylin. Querschnitt einer Retinula durch das auGerste distale Ende 
 (entsprechend der Fig. \M). Das centrale Rhabdomer ist gegen beide Plasmastrange 
 cii und clH der centralen Zelle vorgewolbt, wodurch der Querschnitt des Rhabdoms 
 die Form eines Kreuzes erhalt. 
 
 Fig. 3. Leiobunum rotundum Latr. (Immers. 1/18, IV). Alkohol. Pikrin- 
 saure, Hamatoxylin. Querschnitt einer Retinula auf der Grenze zwischen dem proxi- 
 malen und distalen Abschnitt (wie Fig. 1 K). Das centrale Rhabdomer ist in die 
 drei Plasmastrange c 1 , c 11 , c 111 der centralen Zelle vorgewolbt. Die Strahlen der 
 peripheren Zellen p 1 , p 11 und p in sind jetzt noch vorhanden, wodurch das Rhab- 
 dom eine sechsstrahlige Gestalt erhalt. 
 
 Fig. 4. Leiobunum rotundum Latr. (Immers. 1/18, IV). Alkohol. Pikrin- 
 saure, Hamatoxylin. Querschnitt durch das auGerste distale Ende einer Retinula; 
 jedes der peripheren Rhabdomere ist in die Substanz des centralen Rhabdomers 
 eingedrungen und hat das letztere in drei Stiicke gespalten, die die Form von 
 Strahlen angenommen haben. 
 
 Fig. 5. Opilio parietinus de Geer. Schema des distalen Theiles zweier 
 Zeitschrift f. wissensch. Zoologie. LVIII. Bd. 
 
 4 
 
50 
 
 Fred. Purcell, 
 
 Retinulae, kombinirt in derselben Weise wie Fig. 1. Die als 1 und 2 bezeichneten 
 Retinulae entsprechen in der Querschnittserie 5 A bis 5 H den Ret. / und Ret. 2- Die 
 Rhabdome sind in 12 Schnitte zerlegt, von denen vier dem distalen Abschnitt zu- 
 kommen, der hier durch Briicken mit den benachbarten Rhabdomen verbunden ist 
 (vgl. den Text p. 20). In dem proximalen Abschnitt stellt ein schmaler, hellblauer 
 Streifen das centrale Rhabdomer dar. 
 
 Fig. 5,4 bis 5/2. Opilio parietinus de Geer (Immers. 4/12, IV). Alkohol. 
 Pikrinsaure, HSmatoxylin. Acht Querschnitte durch den distalen Theil ein und der- 
 selben Gruppe von vier Retinulae (als 2 bis 4 bezeichnet), von denen Ret. 2 und 2 
 in Fig. 5 rekonstruirt und auf 8 / 10 verkleinert sind. 
 
 Fig. hA und 5 B liegen an der proximalen Seite des Rhabdoms; sie zeigen die 
 charakteristische eckige Gestalt der centralen Zelle, deren Rhabdomer in 5 B sicht- 
 bar ist. 
 
 Fig. 5 C bis 5 j E, durch den proximalen Abschnitt des Rhabdoms, das hier fast 
 ganz aus den dunkelblauen peripheren Rhabdomeren besteht, das centrale Rhab- 
 domer ist nur in Form von schmalen hellblauen Streifen sichtbar. 
 
 Fig. 5Fbis 5 H durch den distalen Abschnitt des Rhabdoms, welcher hier hell- 
 blau gefarbt ist ; die Bildung der die benachbarten Rhabdome verbindenden Briicken 
 ist in 5 F gezeigt, wo ein Theil des dunkelblauen proximalen Abschnittes noch zu 
 sehen ist (in Ret. 3 und 4). 
 
 Fig. 5 G zeigt nur Ret. 4. 
 
 Fig. 522 zeigt das Netzwerk von Rhabdomsubstanz ; nur jene, welche zu den 
 Retinulae 2 — 4 gehoren, sind in der Figur blau kolorirt und stehen mittels 1 3 Briicken 
 mit den benachbarten Rhabdomen in Verbindung. Die letzteren sind zur besseren 
 Unterscheidung nur mit einem grauen Ton versehen. Nur die Zellen, welche der 
 Ret. 4 angehoren, sind eingezeichnet, um das starke Anwachsen der centralen Zelle 
 und die hiermit einhergehende GroBenabnahme der peripheren Zellen zu zeigen. 
 
 Tafel II. 
 
 Fig. 6. Acantholophus hispidus Herbst. Schema des distalen Theilcs 
 einer Retinula, kombinirt, in gleicher Weise wie Fig. 1, aus der Serie von Quer- 
 schnitten Fig. 6^4 bis 6G; nur die centrale Zelle (c) und die peripheren Zellen p 2 und 
 p 3 sind zu sehen. Das Rhabdom ist in 11 Schnitte zerlegt, von denen drei dem 
 distalen Abschnitt angehoren. 
 
 Fig. GA bis 6G. Acantholophus hispidus (Immers. 1/18, IV). Alkohol. 
 Pikrinsaure, Hamatoxylin. Acht Querschnitte durch den distalen Theil einer und 
 derselben Retinula (rekonstruirt und reducirt auf die Halfte in Fig. 6). Die Buch- 
 staben p 1 , p 2 , p 3 , c 1 , c 2 , c 3 bezeichnen immer dieselben Zellen in jedem Schnitf, 
 
 Fig. 6^4 liegt unmittelbar proximal vom Rhabdom. 
 
 Fig. 62? bis 6E gehen durch den proximalen Abschnitt, der hauptsachlich aus 
 den dunkelblauen peripheren Rhabdomeren besteht; Reste von dem hellblauen cen- 
 tralen Rhabdomer sind in 6 D und 6C zu sehen. Die hellblauen Theile, welche mit 
 9 1 , q 1 und <? 3 bezeichnet sind, mit der Mittellinie in ihnen, gehoren den peripheren 
 und nicht dem centralen Rhabdomer an. 
 
 Fig. 6 F f bis 6G sind durch den distalen Abschnitt des Rhabdoms gefiihrt. Zu 
 bemerken ist das plbtzliche Anwachsen der centralen Zelle, welches besonders 
 deutlich in Fig. 6F' und 6 F" hervortritt. Diese beiden Figuren slellen die proxi- 
 male und die distale Flache ein und desselben Schnittes dar. 
 
 Fig. 7. Acantholophus hispidus (Immers. 1/18, IV). Alkohol. Pikrin- 
 
Uber den Bau der Phalangidenaugen. 
 
 51 
 
 stiure, Hamatoxylin. Querschnitt durch den diinnsten Theil eines besonders stark 
 verschmalerten Rhabdoms. Seine Lage entspricht Fig. 6 E. 
 
 Fig. 8. Phalangium opilio L. (Immers. 4/18, IV). Alkohol. Pikrinsiiure, 
 Hamatoxylin. Querschnitt durch das auBerste distale Ende einer Retinula; die 
 peripheren Rhabdomere sind dunkelblau kolorirt. 
 
 Fig. 9. Platybunus tr i a n gu 1 a ris Herbst (Immers. 1/18, IV). Glanzkry- 
 stalle aus der Matrix der Retinalkapsel. 
 
 Fig. 10. Acantholophus hispidus (Immers. 1/4 2, I). FLEMMiNG’sche Fliis- 
 sigkeit -j- Alkohol bei 45° C. Schnitt durch ein ganzes Auge, welcher parallel der 
 Langsachse des Korpers, und zwar in derjenigen Richtung, welche in Fig. 11 durch 
 die punktirte Linie Fig. 10 bezeichnet ist, gefiihrt wurde. 
 
 Fig. 14. Acantholophus hispidus (Immers. 1/42, I). FLEMMiNG’sche Fltis- 
 sigkeit -f- Alkohol kalt. Schnitt durch ein ganzes Auge. Der Schnitt ist genau quer 
 zur Langsachse des Korpers und rechtwinkelig zum Schnitt der Fig. 10 gefiihrt. 
 Der Schnitt enthalt die Sehachse und theilt das Auge in zwei symmetrische Halften. 
 Der Pfeil zeigt die Medianebene des Korpers an, und die punktirte Linie die Rich- 
 tung des Schnittes von Fig. 4 0. Rt l und N.op 1 bezeichnen die Lage der Retina und 
 des N. opticus des anderen Auges, welches nicht eingezeichnet wurde. Sowohl 
 diese Figur wie die vorhergehende sind nicht kombinirt, sondern mit der Camera 
 genau je nach einem einzigen Schnitt gezeichnet. 
 
 Buchstaben der Fig. 10 und 11. 
 bm, Basalmembran der Hypodermis; 
 
 Ct l , huBere -| 
 
 Ct 2 , mittlerei Schicht der Korpercuticula ; 
 
 Cf 3 , innere J 
 Ft, Fettgewebe; 
 
 Gl, Glaskorper; 
 
 Hy, Zellenlager der Hypodermis, mit Glanzkrystallen gefullt; 
 
 Hy' } do., ohne Krystalle ; 
 k, Kerne der Retinazellen ; 
 
 k l , sehr kleine, runde Kerne im proximalen Theil der Retina, wahrschein- 
 lich solche von centralen Zellen ; 
 k 2 , lange, abgeplattete Kerne zwischen den beiden Augen; 
 fc 3 , do., zwischen den Fasern des Opticus; 
 
 /c 4 , do., von der Scheide der Nervenfaserbiindel im Auge; 
 
 L, Linse ; 
 
 Mk, Querschnitt einer Muskelfaser; 
 
 Mx , diinne zellige Matrix der periretinalen Membran ; 
 
 Nv, Nervenfaserschicht ; 
 
 N.op 1 — V, Nervi optici; 
 peri.m, Periretinalmembran ; 
 
 prae.m, Praretinalmembran, beide zusammen bilden die Retinalkapsel ; 
 Pg l , Pigmentzone der Mittelschicht der Guticula ; 
 
 Pg 2 , Pigmentzone aus an der Peripherie des Glaskorpers gelegenen Hypo- 
 dermiszellen gebildet; 
 
 Pg 3 , periretinaler Pigmentring aus randstandigen, nicht stdbchentragen- 
 den Zellen gebildet, zugleicb ein Theil der Matrix der Periretinal- 
 membran; 
 
 4 * 
 
52 
 
 Fred. Purcell, 
 
 ph, stark lichtbrechende, den Phaospharen ahnliche Korper in den Hypo- 
 dermiszellen ; 
 
 Por, groBe Porenkanale der Cuticula ; 
 
 por, feine Porenkanale der Cuticula und Linse ; 
 
 Rm, Rhabdom; 
 
 Rt, Retina; 
 
 Tr, Trachea; 
 
 x, Stelle, wo die Basalmembran der Hypodermis und die periretinale 
 Membran sich trennen ; 
 
 zw.g, Gewebe, das den dreieckigen Raum zwischen der Retinalkapsel und 
 den beiden Augen ausfullt. 
 
 Fig. 12. Opilio parietinus (Immers. 1/12, I). Alkohol. Pikrinsaure. 
 Schnitt durch die Retina eines rechten Auges, senkrecht zur Sehachse geftihrt ; er 
 zeigt die Anordnung der Rhabdome. 
 
 Fig. 13. Acantholophus hispidus (Immers. 1/IB, IV). Alkohol. Pikrin- 
 saure bei 35° C. Proximaler Theil von drei Retinulazellen mit Phaospharen [Ph), 
 welche wie in einer Vacuole (vc) liegen ; die Plasmastruktur ist nur in eine der Zellen 
 eingezeichnet. 
 
 Fig. 1 kA. Opilio parietinus (Immers. 1/18, IV). Alkohol. Pikrinsaure. 
 Querschnitt durch den proximalen Abschnitt eines Rbabdoms, welcher die Waben- 
 struktur zeigt; sp, kiinstlicher Spalt in der Mittellamelle [ml). 
 
 Fig. 14 B. Opilio parietinus (Immers. 1/18, IV). Alkohol. Pikrins. gesattigt 
 bei 45° C. Querschnitt durch den proximalen Theil des Rhabdoms. Das centrale 
 Rhabdomer ( cr ) ist durch gelostes Pigment schwarz gefarbt, wahrend die periphe- 
 ren Rhabdomere [pr) ungefarbt sind. 
 
 Fig. 15. Opilio parietinus (Immers. 1/18, IV). Alkohol. Pikrinsaure, Hama- 
 toxylin. Proximaler Theil eines abgeplatteten Rhabdoms, gesehen von der schma- 
 len seitlichen Kante; ql, Querlamellen (Querlinien). 
 
 Fig. 16 u. 17. Acantholophus hispidus (Immers. 1/18, IV). Alkohol. 
 Pikrinsaure, gesattigt bei 45° C. Querschnitle durch den proximalen Theil zweier 
 Rhabdome. Das centrale Rhabdomer (cr) ist durch gelostes Pigment schwarz ge- 
 farbt, wahrend die peripheren Rhabdomere ungefarbt geblieben sind. 
 
 Fig. 18. Acantholophus hispidus (Immers. 1/18, IV). Alkohol. Pikrin- 
 saure, Hamatoxylin. Querschnitt durch das proximale Ende eines Rhabdoms. 
 
 Fig. 19. Leiobunum rotundum Latr. (Immers. 1/18, IV). Alkohol. Pikrin- 
 saure, Hamatoxylin. Querschnitt durch das proximale Ende eines Rhabdoms; das 
 centrale Rhabdomer (cr) ist hell gelassen. 
 
 Fig. 20. Schema, welches die Anordnung der Protoplasmawaben zeigt. 
 
 Fig. 21 — 24. Acantholophus hispidus. Retinazellen, macerirt in Hal- 
 LER’scher Flussigkeit. 
 
 Fig. 21 (F, I) Theil einer stark pigmentirten centralen Zelle (c) und einer peri- 
 pheren Zelle [p), welche dem Rhabdom anliegt. 
 
 Fig. 22 (F, I). Eine Retinula; der den Kern enthaltende Theil einer der peri- 
 pheren Zellen [pi) ist abgebrochen. 
 
 Fig. 23 (Immers. 1/12, II). Eine periphere Zelle mit einem langen Stuck einer 
 anhangenden Nervenfaser [nf) t bei x ist das Rhabdomer abgerissen. 
 
 Fig. 24 (F, I). Vier Retinazellen mit Stucken von anhangenden Nerven- 
 fasern [nf). 
 
 Fig. 25 (Immers. 1/18, IV). Alkohol. Pikrinsaure, Hamatoxylin. Ubersicht der 
 
Uber den Bau der Phalangidenaugen. 
 
 53 
 
 Rhabdome von acht Phalangidenarten bei gleicher VergroDerung gezeichnet. Die 
 Schnitte sind durch den dicksten Theil (in der Nahe des proximalen Endes) des 
 Rhabdoms gefuhrt und zeigen die typischen Formen, die bei den verschiedenen 
 Arten vorkommen. 
 
 A, platte Rhabdome der Acantholophusgruppe, welche vorzugsweise im 
 vorderen lateralen Theil der Retina zu finden sind; 
 
 B, dreistrahlige Rhabdome bei alien Arten auCer Platybunus ; 
 
 C, kompakte dreiseitige Rhabdome von Phalangium, Acantholophus und 
 Oligolophus; bei den beiden letzten Gattungen vorzugsweise im hin- 
 teren Theil der Retina ; 
 
 Dy kompakte cylindrische Rhabdome von Platybunus. Die eine drei- 
 strahlige Figur bildenden Linien in den Rhabdomen stellen nur bei 
 Platybunus die Grenzen zwischen den Rhabdomeren dar, bei alien 
 anderen Arten aber sind sie die p. 24 beschriebenen Mittellinien. 
 
Zeitsch 7 
 
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Tall. 
 
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 10. H? 
 
 11. H-° 
 
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ON THE 
 
 . ... 3 ^ 4 'f 
 
 v 
 
 
 MORPHOLOGY 
 
 OF THE 
 
 Compound Eyes oe Arthropods. 
 
 BY 
 
 S. WATASE, 
 
 FELLOW OF THE JOHNS HOPKINS UNIVERSITY* BALTIMORE. 
 
 j'.» Si , 
 
 Reprinted from the “ Studies from the Biological Laboratory, Johns Hopkins University, ' 
 
 Vol. IV., No. 6. 
 
 BALTIMORE : 
 
 PRESS OF ISAAC FRIEDEN W ALI), 
 
 32 S. Paca Street. 
 
ON THE MORPHOLOGY OF THE COMPOUND 
 EYES OF ARTHROPODS. By S. WATASE, Fellow of 
 the Johns Hopkins University. With Plates XXIX-XXXV. 
 
 Contents. Page. 
 
 I. Introduction 287 
 
 II. Morphology of the Ommatidium. 
 
 a. Serolis 290 
 
 b. Talorchestia 295 
 
 c. Cambarus 297 
 
 d. Homarus 299 
 
 e. Callinedes 299 
 
 III. Compound (Lateral) Eye of Liinulus. 
 
 a. General Sketch of the Compound Eye 301 
 
 b. Structure of the Ommatidium 302 
 
 c. Development of the Compound Eye 307 
 
 IV. Phytogeny of the Ommatidium 313 
 
 V. Summary 323 
 
 VI. Appendix : The Compound Eye of Echinoderms 324 
 
 VII. Explanation of Figures 327-334 
 
 I. — Introduction. 
 
 Early in the spring of 1888, Prof. W. K. Brooks kindly placed 
 at my disposal a valuable set of specimens of Liinulus ,* with a 
 suggestion that I should work on the eyes in order to continue 
 the research upon which the late Dr. A. T. Bruce was engaged 
 at the time of his death. 
 
 Through the kindness of Prof. M. McDonald, the United States 
 Commissioner of Fish and Fisheries, I was enabled to continue 
 my work on the subject during the summer of the same year at 
 the Woods Holl Laboratory of the Fish Commission, where the 
 fresh specimens for the study of Limulus at all stages of develop- 
 ment may be obtained without difficulty. To these gentlemen 
 are due my sincere thanks and gratitude ; through their kind- 
 ness and encouragement I have been enabled to prosecute my 
 
 1 These specimens were mostly preserved by Dr. J. C. Hemmeter, when the 
 Chesapeake Zoological Laboratory was in session at Beaufort, N. C., in 1885. 
 They were in an excellent state of preservation. 
 
288 
 
 S. WATASE. 
 
 work. I am also indebted to Prof. John A. Ryder and Dr. E. A. 
 Andrews for various courtesies. 
 
 It soon became evident that the eye of Limulus is, as we 
 might expect from its great antiquity, a very primitive visual 
 organ, presenting in a simple form a most interesting type. 
 
 For the further comprehension of its structure and significance 
 it became necessary to extend my researches to several other 
 Arthropods. Among the forms studied in this connection are 
 the following, besides five other pelagic Arthropods from the 
 Gulf Stream, whose generic names have not yet been ascertained : 
 Pranchipus , Estheria , Talorchestia,Hyperia , Phronima , /Serolis, 
 Ligia , Asellus , Notonecta , Libellula , Agmon , Gryllus , Corethra , 
 Cambarus , Homarus , Hippg, Alpheus , Gebia , Penaeus , Squilla, 
 Callinectes , Gonodactylus and Lucifer. 
 
 While only a few selected types are here described, all the 
 above named forms, representing three great groups of Arthro- 
 pods, Insects, Crustacea and Arachnids, were carefully studied, 
 and all the results have been embodied in the general observa- 
 tions which follow, although the principal subject of the paper is 
 the eye of Limulus. 
 
 The points which are at present receiving marked attention 
 among the workers on the morphology of the compound eyes 
 of Arthropods may be put in four principal categories, viz. 
 (1) the mode of termination of the optic nerve fibre in the 
 retinal cell; (2) the mode of formation of different “layers” in 
 the retina; (3) the phytogeny of the compound eye; and (J) the 
 homology of the optic ganglia. 
 
 The problem therefore involves several inquiries. The funda- 
 mental step is to determine to what morphological category 
 the compound eye of the Arthropod belongs. To the elucidation 
 of this point our histological inquiries must be directed, while 
 our phylogenetic consideration of the eye may fitly be reserved 
 until we have learned something of its structure. 
 
 The first thing which confronts us is this : What is the 
 ommatidium or “ eyelet,” the repetition of which, often several 
 thousandfold, gives rise to the compound eye? What is the 
 significance of the characteristic arrangement of its component 
 cells ? Is it possible to reduce it to a simpler form, or to express 
 its structure and significance briefly ? 
 
COMPOUND EYES OF ARTHROPODS. 
 
 289 
 
 Until these questions be answered satisfactorily, the most 
 laborious technique of histology is often powerless to discrim- 
 inate the essential from the secondary features in the component 
 cells of a given ommatidium ; and in the absence of an accurate 
 conception of its fundamental structure, the various modifications 
 of the cells and fibres of an ommatidium may be made to assume 
 any significance to suit our speculative ideas. 
 
 With these considerations in mind, I have sought for an 
 ommatidium in which all the features of its structural elements 
 may be made out plainly, and from wdiich we may derive the 
 ommatidial elements of a more complex type. 
 
 Although the number of forms studied in this connection is 
 not quite so extensive as I wish, I feel justified in believing that 
 any view which explains the structural peculiarities of the 
 ommatidia of each and all of the forms in the list above given 
 and reduces them to the same fundamental structure, may fairly 
 be considered as in all probability generally applicable to the 
 eyes of Arthropods. 
 
 In what follows, the attempt will be made to approach the 
 subject from this standpoint, viz. the consideration of the 
 ommatidium as the morphological unit of the compound eye in 
 Arthropods, just as each little circle of rods with a cone in its 
 centre may be considered as the morphological unit of the 
 “ mosaic layer ” (Henle) of the human retina. 
 
 II. — The Morphology of the Ommatidium. 
 
 Following the plan given in the Introduction, I will consider 
 the morphology of the ommatidium, taking it as the structural 
 unit of the compound eye. After the nature of this unit has been 
 considered, we may proceed to examine the state of aggregation 
 of the units in the different forms of the Arthropods. For this 
 purpose it is convenient to select one particular ommatidium 
 which may be made the basis of comparison. Accordingly, the 
 ommatidium of Serolis will be considered at the outset, and the 
 attempt will be made to homologize the ommatidial elements of 
 Serolis with those of other forms. I am indebted to Prof. M. 
 McDonald for the opportunity to study the eyes of the four 
 South American forms of Serolis recently brought back by the 
 U. S. Fish Commission steamer Albatross. 
 
290 
 
 S. WATASE. 
 
 a. Serolis. 
 
 (Figs. 1, la, lb, 2, PI. XXIX.) 
 
 The general structure of the eyes of Serolis has already been 
 made known by Beddard. 1 Although I have verified all the 
 chief results of his research, I shall speak of a few details which 
 seem to me specially significant. 
 
 The cells of the ommatidium are arranged in three principal 
 strata, a , b, c (Fig. 1 a, PL XXIX). Each stratum of cells is 
 characterized by the special product which it secretes, viz. 1, 2 
 and 3. 
 
 The outermost stratum (a) secretes upon its external surface 
 the chitin wdiich constitutes the cornea or the corneal facet ( c , 
 Fig. 1, PI. XXIX). 
 
 The cells themselves have been designated as the corneagen by 
 Patten. 2 
 
 The corneagen is sometimes difficult to detect in the eye of 
 the adult animal and may easily be overlooked. It is very 
 conspicuous in the young. 
 
 The next stratum of cells (b) is also characterized by the 
 chitinous secretion on the surface nearest the median axis of the 
 ommatidium. The two cells (b) secrete a thick chitinous layer 
 which encloses an ovoidal space of considerable size filled with a 
 transparent liquid substance, (2). The cell (b) has been named 
 vitrella (Lankester and Bourne); 3 4 and the ovoidal chitinous body 
 (2) is the crystalline cone 4 ( c . c , Fig. 1, PI. XXIX.) 
 
 1 Beddard, F. E. Challenger Report, Zoology, Vol. XI, Isopoda: Serolidae. 
 His additional observations on Serolis may be found in the paper by the same 
 author, On the Minute Structure of the Eye in Gymothoidae, Trans. Roy. Soc. 
 Edin. 1887. 
 
 2 Studies on the Eyes of Arthropod : I. Development of the Eyes of Vespa, 
 with observations on the Ocelli of some Insects, p. 193, Journal of Morphology, 
 Yol. I, No. 1, 1887. 
 
 3 The Minute Structure of the Lateral and the Central Eyes of Scorpio and 
 Limulus. Quart. Jour, of Micro. Science, Yol. XXIII, p. 198, 1883. 
 
 “ Vitrella ” is synonymous with “ retinophora ” of Patten ; “ vitreous cell ” 
 and “crystalline cone cell” of authors. The nucleus of the vitrella has been 
 named byClaparede “Semper’s nucleus,” after its discoverer : Zur Morphologic 
 der zusammengesetzten Augen bei den Artliropoden. Von Edouard Claparede. 
 Zeit. f. Wiss. Zool. Bd. X, p. 193, 1860. 
 
 4 It is interesting to notice that the observations made by Clarke on the eye of 
 a Trilobite ( The Structure and Development of the Visual Area in the Trilobite, 
 
COMPOUND EYES OF ARTHROPODS. 
 
 291 
 
 The crystalline cone is purely a dioptric apparatus. A careful 
 study of its structure, both in the adult and in the young embryos 
 of Serolis , has clearly shown that this has nothing to do with the 
 sensory function, as it has no connection whatever with the optic 
 nerve fibres. 
 
 The vitrella, then, agrees wfith the corneagen cell in the 
 capacity for secreting chitin on one portion of its surface. The 
 difference between them consists in this, that while the corneagen. 
 cells secrete chitin towards the exterior of the body, the vitrellae 
 secrete it towards each other, that is, towards the median axis of 
 the ommatidium. 
 
 Next below the vitrella comes the important stratum (<?,Fig.l&) 
 which lies at the bottom of the whole structure, and which sends 
 processes inward {Op. n) to form the optic nerve fibre, which— 
 terminates in the optic ganglion. 
 
 The cells in this structure are also characterized by secreting 
 chitin towards the median axis of the ommatidium. The chitin 
 thus produced invests the colorless portion of the cell ( c ) consti- 
 tuting the rod or the rhabdomere (Lankester and Bourne ). 1 
 
 A large number of transverse striae, running at right angles to 
 the longitudinal axis of the ommatidium, are seen through the 
 transparent cuticle. The cell itself which gives rise to the 
 rhabdomere has been known as retinula (Grenacher ). 2 The 
 
 Phacops Rana, Green, Journal of Morphology, Vol. II, No. 2, 1888) show, so 
 far as I understand his account, that the lens has a somewhat similar structure 
 to the crystalline cone of Serolis, or, in fact, to the crystalline cone of Isopod 
 Crustacea, as far as it is known. From Clarke’s description I gather that the 
 lens of Phacops is unequally biconvex, the curvature being the greatest on the 
 proximal surface ; and that the lens was hollow , probably filled with some viscid 
 humor. In the absence of a more complete knowledge on the nature of a corneal 
 covering to the eye in the Trilobite it is premature to carry out the homology of 
 the “ crystalline cone ” of Serolis and of the Isopods in general to the “ lens ” 
 of a Trilobite, although there appears to exist a certain resemblance between the 
 two. It is perhaps worthy of mention, in this connection, that there is nothing 
 whatever in the lens-cone of Limulus at any period of its development which 
 shows any resemblance to the lens of a Trilobite as made out by Clarke. The 
 lens-cone of Limulus is the solid, conical projection of the chitinous cuticle of the 
 body , fitting into the open depression of the shin, and in no period of the life 
 history of the animal is it separated from the outermost cuticle of the body. 
 (Compare Grenacher, Lankester and Bourne, and Part III of the present paper 
 on the compound eye of Limulus.) 
 
 1 loc. cit. p. 183. 
 
 2 Untersuchungen uber das Sehorgan der Arthropoden, 1879. 
 
292 
 
 8. WATASE. 
 
 retin ula, then, agrees with the corneagen and vitrella which lie 
 above, in its capacity to secrete chitin on a part of its surface. 
 Thus the three strata of cells, a , b, and <?, agree fundamentally 
 with one another in their capacities to secrete chitinous structures, 
 and these are respectively known as the cornea (1), the crystal- 
 line cone (2), and the rhabdomere (3). 
 
 Hence, aside from the general homology which exists between 
 the cells in the optic area, all being derived from a common 
 source, the ectoderm, there exists a special homology between 
 the cells in the three strata which constitute the important parts 
 of the ommatidium, all being characterized by the capacity for 
 secreting chitin from a part of their surface. 
 
 In this respect these ommatidial cells are essentially like the 
 general ectodermal cells which secrete the chitinous covering of 
 the body from their distal extremities. 
 
 In carrying this analysis further we find that the outer surface 
 of the corneagen is homologous with the axial surface of the 
 vitrella, and that of the retinula and the three chitinous structures, 
 cornea , crystalline cone and rhabdomere , are homologous with 
 each other. 
 
 If this interpretation be established by the further discussion 
 of the ommatidia of other forms, it follows that the crystalline 
 cone and rhabdomere are really homologous with the chitinous 
 investment of the body, and the narrow axial space of the 
 ommatidium represented by the line x must therefore have the 
 same significance as the space outside of the body. . Morpholo- 
 gically speaking, the crystalline cone and rhabdomere are just 
 as much a part of external surface as the cornea itself. Diagram- 
 atically, the three strata of the ommatidium may be represented 
 as in Fig. 2, PI. XXIX. 
 
 According to this view, the ommatidium of Serolis may be 
 regarded as an open pit of the ectoderm, the walls and margin 
 of which are covered by the product of their own secretion, the 
 chitin. The chitin in the walls and margin of the pit becomes 
 continuous with the general chitinous covering of the body, as 
 the rows of the ommatidial cells themselves become continuous 
 with the general ectoderm. 
 
 At the bottom of the ommatidial pit there exists a pair of 
 colorless “ hyaline cells” (Beddard), with their upper processes 
 
COMPOUND EYES OF ARTHROPODS. 
 
 293 
 
 inserted into the axial space between the four rhabdomeres. 
 Each cell has a peculiar refractive nucleus. It lacks any con- 
 nection with the optic nerve fibre, and is suspended from the 
 bottom of the pit at a considerable distance above the basement 
 membrane. The “hyaline cell” is a modified ectodermic cell, 
 and is homologous with the rest of the cells which enter into 
 the formation of the ommatidium. The cell, however, does not 
 develop pigment granules in itself, nor establish any connection 
 with the central nervous system as does the retinula cell, nor 
 does it secrete chitin on a part of its surface as do the rest of the 
 ommatidial cells we have considered already. As to the function 
 of this cell I am not able to say anything definitely. It cannot 
 be sensory, having no connection with the nerve centre. When 
 we see, however, that in some species of Serolis the lower ends of 
 the four rhabdomeres are deeply imbedded in the substance of the 
 “ hyaline cell,” or, in other words, that the rhabdomeres are 
 firmly held together by the substance of the “hyaline cell” 
 at the point where they meet, it becomes probable that the cell 
 in question may serve the purpose of a mechanical support, 
 adding firmness to the whole structure. To this point I will 
 recur later on. 
 
 In addition to the cells which form the essential part of the 
 ommatidium there exists a number of pigmented cells, ( pg . c. 
 Fig. 1 a, PI. XXIX). The pigment cells are greatly developed 
 around the vitrellae and closely invest the dioptric portion of the 
 ommatidium from outside. These pigmented cells also are modi- 
 fied ectoderm cells which lie between the adjacent ommatidial 
 pits. 
 
 The morphological unit of the compound eye of Serolis may 
 then be described as consisting of a group of ectodermic cells 
 lying outside of the basement membrane, and so arranged as to 
 form the walls of an open tubular depression. This group of cells 
 undergoes special differentiations at different levels of the pit. 
 The uppermost group, consisting of two flattened cells, consti- 
 tutes the corneagen, which secretes the cornea externally; the 
 second group of cells, consisting of two large cells, the vitrellae, 
 secretes chitin towards the lumen of the tube, thus forming the 
 crystalline cone; the third group of cells, the retin ulae, con- 
 sisting of four cells, secretes the rhabdomeres. The retinulae 
 
294 
 
 S. WATASE. 
 
 are the only cells which are connected with the nerve centre. 
 In addition to the three elements above named, the fourth 
 element, in the form of two “ hyaline cells,” exists, plugging, as it 
 were, from the inside the imperfect bottom of the depression. 
 A number of pigment cells enveloping more or less completely 
 the above mentioned group of cells from the outside constitutes 
 the fifth element of the ommatidium. 
 
 A glance at the diagram (Fig. 1 b, PI. XXIX) will make 
 this point clear. The diagram is supposed to he the plan of the 
 constructive elements of the ommatidium of Serolis as they 
 would appear when seen from the exterior, in the direction of 
 the arrow, Fig. la, PI. XXIX, if they actually formed an 
 open pit like that shown in Fig. 2. 
 
 The innermost body which appears in the depression is the pair 
 of “hyaline cells,” ( d)\ next above and therefore in the outer 
 circle to ( d ) are the retinulae ( c ), the rhabdomeres being repre- 
 sented by the yellow edges. Above the retinulae will come the 
 two extremely enlarged cells, the vitrellae (Z>), whose chitin- 
 secreting surface is marked by the yellow color. Next above (h) 
 comes a pair of corneagen cells (a), whose chitin-secreting surface 
 is also represented by the yellow color as in the cells forming the 
 two inner series. The outermost of all is a pair of extremely 
 flattened, pigmented cells, ensheathing completely the dioptric 
 portion of the ommatidium. 
 
 The diagram is not unlike the one used in illustrating the 
 arrangement of the different parts of a flower. This does not 
 seem surprising when we remember that the nature of the prob- 
 lem in both cases is identical. Just as sepals, petals, stamens and 
 pistils of a flower are considered as the modified leaves clustered 
 round an extremely short stem or axis in whorls or in spirals, so 
 the corneagen, vitrellae, retinulae, etc., are the modified ectoderm 
 cells arranged at different heights on the ommatidial axis. There 
 is, however, this difference between them, that while in a flower 
 the parts concerned in the formation of the organ are themselves 
 aggregations of many cells, those in the Arthropod ommatidium 
 are highly differentiated individual cells; and while in the case 
 of the flower it is the clustering of different parts around an 
 extremely shortened projecting axis, in the ommatidium of the 
 Arthropod it is the distribution of parts along an elongated and 
 sunken one. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 295 
 
 Although the subject will be much more intelligible after • 
 other types have been described, it will not be altogether out of 
 place here to mention briefly my view of the steps through which 
 the characteristic arrangement of parts in an ommatidium like 
 that of Serolis might have been brought about. Suppose a 
 circular area of the skin to be divided into five zones, one lying 
 within the other. Mark the innermost circle as (1), and the next 
 zone as (2), and the middle as (3), the fourth as (4) and the outer- 
 most as (5). Suppose, further, this circular area of the flat 
 ectodermal surface of the body to sink down as a conical pit 
 with its open base turned towards the exterior. The cell or 
 cells lying in the innermost circle (1) in the middle of the area 
 will sink deepest, while the outermost circle (5) wflll retain its 
 original level. The cells lying in the innermost circle (1) are 
 the “hyaline cells”; those lying within the second zone (2) are 
 the retinulae ; those lying in (3) are the vitrellae; those within 
 (4) are the corneagen, and those within (5) are the group of 
 pigmented cells which surround the dioptric portion of the 
 ommatidium. 
 
 All that is necessary to convert this diagram into the omma- 
 tidium of Serolis is to reduce the lumen of the pit until it finally 
 disappears and the cells facing the lumen of the tube come into 
 contact with each other, or else remain separated by the chitinous 
 structure; a faint line like that shown by x in Figs. 1, 3 and 4 
 being all that remains of the axial cavity, like that shown in 
 Fig. 2. 
 
 The compound eye of Serolis is in this view nothing but a 
 collection of these depressions in the skin, which by virtue of 
 their aggregation attain the morphological and physiological 
 value of an organ. 
 
 b. Talorchestia. 
 
 (Figs. 3, 3 a, PI. XXIX.) 
 
 A glance at the figures will at once show the fundamental 
 similarity of the ommatidium of this Amphipod and that of 
 Serolis. Fig. 3, PL XXIX, shows a single ommatidium of 
 Talorchestia. The outermost covering c is the corneal facet. 
 The tw r o cells which lie beneath it are the corneagen ( c . g). 
 These cells are not so conspicuous in the adult as in the young 
 
296 
 
 S. WATASE. 
 
 animals; nevertheless, with care one can easily make them out. 
 Beneath the corneagen comes the stratum consisting of two cells, 
 the vitrellae, which secrete an enormous cuneiform, chitinous 
 structure, the crystalline cone (e. c) with its more pointed end 
 turned inward. The crystalline cone consists of halves, each 
 corresponding to a single vitrella cell which secretes it on its 
 median, axial surface. The line x, as in Serolis , shows the 
 morphological lumen of the ommatidial cavity. 
 
 Next below this stratum comes the group of four cells, the 
 retinulae (jRt). The chitinous edges of the retinulae meet in the 
 median axis and form the rhabdom {Bb). Transverse striations, 
 rather broad and coarse in their nature, are seen through the 
 transparent cuticle, the rhabdomere. At the lower part of a 
 retinula cell, near the basement membrane, is situated its nucleus 
 ( n ); it may be found below the membrane. 
 
 The retinula cell undergoes a curious modification at the upper 
 end. Each of the retinula cells sends out a flattened process 
 which is highly pigmented and which completely ensheaths the 
 dioptric portion of the ommatidium from the outside. In fact, 
 the place of pigment cells ( pg . c) in Serolis is taken by the 
 modified upper expansion of the retinula cells. Outside of this 
 pigmented sheath there exists a number of non-pigmented, 
 elongated ectodermal cells {p. e ) which pack the interspaces 
 between the adjacent ommatidia. 
 
 If we plot out the whole structure into a diagram, in the same 
 way as we did the ommatidia of Serolis , it will assume the 
 appearance shown in Fig. 3 a, PL XXIN. The innermost cells, 
 the u hyaline cells ” of Serolis , are not represented ; the cells in 
 the second circle are greatly developed meeting one another 
 with their chitin -secreting edges, the rhabdomeres, while the 
 upper extremity of each retinula cell envelops the crystalline 
 cone from the outside, thus physiologically taking the place of 
 pigment cells, {pg. c) or 5 in Serolis , Fig. 1 b, PL XXIX. 
 
 In the third circle the two vitrellae are found with their chitin- 
 secreting surface marked yellow. The cells in the fourth circle 
 constitute the corneagen ( c . g)\ The fifth circle of cells is 
 represented by the packing cells {p. c). 
 
COMPOUND EYES OF ARTHROPODS. 
 
 297 
 
 c. Cambarus. 
 
 (Fig. 4, PL XXIX; Fig. 35, PL XXXI; Figs. 71, 79, Pl. XXXV.) 
 
 In Fig. 4, PI. XXIX, three ommatidia of Cambarus are 
 represented, a , b and c. The ommatidium may be described as 
 in the two preceding cases, as consisting of three strata of cells, 
 each stratum characterized by the special chitinous product 
 which it secretes. The outermost stratum of cells, the corneagen 
 (c.g) 1 2 secretes a slightly biconvex corneal facet ( c ). 
 
 Next below the corneagen comes the stratum of vitrellae, 
 consisting of four cells, each with an extremely elongated internal 
 process. The median surface of the vitrella which corresponds to 
 the chi ting- secreting surface of the vitrella in Serolis and Talor- 
 chestia secretes a perfectly transparent, homogeneous substance, 
 rather refractive in its nature, forming the crystalline cone 
 ( c . c). I cannot say anything of the chemical nature of this 
 substance in its relation to the chitin. The cells which secrete 
 this substance in the crayfish are morphologically identical with 
 the vitrellae in Serolis and Talorchestia which secrete chitin, 
 and the use this body subserves in the ommatidium of the cray- 
 fish is identical with that of the chitinous body, the crystalline 
 cone, in the two Arthropods we have already examined. There 
 can be no question of the homology of this body to the crys- 
 talline cone of other Arthropods. I have therefore represented 
 this body in the yellow color as in Serolis and Talorchestia. 
 The outline of the vitrella cell in the figure has been made 
 diagramatic to a certain extent. 
 
 Next below the vitrellae come the retin ulae, the whole being 
 aggregated into a spindle-shaped bundle.^ On the distal end of 
 
 1 Reichenbach ( Studien zur Entwicldungsgeschichte des Flusskrebses, p. 93) 
 speaks of these cells as “ Sernper’schen Zellen.” In examining his figure 
 (Fig. 225, Taf. XIV) we find what he regards as “ Semper’s cells ” are those with 
 “ Semper’schen Kerne” (S. K.) The term “Semper’s nucleus” as originally 
 used by Claparede and adopted by others does not mean the nucleus of the 
 corneagen, as Reichenbach uses it, but means that of the vitrella. In fact the 
 existence of corneagen has been ignored by several earlier writers. It was 
 Patten who emphasized its importance. 
 
 2 This spindle stratum does not come from any part of the “ augenfalte ” of 
 Reichenbach, but arises in the region where he locates the ectodermal and 
 mesodermal pigment cells, loc. cit. Fig. 225, Taf. XIV. 
 
298 
 
 S. WATASE. 
 
 the rhabdom the proximal ends of the vitrellae apparently 
 terminate. 1 
 
 When the spindle is macerated and each of its component 
 elements is examined individually, it is found that each retinula 
 develops on its median surface a thick chitinous border which 
 shows a wavy, corrugated appearance. This chitinous border 
 attains its greatest thickness in the midst of the retinula cell (Fig. 
 35, PL XXXI). When the chitin-secreting edges of a number 
 of these retinula cells meet they form a spindle-shaped bundle, 
 the thickest part of the rhabdom ere corresponding to the most 
 bulging part of the spindle. The lower extremities of the 
 retinula cells Send out nerve processes, each of which forms an 
 optic nerve fibre, ( Op.n , Fig. 4, PL XXXI). Fine elongated 
 cells filled with yellow pigment granules are found between the 
 adjacent retinula bundles (ep, Fig. 4). They are also the modi- 
 fied ectodermal cells. The pigment cells which occur around 
 the vitrellae form a complete envelope around the dioptric 
 portion of each ommatidium. 
 
 Thus even with this complicated type of ommatidium the 
 plan of structure is fundamentally alike to that of Serolis. The 
 central element, the “hyaline cells” of Serolis, is not represented 
 in the crayfish. To this special point I shall recur. 
 
 If w r e plot out the ommatidial elements of the crayfish as we 
 did those of Serolis , exactly a similar kind of diagram will be 
 formed. Thus in the centre will come the group of retinulae, 
 
 1 Parker found in Homarus (A Preliminary Account of the Development and 
 Histology of the Eyes in the Lobster . Proceedings of the American Academy, 
 Vol. XXIV, pp. 24-25, 1888) that the lower extremities of vitrellae run over 
 the spindle, and that each terminates on the retinal side of the basement mem- 
 brane. It is probable that the same is true in Cambarus. This fact is perfectly 
 intelligible according to the view which is maintained in the present paper, for 
 the cells found in the higher level of a given ommatidium are morphologically 
 situated in the outside of those found in the lower. Patten’s account ( Eyes 
 of Molluscs and Arthropods, Mittheilungen aus der Zoologischen Station zu 
 Neapel, Bd. VI, Heft 4, 1886) that the rhabdom is the continuation of the 
 retinophorae (vitrellae) is not true. I macerated the “spindle” of Penaeus, 
 upon which Patten bases his observations, and found that it is composed of a 
 number of pigmented retinula cells, each with its chitinous border as in 
 Cambarus (Fig. 35, PL XXXI) or in Homarus (Fig. 34, PI. XXXI). Grenacher 
 is right in regarding the rhabdom as the secretion product of the retinula 
 cells. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 299 
 
 consisting of seven cells ; then come in the outside four vitrellae 
 arranged in a square ; then two corneagen cells ; and outermost 
 of all, four pigmented cells which completely envelop the 
 dioptric part of the ommatidium. 
 
 d. Horn ar us. 
 
 (Fig. 34, PI. XXXI.) 
 
 The compound eye of Homarus has quite recently been studied 
 by Parker. 1 I do not find any facts in Parker’s paper which 
 contradict the view maintained in the preceding pages in regard 
 to the nature of the ommatidium. The threefold differentiation 
 of the ectodermal cells in the walls of the ommatidium is just as 
 evident here as in the forms already considered. The problem- 
 atical body, the spindle (Grenacher’s rhabdom), like the corres- 
 ponding structure in Cambarus , resolves itself into a number of 
 individual retinula cells with thick, serrated, chitinous secre- 
 tions as in the case of Cambarus and Penaeus. 
 
 In Fig. 34, PI. XXXI, four retinula cells are partly isolated 
 from one another. The lower end of each retinula cell is pro- 
 longed into an optic nerve fibre, while the upper end is devoid 
 of any pigment granules. A large translucent nucleus exists 
 near the distal extremity of the retinula. The distal end of each 
 cell is prolonged into a short, clearly defined, tapering process. 
 The development of the chitinous serrature is greatest in the 
 middle of the chitin-secreting border of the retinula ; when,- 
 therefore, the whole is brought together into a bundle, the 
 general outline of such a structure is spindle-shaped. 
 
 e. Callinectes. 
 
 (Figs. 5, 5a, PI. XXIX; Fig. 37, PI. XXXI; Fig. 72, PI. XXXV.) 
 
 Callinectes presents the same interesting modification which 
 we saw in Talorchestia. We find the corneagen cells and the 
 vitrellae in two successive strata, but no special pigment cells 
 embracing this dioptric mechanism exist. The packing cells 
 which we saw in the intervening spaces of the ommatidia in 
 Talorchestia are not found in Callinectes. 
 
 1 G. H. Parker : A Preliminary Account of the Development and Histology 
 of the Eyes in the Lobster. Proc. Amer. Acad. Yol. XXIV, 1888. 
 
300 
 
 S. WATASE . 
 
 The third stratum of cells, the retinulae, are extremely 
 elongated, each retinula with a clear, straight, finely striated 
 rhabdomere. A somewhat diagramatic representation of the 
 individual retinula cell with its chitinous border is given in 
 Fig. 37, PL XXXI. In Fig. 5 a, PI. XXIX, the upper portions 
 of the two retinula cells are shown ; the rhabdomere just beneath 
 the crystalline cone shows a slight enlargement. The size of the 
 retinula cell at the distal extremity undergoes an enlargement. 
 The enlarged extremities of the retinulae form a pigmented 
 collar around the inner end of the crystalline cone. If the 
 upward extension of this part of the retinula cells be carried 
 still further, a condition which prevails in a retinula like that of 
 Talorchestia will be found. In the freshly teased preparation, 
 one or more refractive globules, ranging from yellow to reddish 
 brown in color, are found in the thickened portion of the 
 retinula. The general shape of the ommatidium as determined 
 by the shape of the corneal facet is hexagonal. 
 
 For the general discussion of the subject I do not find it 
 necessary to go into further details of comparison, nor to enume- 
 rate more examples from different Arthropods. In the forms I 
 have studied in this connection, representing near thirty Arthro- 
 pod genera, I found nothing which invalidated the general inter- 
 pretation of the ommatidial structure of the Arthropod as 
 explained in the preceding pages, and the compound eye of the 
 Arthropod, whether it be sessile or stalked, must be held to 
 represent morphologically a group of ectodermal pits or a 
 bundle of ectodermal tubules. 
 
 III. — The Compound (Lateral) Eye of Limulus. 
 
 The question which next suggests itself is this : Adopting for 
 a moment the view that the ommatidium of the compound eye 
 is morphologically an ectodermic pit, is there any adult arthro- 
 pod whose eye permanently remains in this condition ? 
 
 I find such an ommatidium in the compound eye of Limulus . 
 
 On this account I will describe the eyes of Limulus somewhat 
 in detail. 1 In the succeeding section of the paper I will point 
 
 1 An outline of this portion of the paper will be found in the March number 
 of the University Circular , 1889, Johns Hopkins University, under the title 
 The Structure and Development of the Eyes of the Limulus. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 301 
 
 out how an ommatidium like that of Eimulus may be converted 
 into one of a more complex type. 
 
 a. General Sketch of the Compound Eye. 
 
 The compound eye of Eimulus is placed in the dorso-lateral 
 angle of the prosomatic shield. In the fully grown animal the 
 outline of the eye is bean-shaped, with its longer axis parallel to 
 the longitudinal axis of the body. This bean-shaped area is 
 slightly protuberant from the surrounding level ; the chitinous 
 covering of the body is thinner, softer, and more translucent in 
 the region of the eye than on the rest of the dorsal surface. 
 Through the translucent chitin we see a large number of circular 
 spots, all of a uniform size. On further examination we find 
 that each spot corresponds to a conical projection of the chitin, 
 which fits into a cavity in the skin. This conical thickening of 
 the chitin constitutes the lens or lens-cone , and the group of cells 
 forming the walls and bottom of the open cavity constitute the 
 essential part of the ommatidium. 
 
 The cells which form the walls of the pit are elongated and 
 highly pigmented, forming the perineural cells (p. n , Fig. 6, 
 PL XXIX) ; those cells which form the bottom of the pit 
 undergo extreme enlargement compared with the perineural 
 cells, and elongate along the longitudinal axis of the pit. They 
 are grouped in a characteristic manner, reminding one of the 
 arrangement of cells in the taste-bulb of a vertebrate. This 
 bulb-like group of cells is the essential part of the retina. In 
 my preliminary account of the subject 1 I called this group of 
 cells an ommatidium. Properly speaking, the term ommatidium 
 includes the whole group of cells forming the walls and bottom 
 of the pit, together with the lens-cone and the thick chitinous 
 covering corresponding to the area occupied by a single pit. In 
 this latter sense the term ommatidium will be used in the 
 present paper, and the bulb-like group of cells at the bottom of 
 the pit will be designated as the sensory part of the ommati- 
 dium. 
 
 In the sensory part of the ommatidium, which consists of two 
 portions, a central a and a peripheral b , the arrangement of the 
 cells is quite regular. 
 
 1 loc. cit. p. 70. 
 
302 
 
 S. WATASE. 
 
 The central part always consists of a single ganglion cell 
 ( G , Fig. 6, PL XXIX), and the peripheral of the rod-bearing 
 pigmented cells (. Rt , Fig. 6) which surround the former in the 
 centre. As to the minute structure of these elements I shall 
 speak later on. The cells composing the sensory portion of the 
 ommatidium send out from their proximal ends nervous processes, 
 which, forming a bundle, are seen to emanate from the basal end 
 of the ommatidium. The number of nerve fibres at the begin- 
 ning of the bundle and that of the cells composing the sensory 
 bulb of the ommatidium always correspond. These bundles, 
 however, soon break up as they penetrate deeper, and become 
 mixed up with the fibres from the neighboring bundles, forming 
 a complex plexus underneath the ommatidial area. Scattered 
 in the plexus are found a number of thickenings, which, in gold- 
 chloride preparations, appear as darkly stained masses of proto- 
 plasm occurring at the nodes of junction of the several inter- 
 crossing fibres (Figs. 44 and 45, PI. XXXII). In the fresh 
 state these enlargements contain a variable amount of yellow 
 granules imbedded in the mass of the protoplasm. In the 
 figures above referred to, mere outlines of such thickenings are 
 given. 
 
 After the nerve fibres have formed the plexus they again 
 come out in bundles, and piercing through the perforations in the 
 basket-like chitinous support which underlies the ommatidial 
 region, travel forward, slightly inward, then downward and back- 
 ward until they terminate in the optic ganglion in the brain. 
 
 b. The Structure of the Ommatidium. 
 
 In the general sketch of the compound eye of Limulus above 
 given, the structure of its morphological unit, the ommatidium, 
 has been briefly described. It consists of two parts, (1) the 
 sensory and (2) the dioptric part, which is also partly protective. 
 
 The sensory part consists of two factors, (a) the central ele- 
 ment or the ganglion cell (6r, Fig. 6, PI. XXIX), and ( b ) the 
 peripheral elements or the retinulae {JEt, Fig. 6). These two 
 kinds of cells are the only ones in which nerve fibres terminate. 
 
 The retinula cells group themselves in the shape of a bulb, or 
 something as the several segments do in an orange, each segment 
 corresponding to a single rod-bearing cell or retinula (Fig. 10, 
 
COMPOUND EYES OF ARTHROPODS. 
 
 303 
 
 Et , PI. XXX). Along the axial side of the retinula cell there 
 exists a definite longitudinal tract which is free from the pigment 
 granules. On the outside of this colorless region there exists a 
 thin layer of chitin. This chitinous covering on the outside of 
 the retinula constitutes the rod or the rhabdomere (Eb, Fig. 10, 
 PL XXX). When the non-pigmented portion of the retinula 
 cell is examined with a high power, we find the whole structure 
 to be transversely striated. This striated appearance seems to be 
 produced by the existence of an enormous number of transverse 
 fibrils lying in the inside of the chitinous cuticle. That these 
 striae are due to the existence of independent fibrils is further 
 shown by the fact that by careful focusing up and down of the 
 microscope some striae can be directly traced out beyond the 
 limit of the chitinous covering, in the form of independent fibrils 
 (Tr.fb, Fig. 10, PL XXX). 
 
 When we make a transverse section of the retin ulae, each cell 
 will present a wedge-shaped outline, with its pointed end turned 
 toward the axis of the ommatidium ; the non-pigmented part of 
 the cell in its relation to the cuticular secretion on the outside 
 will be seen from the diagram (Fig. 6a, b, Eb, PL XXIX). 
 
 The number of retinula cells entering into a single ommati- 
 dium is quite variable. Thus, Grenadier 1 counts fifteen ; Lan- 
 kester and Bourne 2 count ten ; I have found some with eleven, 
 others with nine ; in one ommatidium I counted as many as 
 nineteen. 
 
 I am not able to say under what conditions this numerical 
 variation takes place. Whatever significance there may exist in 
 this numerical variation, it does not alter in the least, as in the 
 case of different elements in other ommatidia we have already 
 considered, the general idea that the ommatidium of the Arthro- 
 pod is morphologically a depression in the skin. 
 
 Each ommatidium contains a single ganglion cell. Only in 
 one case did I see two ganglion cells in a single ommatidium. 
 The ganglion cell is situated morphologically in the centre of 
 the sensory part of the ommatidium. The retinula cells com- 
 pletely envelop this ganglion cell from the outside, in a similar 
 way as the rods encircle the cone in the centre in the retinae of 
 some vertebrates. 
 
 loc. cit. Fig. 125, Taf. XI. 
 
 Hoc. cit. Fig. 10, PI. XI. 
 
304 
 
 S. WATASE. 
 
 The main body of the ganglion cell where the nucleus is 
 situated is found in the lower part of the ommatidium, occupy- 
 ing in most cases an eccentric position. Figs. 13, 14 and 15, 
 PI. XXX, show three ganglion cells completely isolated from 
 the rest of the retinal cells. In Fig. 10, PI. XXX, the relation 
 of the ganglion cell to the retinula cell is well shown. 
 
 The ganglion cell is distinguished by constant and well marked 
 features from the rest of the ommatidial cells. The cell is 
 bi-polar, the one process going upward and outward through the 
 axial channel of the ommatidium (the axial canal of the rhab- 
 dom), and the other going downward and inward to the sub- 
 ommatidial plexus and thence to the brain. For the sake of 
 convenience the former will be designated as the axial process 
 {Ax.p, Figs. 13, 14, etc.), and the latter as the optic nerve process 
 {Op. n, Figs. 13, 14, etc.) of the ganglion cell. 
 
 The axial process gradually tapers to a narrow point. The 
 process consists of a bundle of extremely fine fibrils, which are 
 distinctly seen as a series of longitudinal striae. For some 
 mechanical causes, the distal process was seen split into three 
 main branches in one specimen (Fig. 15, PI. XXX); in another 
 specimen (Fig. 10), the topmost end of the axial process was seen 
 completely macerated into a number of component fibrils, the 
 whole showing a brush-like appearance. 
 
 The large, translucent, spherical nucleus {JV, Figs. 10, 13, 14, 
 15, PI. XXX), with its nucleolus, is found in the substance of 
 the cell body proper. The position of the nucleus is very often 
 eccentric, as in Fig. 14. The nucleolus is surrounded by con- 
 centric markings in all cases. Between the nucleus and the 
 proximal process there is always a larger or smaller patch of 
 pigment granules (. Pg.p , Figs. 10, 13, etc.). The color of the 
 granules ranges from light yellow to dark brown or even to jet 
 black. The yellow-colored pigment is diffused more or less 
 widely through the body proper of the ganglion cell. Even 
 in the same patch of pigment in the cell there are different 
 gradations in the color. From the nature of the drawing used 
 in the present paper I cannot show these gradations. Pigment 
 patches may exist in other parts of the cell, but they never form 
 so constant a characteristic as the group between the proximal 
 process and the nucleus. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 305 
 
 The third element which figures prominently around the 
 sensory cells of the ommatidium is the eipithelial cell. The cell 
 may either be pigmented or non-pigmented. In Fig. 11, Pl. 
 XXX, a group of four retinula cells is shown partly isolated 
 from one another. Within the interspaces between the adjacent 
 retinula cells will be found a number of pigmented epithelial 
 cells, extremely elongated, many of them extending the whole 
 length of the retinula cell. In Fig. 12, PI. XXX, a number of 
 highly attenuated epithelial cells (Ep) is shown closely clinging 
 to the outer surface of the retinula cell. In Figs. 10, 13, 14 and 
 17 highly attenuated epithelial cells (Ep\ both of the pigmented 
 and the non-pigmented kind, are shown, as they occur either 
 around the ganglion cell or around the retinula cell. 
 
 In no case are these epithelial cells found to establish any 
 organic connection with any part of the sensory cells or of their 
 processes. These attenuated epithelial cells do not differ in any 
 essential way from the ordinary ectodermal cells which are found 
 in the walls of the ommatidium or in the general ectoderm. 
 
 The epithelial cells as they further depart from the periphery of 
 the group of sensory cells, become stouter and thicker. One 
 end of each epithelial cell is usually devoid of pigment granules; 
 this non-pigmented end of the cell secretes the chitin which 
 forms the lens-cone. We have already seen that the portion 
 which secretes the chitinous covering, the rhabdomere, of the 
 retinula cell is devoid of pigment granules. In this respect the 
 epithelial cells which form the wall of the pit and the neuro- 
 epithelial cells which form the bottom of the pit closely agree 
 with each other. 
 
 A glance at the series of figures (Figs. 17-33, PI. XXXI) 
 will show the degrees of modification undergone by the epithe- 
 lial cells teased out from the different parts of the ommatidium. 
 Those cells (Figs. 17 and 18) which lie at the bottom of the pit 
 undergo the greatest modification. Fig. 17 is the central 
 ganglion cell of the ommatidium, with a few slender epithelial 
 cells around it. Fig. 18 is the retinula cell with a few pigmented 
 epithelial cells clinging to its side. Both differ from the rest of 
 the epithelial cells in their possession of nerve processes. In some 
 epithelial cells, as in Figs. 28 and 25, PI. XXXI, the degree of 
 attenuation is so very great that the protoplasmic character of 
 
306 
 
 S. WATASE. 
 
 the cell body is hardly recognizable. The cell body is repre- 
 sented by an extremely slender, bristle-like filament, with no 
 pigment granules in the inside. In others this hyaline cell body 
 is divided into a number of branches, as shown in Figs. 21 and 24, 
 PI. XXXI. In other ones, one-half of the cell body is reduced 
 to a hyaline filament, while the other half retains a protoplasmic 
 nature, with a greater or less amount of pigment granules as in 
 Figs. 19, 20 and 24, PI. XXXI; while in still others the proto- 
 plasmic substance is mostly gathered around the nuclei and the 
 two extremities reduced to hyaline structures with no pigment 
 granules in them, as in Figs. 22, 24, 27, 29 and 30, PL XXXI. 
 
 The contents of the ommatidium of Limulus may be put, then, 
 under two categories: (1) the epithelial cells; (2) the products 
 of secretion of these cells. The first may be classified into two 
 principal divisions: (a) the neuro-epithelium, (b) the ordinary 
 epithelium. The neuro-epithelium is of two kinds, the central , 
 ganglion cell and the peripheral, retinula cells. The ordinary 
 epithelium shows all varieties of modification, and exists either 
 in the interspaces of the neuro-epithelium or around it. The 
 products of secretion may also be put under two heads : (a) the 
 chitin secreted by the neuro-epithelium (the retinula cells) 
 forming the rhabdom ; (b) the chitin secreted by the ordinary 
 epithelium forming the lens-cone, and the entire chitinous struc- 
 ture lying over the pit. 
 
 The basement membrane (B. M , Fig. 6, PI. XXIX) under- 
 lies the whole ommatidium, separating it completely from the 
 underlying mesodermic tissue {Ms, Fig. 6, PI. XXIX). At 
 the bottom of the ommatidium the basement membrane follows 
 the course of nerve fibres issuing from the bases of the neuro- 
 epithelial cells, and thus forms a complete sac to the bundle of 
 nerve fibres. When the bundle breaks up in the plexus the base- 
 ment membrane sheath of the optic nerve fibres becomes indistinct. 
 
 A study of transverse sections of the ommatidium at different 
 heights will show the relative positions of the component elements 
 a little more clearly. In Fig. 6a, PI. XXIX, transverse sections 
 of the ommatidium cut at four different levels are shown, and 
 will be intelligible if one compares them with Fig. 6, PI. XXIX. 
 
 In {a) Fig. 6a, the plane of section passes near the opening of 
 the pit. In the centre is found a portion of the lens (Z), and 
 
COMPOUND EYE 8 OF ARTHROPODS. 
 
 307 
 
 surrounding it a circular row of columnar pigmented epithelium. 
 The lens is secreted by the surrounding columnar cells. Around 
 the whole ommatidium there exists a rather thick, well defined 
 basement membrane (B. M) which separates the ommatidium 
 from the mesodermic tissues (Ms). 
 
 In (b) Fig. 6a, a section at about the level of the letters JRt , in 
 Fig. 6, passing through the sensory portion is shown. The main 
 contents of the ommatidial capsule (B. M) are the retinula cells 
 (Rt). The axial moiety of each retinula cell is free from the 
 pigment granules, and a thin cuticular covering is secreted 
 around this portion of the cell. This cuticular covering consti- 
 tutes the rhabdomere (Rb). Sections of a number of pigmented 
 cells are seen lying between the retinulae and the basement 
 membrane ; these are sections of the perineural epithelial cells. 
 
 In (c), Fig. 6a, a section passing through the body of the 
 ganglion cell (G) is shown ; the ganglion cell is surrounded by 
 a circle of retinula cells, which are now gradually tapering, each 
 into a slender nerve process. The ganglion cell takes a more 
 eccentric position in (d) than in ( c ). The retinula cells (Rt) in 
 (c) and (d) are represented by < the circle of nerve fibres sur- 
 rounding the ganglion cell (G). 
 
 In (e), Fig. 6a, the section passes through the bottom of the 
 ommatidium ; all of the sensory cells are reduced into nerve 
 fibres which proceed one from the bottom of each cell. The 
 basement membrane (B. M) is still distinctly seen, forming the 
 sheath of the nerve bundle. The interspaces of the adjacent 
 ommatidia are occupied by the mesodermic connective tissue. 
 
 If we plot out the arrangement of the component elements in 
 the ommatidium of Limulus, the appearance shown in Fig. 7 
 will be presented. The epithelial cells (Ep) forming the walls 
 of the pit are drawn with their chitin-secreting surfaces turned 
 towards the axis of the ommatidium, which is morphologically 
 the exterior surface of the body. 
 
 c. The Development of the Compound Eye. 
 
 The first rudiments of the compound eye appear extremely 
 early in the embryonic stage as a pair of ectodermal thickenings 
 above the level of the “ dorsal organ.” In Fig. 39, PI. 
 XXXII, * shows the place where the compound eye makes its 
 
308 
 
 S. WATASE. 
 
 appearance. In this stage the “ dorsal organ ” ( D . 0) is already 
 very prominent. The eye itself does not show its characteristic 
 invagination, but is represented by a slightly thickened patch of 
 the ectoderm ; except in the microscopic examination of the sec- 
 tions it is hardly recognizable. The section Fig. 42, PI. 
 XXXII, shows an embryo considerably more advanced than 
 Fig. 39. The plane of section passes obliquely forward from 
 behind, and cuts both the eye (A) and the “ dorsal organ ” 
 (. D . O). The most noticeable feature in this stage of the devel- 
 opment of the eye is the existence of the lateral invaginations at 
 the edges of the ocular area. The section shows three portions, 
 the dorsal fold (d.f) and ventral fold ( v.f), with the median 
 portion (A) between them. The groove along which the 
 ectoderm invaginates is V-shaped, with the pointed end turned 
 towards the posterior end of the body. As the invagination 
 becomes deeper, the pointed end of the V becomes prolonged into 
 a tube, which w T orks its way beneath the general surface of the 
 ectoderm. This tubular invagination of the posterior part of 
 the eye will be called the median fold of involution ( m.f ) in 
 our subsequent descriptions. 
 
 The whole outline of the invaginated area may be represented 
 by Y, the area intercepted by the two divergent branches of the 
 
 Y being the ommatidial portion, while the posterior stem of the 
 
 Y is the continuation of the lateral depressions, the median fold, 
 extending backward and downward beneath the surface of the 
 general ectoderm. 
 
 The opening of the tube is at the junction of the two divergent 
 branches of the Y with the posterior median stem. 
 
 Figs. 47-56, PI. XXXIII, represent a series of transverse 
 sections of the compound eye in the stage represented by Fig. 
 42, PL XXXII. Fig. 47 is the section passing through the 
 anterior end of the ocular area. The ectodermic cells are con- 
 siderably thicker than the general ectoderm in this region. The 
 section which comes immediately behind this is not given in the 
 plate. Fig. 48 shows essential characters of the section which 
 immediately follows the section Fig. 47. At two portions of the 
 thickened ectoderm, <f./*and v.f, the cells arrange themselves 
 in radial fashion, accompanied by a characteristic change in each 
 nucleus, which becomes coarsely granular and peculiarly trans- 
 lucent. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 309 
 
 In examining the next section, Fig. 49, the radial arrange- 
 ment of cells in the above section is shown to be the beginning 
 of invagination. In Fig. 49 the lateral invaginations d.f and 
 v.f have each a well developed lumen. In the median side the 
 walls of the invaginations become continuous with the group of 
 finely divided cells, each with darkly stained nucleus. This 
 group of cells give rise to ommatidia. The outer wall of each 
 invagination becomes continuous with the columnal epithelial 
 covering of the body. 
 
 Figs. 50 and 51 show essentially the same thing. The omma- 
 tidial area (Om) in Fig. 51 becomes wider than in any of the 
 preceding sections. 
 
 Just at the point where the maximum width of the omma- 
 tidial area is reached (Fig. 51), it again begins to become narrow, 
 for in the section (Fig. 52) which comes immediately behind that 
 shown in (Fig. 51), the ommatidial area is narrower compared 
 with that showm in Fig. 51, and the two lateral folds of invagi- 
 nations approach each other, resulting in a fusion in the median 
 line in Fig. 53. In Fig. 54 the fused lateral folds, resulting in 
 the formation of the median fold (m.f), begins to work its way 
 beneath the level of the skin. In Fig. 55 a few cells (m. f ), 
 forming the posterior extremity of the median fold beneath the 
 skin, are found. The section (Fig. 56) passing behind that shown 
 in Fig. 55 shows no trace of invagination of any kind. 
 
 The anterior end of the eye, then, begins with Fig. 47 ; two 
 sections behind it a beginning of lateral invaginations becomes 
 discernible ; at Fig. 53 the two lateral folds fuse, and at the 
 ninth section (Fig. 54) they form a median tube ( m.f ), and at 
 Fig. 56 the posterior boundary of the ommatidial area terminates. 
 
 In Figs. 45 a and 4 6a are shown the longitudinal sections of 
 the eye. The ommatidial cells in the anterior portion of the eye 
 send out nerve fibres which form the optic nerve ( Ojp . n). The 
 basement membrane {B.M) which underlies the ectodermal cells 
 forms a complete sheath to the bundle of optic nerve fibres going 
 to the optic ganglia of the brain, which are formed by inde- 
 pendent involutions on the ventral side of the body. 1 The origin 
 
 1 The optic ganglia arise as ectodermic involutions in the pre-oral region of 
 the animal. Owing to the existence of a great cephalic flexure in Limulus, that 
 side where the optic ganglia as well as the median eye originate has for the sake 
 of convenience been called the “ ventral side.” 
 
310 
 
 & WATASE. 
 
 of the optic nerve fibres is further shown in Fig. 43, PL XXXII. 
 The figure shows an oblique transverse section through the 
 ornmatidial region of a more advanced embryo than the one 
 shown in Fig. 45 «, PL XXXIII. The plane of section passes 
 through the ornmatidial area and through the bundle of the optic 
 nerves, which run forward. The migration of the ectodermal 
 cells seems to take place from the periphery to the inside of the 
 body. The basement membrane (B. M) forms a continuous 
 sheath around this migratory stream of ectodermal cells. 
 
 In Fig. 40, PL XXXII, is shown an embryo in the so-called 
 “ trilobite stage.” A pair of round protuberant organs (B. 0) in 
 front of the eyes (E) are the “ dorsal organs.” The eye is distinctly 
 marked in this stage, owing to the formation of pigment granules 
 in the component cells. Except in the general enlargement of 
 the organ and the formation of the pigment granules in the 
 cells, the eye in this stage does not offer any material difference 
 from the condition seen in the stage which we have already 
 examined (Figs. 47—56). After the first moult of the chitinous 
 covering from the condition represented in Fig. 40, PL XXXII, 
 it passes into a more advanced condition when the essential 
 Limuloid features characteristic of the adult become distinctly 
 discernible. Fig. 41, Pl. XXXII, represents the young Limulus 
 at its first stage in the acquisition of the external Limuloid 
 features. The whole body is translucent ; the chitinous covering 
 of the body is yet extremely thin, and the dendritic ramifications 
 of the “liver” containing the remnant of the food-yolk of various 
 hues, sometimes green, at other times yellow or pink, shine 
 through the transparent tissues of the body. 
 
 The compound eye (A) shows an interesting feature in con- 
 nection with the formation of the ornmatidial area. 
 
 Figs. 57-64, PL XXXIV, show a series of transverse sections 
 through the compound eye in the stage shown in Fig. 41, PL 
 XXXII. 
 
 Fig. 57, Pl. XXXIV, represents a section passing through the 
 anterior part of the eye. The ectoderm in the ornmatidial area 
 is thrown into a number of folds. The cells lying in the valley 
 of the fold are destined to give rise to the ommatidium. In the 
 dorsal margin of the ornmatidial area there exists a thick layer 
 of ectoderm, consisting of a number of large cells, each with 
 
COMPOUND EYES OF ARTHROPODS. 
 
 311 
 
 granular protoplasm and a large nucleus. At the basal end of 
 each cell there exists an accumulation of transparent liquid sub- 
 stance. Beneath ommatidial area a well defined basement mem- 
 brane (JS. 31), which is the continuation of the same membrane 
 existing under the general ectoderm of the body, sharply marks 
 out the ommatidial structure from the mesodermic tissues. 
 
 In Fig. 58, which is taken a few sections behind the preceding, 
 is shown the ommatidial area at its widest part. Four omma- 
 tidia formed by the undulating folds of the ectoderm are shown, 
 each depression being accompanied by a corresponding thick- 
 ening of the chitin, forming the rudiment of the lens-cone ( C ). 
 
 On both sides of the ommatidial area we find the lateral 
 invagination of the ectoderm. The cells forming the invaginated 
 folds undergo enormous enlargement, and in the basal end of* 
 each cell is shown a large accumulation of transparent fiuid. 
 The cells forming the inner walls of the dorsal and ventral folds 
 of invaginations undergo various metamorphoses. Each of them 
 becomes divided into a number of smaller cells, each of these 
 segments being packed with extremely fine pigment granules. 
 As the cells in the inner walls of the dorsal and ventral folds 
 ( d.f , v.f) become divided up into a number of smaller pigmented 
 segments, they are constantly pushed out towards the surface, 
 where they, grouping themselves into a number of ommatidia, 
 increase the general surface of the ommatidial area. Thus, 
 the older ommatidia are found in the middle and the younger 
 ommatidia are to be found around the marginal portion of the 
 ocular area. 
 
 In Fig. 59 a section is shown in which the ommatidial area is 
 narrower than the preceding. In Figs. 60 and 61 this narrowing 
 of the ommatidial portion continues, and in Fig. 62 the dorsal 
 and the ventral folds meet in the median line. In Fig. 63 the 
 median fold becomes completely buried beneath the general 
 ectoderm. In Fig. 64 the section passes near the posterior 
 extremity of the median fold, where the reduction in the size of 
 the tube as well as its increased distance from the level of the 
 general ectoderm is clearly discernible. 
 
 In all the figures above described Pg. c means the pigment 
 cells, which in the state of nature are highly pigmented, each 
 with a small eccentric nucleus. Each section has been carefully 
 
312 
 
 S. WATASE. 
 
 depigmented, without which the process of the formation of the 
 ommatidia and the increase of the ocular area is perfectly unin- 
 telligible. Those cells in the figures which have no granular 
 structure are, in the state of nature, charged with a large quan- 
 tity of pigment granules, and those large cells forming the walls 
 of the lateral folds are granular and free from pigment granules. 
 
 Transverse sections are not fitted for the study of the forma- 
 tion of the nerve fibres. In this stage, as in the preceding ones, 
 each ommatidium sends out nerve fibres from its basal end 
 towards the interior of the body. Besides the cells in the bottom 
 of each ommatidial pit, each gigantic cell of the outer walls of 
 the dorsal and ventral folds sends out a nerve process from its 
 basal end, which runs inward and forward, joins the nerve fibres 
 *from the bottom of each ommatidium, forming a large bundle, 
 and goes to the brain. 
 
 The invagination of the lateral and posterior margin of the 
 ocular area is usually not permanent. With a growth of the 
 animal and an increase of the surface of the ocular area, the 
 invaginated folds, both on the sides as well as behind the ocular 
 area, become stretched out in the adult animals and leave no 
 trace of their existence. Occasionally, however, one meets in the 
 adult with a remnant of the involuted tube in the posterior part 
 of the eye, which exists as a ball of pigment cells buried in the 
 midst of the mesodermic tissue. 
 
 The history of the ommatidial cells in the compound eye of 
 Limulus may then be stated in the following way : 
 
 (1) . A stage of undifferentiated ectodermal cells. 
 
 (2) . Thickening of these undifferentiated cells, accompanied 
 with the invagination beneath the surface. 
 
 (3) . Extreme enlargement of the invaginated ectoderm, fol- 
 lowed by the acquisition of pigment granules and by division. 
 
 (4) . Pushing out of the resulting small pigmented cells towards 
 the surface wdiere they differentiated into two fundamental 
 groups : (a) Ordinary epithelial cells, which secrete the chitinous 
 cuticle over the ocular area, including the lens, or pack the inter- 
 spaces of the neuro-epithelium ; (b) neuro-epithelium, consisting 
 of the rod-bearing retin ulae and the central ganglion cells, the 
 two forming the sensory portion of the ommatidium. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 313 
 
 IV. — Phylogeny of the Ommatidium. 
 
 I do not pretend to enter into a detailed discussion of this 
 difficult problem. In order, however, to give a more intelligent 
 idea of what I have thus far been considering, I will devote the 
 following pages to the reconsideration of facts given in the 
 preceding pages from a more comprehensive standpoint. 
 
 Balfour 1 has given a sketch of the possible evolution of a visual 
 organ. He starts with a simple organism in which a spot on 
 the surface of the body may become spontaneously pigmented 
 and therefore become specially sensitive to light. The cuticular 
 covering of the body may become thickened at this spot and act 
 as an apparatus for condensing the light upon the pigmented 
 spot lying beneath it. He further expresses his view elsewhere 2 
 that the lens-like dioptric apparatus of the eye, formed either 
 as a thickening of the cuticle or as a mass of cells, was at first 
 formed simply to concentrate the light on the sensitive spot ; the 
 power to throw an image of external objects on the perceptive 
 part of the eye was acquired gradually afterward. 
 
 The part which is played by pigment in the physiology 
 of vision is considered a most obscure problem. I quote the 
 following, clearly put forward by Foster , 3 as a physiological 
 aspect of the question bearing upon the discussion at issue: 
 “ But in order that light may produce chemical effects (upon 
 protoplasm), it must be absorbed ; it must be spent in doing the 
 chemical work. Accordingly, the first step towards the forma- 
 tion of an organ of vision is the differentiation of* a portion of 
 protoplasm into a pigment at once capable of absorbing light 
 and sensitive to light — i. e. undergoing decomposition upon 
 exposure to light. An organism, a portion of whose protoplasm 
 had thus become differentiated into such a pigment, would be 
 able to react towards light. The light falling on the organism 
 would be in part absorbed by the pigment, and the rays thus 
 absorbed would produce a chemical action and set free chemical 
 
 1 F. M. Balfour: Address to the Department of Anatomy and Physiology, 
 British Association, 1880 ; Nature , Vol. XXII, p. 417, 1880. 
 
 2 Comparative Embryology , Vol. II, Chapter XVI, Organs of Vision, p. 470. 
 
 S M. Foster: A Text Book of Physiology , 4th Edition, Book III, Chap. II: 
 Sight ; The Photochemistry of Retina, pp. 515-516. 
 
314 
 
 8. WAT A SB. 
 
 substances which before were not present. We have only to 
 suppose that the chemical substances are of such a nature as to 
 act as a stimulus to the protoplasm of other parts of the organism 
 (and we have manifold evidence of the exquisite sensitiveness of 
 protoplasm in general to chemical stimuli), in order to-see how 
 rays of light falling on the organism might excite movements in 
 it, or modify movements which were being carried on, or might 
 otherwise affect the organism in whole or in part. Such consid- 
 erations as the foregoing may be applied to even the complex 
 organ of vision of the higher animals. If we suppose that the 
 actual terminations of the optic nerve are surrounded by 
 substances sensitive to light, then it becomes easy to imagine 
 how light, falling on these sensitive substances, should set free 
 chemical bodies possessed of the property of acting as stimuli to 
 the actual nerve-endings, and thus give rise to visual impulses in 
 the optic fibres.” 
 
 Lubbock 1 advances essentially the same idea as Balfour’s in his 
 recent work on the subject, illustrated with some lucid diagrams. 
 “ In the simple forms,” he says, “ the whole surface is more or 
 less sensitive. Suppose, however, some solid and opaque particles 
 of pigment deposited in certain cells of the skin. Their opacity 
 would arrest and absorb the light, thus increasing its effect, while 
 their solidity would enhance the effect of external stimulus. A 
 further step might be a depression in the skin at this point, which 
 would serve somewhat to protect these differentiated and more 
 sensitive cells, while the deeper this depression the greater would 
 be the protection.” 
 
 That such steps of gradual development of visual organs have 
 actually taken place in some forms is quite probable. In 
 Arthropods, it seems to me worthy of remark that the omma- 
 tidium of the lateral eye of Limulus makes the nearest approach 
 to this primitive condition. It is nothing more and nothing less 
 than a depression in the skin, with the thickened chitinous cuticle 
 fitting in the open cavity and acting as a lens to condense the light. 
 The cells which form the sensory part of the structure are modi- 
 fied ectodermic cells, and, like the rest of the ectodermic cells 
 lying on the surface of the body, secrete the chitinous cuticle on 
 
 1 On the Senses, Instinct, and Intelligence of Animals , Inter. Scie. Series, 
 Vol. LX1X, 1888. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 315 
 
 a part of their surface. Assuming the surface where the chitin 
 is secreted to be the exterior, we may describe the ommatidium 
 of the lateral eye of Limulus as a group of modified ectodermic 
 cells aggregated around and beneath the funnel-shaped depres- 
 sion in the skin. A glance at the diagrams (Fig. 6, PI. XXIX ; 
 Figs. 65 and 66, PI. XXXV) will show this clearly. The sensory 
 cells of the ommatidium come into direct contact with the conical 
 lens, which is the thickened part of the general cuticle; or, to 
 express this in the phraseology of Lankester and Bourne, the 
 ommatidium in the lateral eye of Limulus is “ epistatic.” The 
 cornea and crystalline cone as such have no separate existence in 
 this stage. 
 
 Suppose such an ommatidium to become duplicated until a 
 considerable number be formed, as we may safely imagine 
 to have been the case, from the general tendency in the perfec- 
 tion of a visual organ. What will be the result? The first effect 
 of such an increase of the number of ommatidia in a given area 
 will be the lengthening of each unit in the direction of the 
 ommatidial axis, and the cells ( V, Fig. 67, PI. XXXV) which 
 were situated directly on the outside of the retinulae will travel 
 over and above the sensory portion (Bt and G , Fig. 67). The 
 distal ends of such cells ( V) which were thus pushed over will 
 meet one another in the median or the u optic axis ” of the 
 ommatidium ; further, they will continue to secrete chitin ( c . c , 
 Fig. 67) from their original chitin-secreting surfaces which are 
 now median and axial. The chitin thus secreted will have an 
 independent existence from the cornea, thus forming the rudi- 
 ment of the crystalline cone, and the cells themselves will form 
 the vitrellae ( F, Figs. 67, 68, etc.). Finally, as the deepening 
 still further goes on, the corneal lens ( G ) and crystalline cone 
 ( c . c} will be entirely separated, thus producing a condition 
 somewhat similar to that which obtains in Serolis (Figs. 69 
 and 70.) 
 
 From this point onward, the three chitinous structures, cornea , 
 crystalline cone , and rhabdomere , undergo a different develop- 
 ment in different Arthropods. In some the crystalline cone 
 assumes a transparent semi-liquid state, while the whole cell 
 becomes extremely elongated, forming the crystalline cone of 
 certain Crustacea (Fig. 71, PI. XXXV); it may form a hard 
 
316 
 
 8. WATASE. 
 
 chitinous ball as in Serolis (Fig. 70, PI. XXX Y) ; or a cuneiform 
 chitinons structure, as in Talorchestia (Fig. 73, PI. XXXY) ; or 
 finally the whole cell may remain as a clear, transparent body, 
 as in several insects, forming Grenadier’s “ aconous type ” of the 
 compound eye. 
 
 The forms assumed by the rhabdomeres in different Arthro- 
 pods are equally diverse. The rhabdomere may exist as a plain 
 cuticular- covering over the non-pigmented part of the retinula, 
 as in Limulus or in Serolis (Fig. 10, Bb, PI. XXIX ; Fig. 38, 
 Bb, PI. XXXI, etc.) ; it may become extremely elongated and 
 narrow as in Musca or in Callineetes (Fig. 5, Bb , PI. XXIX ; 
 Fig. 37, Bb , PI. XXXI; Fig. 72, Bb , PI. XXXY); it may 
 become transversely folded as in Cambarus (Fig. 35, Bb , PI. 
 XXXI; Fig. 4, Bb, PL XXIX; Fig. 71, Bb, PI. XXXY); 
 these transverse folds may become still finer, showing the 
 chitinous serrature along the axial edge of the retinula, as in 
 Penaeus and Homarus (Fig. 34, Bb, PI. XXXI) ; or this trans- 
 verse serrature may become extremely fine and regular, as in 
 Squilla (Fig. 36, PI. XXXI). 
 
 The cornea undergoes equally diverse modifications according 
 as it is purely protective, or partly protective and partly dioptric 
 in function. The range of variation is shown by the degrees of 
 curvatures and by the varieties of its thickness. In several of 
 the decapod Crustacea which I have examined, as Penaeus , 
 Cambarus, Homarus, Callineetes, Gebia, etc., the curvatures of 
 the individual cornea on both surfaces are very slight ; it is 
 biconvex in an extremely small degree. In Talorchestia both 
 surfaces of the cornea are parallel. In Serolis, four species of 
 which I have studied, all having well developed compound eyes, 
 there exists a considerable difference in different species in the 
 nature of the cornea. In some the curvature on the proximal 
 surface is very strong and the whole structure is quite thick, 
 while in others the cornea is rather thin and a slight development 
 of curvatures exists. This is interesting, showing that even 
 within the group of nearly allied species there are considerable 
 differences in this respect. 
 
 This fact is easy to understand when we remember the func- 
 tional property of the cornea and the crystalline cone. As has 
 been noticed already, the cr} T stalline cone is always dioptric in 
 
COMPOUND EYES OF ARTHROPODS. 
 
 31 7 
 
 function, while the cornea may be partly protective and partly 
 dioptric, or wholly protective. When the cornea becomes partly 
 dioptric, as in Serolis and in several other Arthropods, the diop- 
 tric function in an individual ornmatidium comes to be performed 
 by two structures, the crystalline cone and the corneal lens. 
 When the two structures act together for the same end at the 
 same time, it is easy to see how a certain trivial peculiarity of 
 the one may induce a correlative modification of the other, and 
 how a slight specific peculiarity may appear exaggerated in the 
 thickness or in the degree of curvature of the corneal lenses in 
 different species. 
 
 After so much has been said in regard to the unity of structure 
 of the ornmatidium in different Arthropods, one important point 
 awaits our consideration, viz. the homology and fate of the cen- 
 tral ganglion cell found in the ornmatidium of Limulus. Unless 
 a great many forms of ommatidia in different Arthropods be 
 compared, a discussion on this point appears to be unprofitable. 
 The consideration which follows is therefore a purely provisional 
 one. 
 
 There can be no question that the central ganglion cell is an 
 important factor in the ornmatidium of Limulus , nor can we 
 doubt the existence of a fundamental homology between the 
 retinulae of Limulus and those of all the other Arthropods which 
 I have examined. With the exception of a few problematical 
 bodies, such as the “ hyaline cells” of Serolis , there are no struc- 
 tures in the ommatidia of most Arthropods which correspond to 
 the central ganglion cell of Limulus , in spite of the existence of 
 a fundamental homology in the other elements of the ornmatidium. 
 
 What has become of the central ganglionic element of the 
 ornmatidium? Was it lost in the course of the phylogenetic 
 history of a more complex ornmatidium ? Or is it reasonable to 
 suppose that some ommatidia came into existence without it 
 from the beginning? Or, if it were lost at all, is there any 
 evidence which makes this supposition probable ? 
 
 The colorless ganglionic cell and the pigmented rod-bearing 
 cells which surround the former I consider as the two primitive 
 morphological factors in the unit of the sensory part of the 
 Arthropod retina, somewhat in the same way as the circle of 
 
318 
 
 S. W A TASK 
 
 rods with a cone in the centre are the two essential factors in the 
 neuro-epithelial layer of the human retina. In the absence of 
 enough comparative data in Arthropods at present we have to 
 dwell largely on the analogy suggested in the other groups of 
 animals. Whatever be the views as to the fundamental homology 
 of the ommatidium of Lim.ulus to a structural unit of the sensory 
 part of the human retina, a superficial resemblance of the one to 
 the other is certainly very strong. The structural resemblance 
 is paralleled by a physiological one. The place where the light 
 acts in the visual end-organ of Arthropods and of man may 
 alike be considered as consisting of a number of definite groups 
 of cells, each group being a morphological and a physiological unit ; 
 or, in other words, the sensory part of the retinge in both cases 
 consists of a mosaic of several sensitive spots. The image formed 
 on such a surface is therefore a mosaic one, whether in an 
 Arthropod or in a Vertebrate. 
 
 Fundamental as this arrangement appears to be in the human 
 retina, these two factors are liable to variation in their relative 
 distribution in different Vertebrates. In fact, the variation takes 
 place between the two extremes where the rods alone exist on 
 one hand and where the cones alone constitute the essential part 
 of the retina on the other. Thus, according to Schultze, “ either 
 form of percipient element (rod and cone) may be represented by 
 the other” in the Vertebrate. This range of variability in the 
 distribution of the cones and rods occurs even in a single group 
 of Vertebrates, as in mammalia, showing that the variation in 
 the distribution of the essential factors, even within a tolerably 
 well circumscribed group of animals, is sometimes quite exten- 
 sive. The group of Arthropods is a heterogeneous one, and I 
 see no a priori objection to believing in the existence of a phe- 
 nomenon analogous to what we find in Vertebrates, viz. that the 
 two percipient elements represented by the central and the 
 peripheral cells in the ommatidium of Limulus may be differ- 
 ently represented in different Arthropods. 
 
 There is no doubt whatever that the retinula cells are homo- 
 logous throughout the Arthropods. In fact, in most Arthropods 
 which I have examined no other elements but the retinulae 
 have any connection with the optic nerve fibres, and they often 
 undergo an enormous development and acquire most complicated 
 
COMPOUND EYES OF ARTHROPODS. 
 
 319 
 
 structures, as in Homarus or in Penaeus , giving rise to the much 
 discussed “ spindle.” 
 
 But what has become of the central element which is so 
 conspicuous in the ommatidium of Limulus, if the retinulae in 
 all Arthropods are homologous? I believe the central cell is 
 fully functional, judging from its position and from its veritable 
 connection with optic nerve fibre in Limulus. What in other 
 Arthropods strongly reminds one of this cell is the “hyaline cell” 
 at the bottom of the ommatidial pit in Serolis and, according to 
 Beddard, also in the Cymothoidae. One important difference, 
 however, exists between the “hyaline cell” of the Isopods and 
 the central cell in the ommatidium of limulus , viz. that, wdiile 
 in the latter the cell is connected with the optic nerve, the 
 “hyaline cell” in the former has no connection with the central 
 nervous system whatever. Hence the “ hyaline cell ” cannot be 
 sensory, even if it be homologous with the central cell of limulus , 
 which it resembles in its general appearance and in its position. 
 The number of “hyaline cells” in Serolis is always two, while 
 its supposed homologue in Limulus is, as a general rule, only 
 one. This fact does not offer any objection to my view of their 
 homology, when we bear in mind that other elements in different 
 ommatidia, as vitrellae and retinulae, show a wide range of 
 variation so far as their numbers are concerned, and yet they 
 can be considered as perfectly homologous. 
 
 A further embryological and comparative knowledge in regard 
 to the “hyaline cell” in Isopods is necessary for the determination 
 of its exact homology. Meanwhile I would observe that if the 
 central and the peripheral cells which we see in the ommatidium 
 of Limulus may be taken as the two essential factors of the 
 sensory element of the typical Arthropod retina, the case of 
 Serolis may be taken as a loss of balance in the relative develop- 
 ment of these two factors ; the central cells having lost their 
 sensory function and remaining as a sort of supporting mechan- 
 ism. We can imagine this change in the function of the central 
 cell as carried still further, and with the excessive development 
 of the peripheral elements, the retinulae, the central element 
 may finally have disappeared. 
 
 All this is, however, a mere suggestion, and my interpretation 
 of the nature of the Arthropod ommatidium in general does not 
 
320 
 
 S. WATASE. 
 
 lose its force even if this section of my views in regard to the 
 fate and homology of the central cell or cells be proved unten- 
 able. It is quite possible that the ommatidia in which there is 
 no element corresponding to the central cell of Limulus may 
 have originated without it from the beginning. It seems, how- 
 ever, more natural to suppose that such an ommatidium had it 
 originally and lost it later, observing that the simplest form of 
 ommatidium possesses it in its fully functional, sensory form. 
 
 Finally we have to consider the nature of the compound eye 
 as a whole as presented in various types of Arthropods. 
 
 That a certain structure in the body of an animal may repeat 
 itself and give rise to a secondary aggregate, or to a compound 
 organ, is a well known fact ; the repetition of similarly con- 
 structed uriniferous tubules forms the essential part of a verte- 
 brate kidney, or the similar repetition of gill-filaments forms the 
 respiratory organ of a Lamellibranch. Sundry other examples 
 of this nature might be given, but the above two will suffice. 
 Tracing, as I have attempted to do, the most complicated omma- 
 tidium into a simple, open ectodermic pit, there is to my mind 
 no difficulty in believing that the compound eye of the Arthro- 
 pod is one of the most astonishing examples of the formation of 
 an organ by the vegetative repetition of the similar structure. 
 Thus, according to Lubbock, there are about 4000 facets in the 
 compound eye of the house-fly ( Musca ), each facet corres- 
 ponding to a single tubular invagination of the skin, the omma- 
 tidium. There are 4000 independent invaginations in the area 
 in the head of the fly occupied by the compound eye ; in the 
 gadfly {(Estrus) 7000; in the goat-moth ( Cossus ) 11,000; in the 
 death’s-head moth {Sphinx atropos) 12,000 ; in a butterfly 
 (Papillio) 17,000 ; in a dragon fly {EEschnd) 20,000 ; in a small 
 beetle (. Mordella ) as many as 25,000. On the other hand, the 
 number of ommatidia seems to have reached its minimum in 
 certain Copepods, as in Corycceus , where the whole visual organ 
 seems to be represented by a single colossal ommatidium. 
 
 Certain forms of Collembola 1 seem to have a very small num- 
 
 1 Lubbock. Monograph of the Collembola and Thysanura, the Ray Society, 
 1873, p. 57, Pis. LV and LYI. Lubbock uses the term “ ocellus ” to designate 
 a single element of the eye which I here called an ommatidium. If the struc- 
 ture of this “ocellus” differs from the ommatidium of other Arthropods, it 
 has, of course, nothing to do with the discussion at issue. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 333 
 
 the outside of the median fold (m.f ). Counting from the outside 
 we meet with three consecutive strata of basement membrane in this 
 region of the eye. 
 
 Fig. 64. In this we come nearly to the posterior extremity of the 
 median fold ( [m.f ). The median fold is further from the surface 
 basement membrane than in Fig. 63. 
 
 Plate XXXV. 
 
 Figs. 65-69. Diagrams showing the probable evolution of the 
 three-layered ommatidium from the single-layered surface depres- 
 sion in the skin, by the gradual subsidence of the neuro-epithelial 
 elements, Rt and G , Fig. 65. In Fig. 66 the ommatidium of 
 Limulus is represented, which is considered a step further advanced 
 from the condition shown in Fig. 65. The distal end of the 
 retinula (Rt) instead of being pointed toward the exterior as in 
 Fig. 65, in Limulus it points towards the median axis of the 
 ommatidium. The chitinous substance being still secreted on the 
 outside, a distinct body of chitin beneath the lens-cone (C) is 
 formed, the rhabdom ( Rb ). In Fig. 67 this deepening is supposed 
 to have gone still further, resulting in the formation of another inde- 
 pendent chitinous body, the crystalline cone ( C . c). In Fig. 68 this 
 deepening is considered to have advanced still further, the crystalline 
 cone ( C . c ) being entirely separated from the corneal lens (£7) by a 
 distinct stratum of cell, the corneagen (c.g). In Fig. 69 an omma- 
 tidium with three strata of cells, each secreting chitinous substance 
 on the part of their surface, is formed. These three strata of cells 
 are known as the corneagen ( c . g ), the vitrella ( V), and the retinula 
 (Rt). Three chitinous bodies secreted by each group of cells above 
 mentioned are the cornea ( 0 ), the crystalline cone (c. c), and the 
 rhabdomere ( Rb ), respectively. 
 
 Fig. 70. — Serolis. Diagram of the ommatidium of Serolis. Gen- 
 eral arrangement of cells in this is not very different from that shown 
 in Fig. 69. The place of ganglion cell in Fig. 69, G, is taken by a 
 pair (of which only one is shown in the diagram) of transparent 
 “hyaline cells” (H). 
 
 Fig. 71. — Cambarus. This is introduced in comparison with the 
 hypothetical ommatidia. 
 
 Fig. 72. — Callinectes. 
 
 Fig. 73 .—Talorchestia. 
 
 In the last three forms no element corresponding to the central 
 
334 
 
 S. WATASE. 
 
 ganglion cell of Limulus nor to the “ hyaline cell” of Serolis can be 
 found. The sensory element of the ommatidium is represented by 
 the retinulae ( Rt ) only. 
 
 Fig. 74. — Limulus. Diagram of the compound eye of Limulus , 
 the black, heavy line representing the ectoderm and each depression 
 in this layer corresponding to an ommatidium. 
 
 Fig. 75. — Serolis. In the same way as the above, the eye of Serolis 
 may be represented by a series of folds. 
 
 Fig. 76. — Notoneda . 
 
 Fig. 77. — Agrion (Larva). 
 
 Fig. 78. — Branchipus. 
 
 Fig. 79. — Cambarus . 
 
 Fig. 80. — Penaeus. 
 
 Fig. 81. — Lucifer. 
 

 STUDIES FROM BIOL. LAB 
 
 VOL. IV. PLATE XXX 
 
 WAT A SE, del. 
 
 A.H.OEN & CO. Lin. 
 
"J DIES FROM BIOL. LAB. 
 
 VOL. IV PLATE XXX 
 
 A HOEN & C( 
 
COMPOUND EYES OF ARTHROPODS. 
 
 321 
 
 ber of ommatidia ; thus in Templetonia only one ommatidium 
 exists on each side of the head ; Orchesella has six on each side 
 of the head ; Tomocerus , Isopoma , have seven ; Degeeria , Lepi- 
 docyrtus , Smynthurus and Fapirius, eight. In the ants we 
 observe a similar gradation in the number of ommatidia. 
 
 What reasons can we assign for this enormous multiplication 
 of similarly constructed parts? What advantage follows from 
 this arrangement ? If the view of the nature of the compound 
 eye which is put forward in the preceding pages be a true one, 
 Muller’s celebrated theory of mosaic vision is the only one that can 
 account for the enormous multiplication of the similarly formed 
 pits in the skin. The subject has been so fully discussed by 
 Lubbock that I need not enter into details here. “According to 
 his (Johannes Muller’s) view, those rays of light only which pass 
 directly through the crystalline cones, or are reflected from their 
 sides, reach the corresponding nerve fibre. The others fall on 
 and are absorbed by the pigment which separates the different 
 facets. Hence each cone receives light only from a very small 
 portion of the field of vision, and the rays so received are collected 
 into one spot of light. The larger and more convex , therefore , 
 is the eye , the wider 'will be its field of vision / while the smaller 
 and more numerous are the facets , the more distinct will the 
 vision be. In fact, the picture perceived by the insects will be 
 mosaic, in which the number of points will correspond with the 
 number of facets.” 1 The whole explanation of the problem seems 
 to me to be contained in the passage above cited ; and no further 
 comment will be necessary more than a statement that the 
 increase in the number of ommatidia is a decided advantage 
 to their possessor. An eye like that of Lim,ulus might by a 
 slight change be converted into one of a more protuberant nature 
 so as to command a wider field of vision, as we see in some 
 species of Serolis or in some Trilobites ; a slight change again 
 might produce a protuberant ocular area mounted on an ophthal- 
 amic stalk, and accompanied by the accessory apparatus of vision, 
 such as the socket for protection or the set of muscles to move the 
 eye-stalk in different directions so as to command a still wider 
 field of vision. In this connection I may refer to a series of 
 diagrams (Figs. 74-81, PL XXX Y). The black heavy layer repre- 
 
 1 Lubbock : Senses, Instinct and Intelligence, p. 163. 
 
322 
 
 8. WATASE. 
 
 sents the ectoderm, and the region in which the ectoderm is 
 thrown into folds the area of the compound eye. The yellow 
 colored layer outside represents the chitin, and the dotted line 
 beneath the ectoderm, the basement membrane. 
 
 In Limulus (Fig. 74) the ectoderm is thrown into a series of 
 shallow folds, which, when viewed from above, would be a group 
 of shallow pits in the skin. Each pit is an ommatidium. In 
 Serolis (Fig. 75) the invagination of the skin is a little deeper than 
 that of Limulus , and the whole ocular area is more prominent. 
 Fig. 76 represents the condition of the ectodermal folding in 
 Notonecta , and Fig. 77 that of Agrion larva. Fig. 78 represents 
 the eye of Branchipus , only a part of the stalk being shown in 
 the figure. Fig. 79 represents the eye of Gambarus ; Fig. 80 
 that of Penaeus , and Fig. 81 that of Lucifer. 
 
 It must not be understood that the number of folds given in 
 the diagrams have anything to do with the actual number of 
 ommatidia that may exist in the actual specimens ; no more than 
 a morphological expression of the eye in a simplest possible form 
 was intended. If one suppose a single invagination of the skin, 
 say of Fig. 79, be divided into three strata and the cells in the 
 bottom stratum to send out nerve fibres, those in the middle to 
 form the crystalline cone and those in the outermost to form the 
 cornea (Fig. 71), the interpretation of the diagram will be com- 
 plete. 
 
 According to this view the compound eyes of Arthropods, 
 either in the sessile or in the stalked form, are nothing more than 
 a collection of ectodermic pits whose outer open ends face towards 
 the sources of light, and whose inner ends are connected with 
 the central nervous system by the optic nerve fibres. The cells 
 forming the walls of the pit arrange themselves into three strata, 
 in most cases accompanied by three regional functional differen- 
 tiations. Grenadier’s classification of the compound eyes of 
 insects into “acone,” u pseudocone” and “eucone” types refers 
 to the condition of the cells and their products in the middle 
 stratum — the vitrellae. 
 
 Morphologically, then, the compound eye of an Arthropod is 
 strictly single-layered, although, as is evident, the present concep- 
 tion is entirely different from the monostichous theory main- 
 tained by some recent writers. From Limulus to Sguilla we 
 
COMPOUND EYES OF ARTHROPODS . 
 
 323 
 
 have a series of forms showing all degrees of modification in the 
 general structure of the eye as well as the structure of its indi- 
 vidual elements, and there is not here a single form which invali- 
 dates the view maintained in the present paper. Moreover, this 
 view has the advantage of greatly simplifying our conception of 
 these structures, reducing, as it does, all of them to one primitive 
 structure, a depression in the skin, in which several organs of 
 ectodermal nature, often of a very complicated type, find their 
 common morphological origin. And when thus the nature of 
 the unit is reduced into a simple invagination of the skin, the 
 formation of the compound eye appears to be but another 
 instance of the well known method in the formation of a mor- 
 phological organ, namely, the vegetative repetition of a similar 
 structure. 
 
 Y. — Summary. 
 
 In studying the structure of the ommatidium of the compound 
 eye of Serolis it has been found that it may be reduced to a 
 simple ectodermic invagination of the skin. Extending my 
 researches over several other Arthropods, of which Talorchestia , 
 Cambarus , Homarus , and Callinectes were mentioned in the 
 preceding pages, the same interpretation of the ommatidium 
 may be applied without exception. This view of the omma- 
 tidium finds its strongest support in the fact that in Limulus , 
 the ommatidium is an open pit of the skin. 
 
 By supposing that the ommatidial pit of Limulus became 
 deeper and that this was accompanied by modifications in the 
 structure and arrangement of the component cells, we can show 
 the probability of our first supposition that the ommatidium of 
 the compound eye of an Arthropod is an independent invagina- 
 tion of the skin. If this view is correct, the unit of the compound 
 eye of an Arthropod is not, after all, so complex a structure as 
 has been supposed by some ; and the enormous increase in the 
 number of ommatidia in a given area of the skin which results 
 in the formation of the compound eye finds its parallel in the well 
 known method, of the formation of morphological organs, viz. the 
 duplication of a simple unit. 
 
 Baltimore, April 30 , 1889 . 
 
324 
 
 S. WATASE. 
 
 VI. — Appendix. 
 
 The Compound Eye of Echinoderms. 
 
 While the preceding paper on the compound eye of Arthro- 
 pods was passing through the press I improved my oppor- 
 tunity to study the compound eye of Echinoderms at Woods 
 Holl, during the months of June and July, 1889, to see how far 
 the morphological interpretation advanced for the former will 
 hold to the latter. 
 
 The three starfishes common at Woods Holl, viz. Asterias 
 vulgaris , Stimp., Asterias forbesii, Verril, and Cribella sangui- 
 nolenta , Lutken, were studied in this connection. While I 
 reserve the description of the details for a future paper, I will 
 briefly point out some important bearings of the results upon 
 the subject which has been discussed in the preceding pages. 
 
 The general structure of the compound eye of the starfish at the 
 tip of each ray is well known through the writings of Hseckel, 
 Wilson, Hoffmann, Lange, Greeff, Hamann, Carri^re, and some 
 others, although there exist some discrepancies in regard to the 
 histological features of the organ as described by these natura- 
 lists. 
 
 The most remarkable observation on the eye of the Echino- 
 derm among the early writers is that of Goodsir, who, according 
 to Forbes , 1 is said to have examined the eye of Cribella oculata 
 and remarked of its structure that it consisted “ of a red cushion 
 with pits on its surface .” My studies on the compound eyes of the 
 starfishes convinced me of the truth of Goodsir’s statement, and 
 the eye of a starfish is nothing more than a group of ectodermal 
 conical pits with their bases turned towards the exterior. The 
 epithelial cells which form the walls of the conical pit have each 
 a cuticular rod secreted at the tip of its distal extremity. 
 
 These cells send out nerve fibres from their proximal extrem- 
 ities. In the centre of the pit along the longitudinal axis 
 of the invagination there is a space filled with a clear fluid sub- 
 stance. The cuticular secretions of the retinal cells, as well as 
 the colorless fluid substance, constitute together a translucent 
 
 1 E. Forbes : A History of British Starfishes, and other Animals of the class 
 Echinodermata. London, 1841, p. 102. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 325 
 
 refractive body in the centre of each pit or the ommatidium, 
 which has been described by several as the “crystalline cone” in 
 the starfish eye. The whole surface of the eye-bulb is covered by a 
 thin cuticle which is secreted by the cells whose distal extremi- 
 ties lie on the general level of the external surface of the optic 
 bulb. In short, this external cuticular covering of the eye in a 
 starfish corresponds to the cornea , and the cuticular secretions at 
 the distal extremities of the retinal cells forming the walls of the 
 pit, correspond to the crystalline cone and the rhabdom of the 
 compound eye of an Arthropod. The colorless fluid substance 
 contained in the axial space of the invagination in the starfish 
 may be compared to the similar substance in the crystalline cone 
 of the ommatidium of Serolis. The number of ommatidia in the 
 starfish increases at the periphery of the optic tract as in an 
 Arthropod. The spaces between the adjacent ornmatidial invagi- 
 nations are occupied by the supporting epithelial cells. The 
 optic nerve fibres pass into the deepest stratum of the epithelial 
 layer, and running horizontally, join the main branch of the 
 ventral nerve cord of the ray. 
 
 On the whole, the entire morphological arrangement of the 
 parts in the compound eye of an Echinoderm is strikingly 
 similar to that of an Arthropod, and the formation of the visual 
 organ in the Echinoderms, at least in one of the large sub-groups, 
 the Asteridce , by a series of ectodermic invaginations, assumes a 
 wider significance when we take into consideration the fact that 
 the compound eye of the Arthropods has a similar mode of 
 origin, although it is an independent adaptation. 
 
 In the anatomical consideration of the Echinoderm eye I have 
 made no allusion to the so-called “ eye-spot ” on the ocular plate 
 of the sea-urchin. In regard to the function of this structure we 
 are by no means certain that it is visual in nature as it is usually 
 assumed. Valentin failed to discover any trace of dioptric appa- 
 ratus in it. More recently Fredericq 1 could not find any 
 evidence, anatomically and experimentally, that the structure in 
 question is an eye. u La tache de pigment qu’on y decritest une 
 pure fiction 
 
 1 Fredericq, Leon : Contributions a V etude des Echinides. Archives des 
 Zoologie Experi. et Gener., Tome 5, 1876, p. 434. 
 
326 
 
 S. WATASE. 
 
 Hamann , 1 who studied this structure in Echinus, declares that 
 the so-called “ eye-spot ” in the sea-urchin bears a close resem- 
 blance to the terminal feeler at the tip of arm of a starfish, with 
 which he thinks it is homologous and not wfith the eye. He 
 therefore substitutes the term “ intergenital plate ” for the 
 “ocular plate” upon which this problematical organ rests. 
 
 Even if this organ be shown to have some connection with the 
 visual function, as Romanes and Ewart 2 claim on the ground 
 of their experiments, it is safe to say that it belongs to a 
 different category of visual organ, as Hamann’s histological study 
 clearly shows, and has nothing to do with the subject under our 
 consideration, viz. the morphology of the compound eye of the 
 Echinoderm and the Arthropod. 
 
 Entirely apart from the preceding, but which may be mentioned 
 in this connection, is the Sarasins’ discovery 3 of the compound eyes 
 in a sea-urchin {Diadema) from Ceylon. Of their visual nature 
 there can be no doubt, as their experiments on the living animals 
 as well as the anatomical structure of the eyelet show. 
 
 The eyelet of the compound eye in Diadema is very much 
 more complex than that of the starfishes I have examined. I 
 cannot help but believe that a study of development of such an 
 interesting form as Diadema , as well as a more extensive study 
 of the eye in different genera and species of Echinoderms, will 
 throw a great deal of light on the morphology of the compound 
 eye in general. 
 
 In my main paper on the subject I hope to describe in detail 
 the structure and development of the compound eye of the star- 
 fish that I have studied, and also to give an account of the 
 growth of our knowledge of the visual organ of the Echinoderms, 
 both anatomical and experimental, from Tiedman and Ehrenberg 
 down to the present day. 
 
 1 Hamann, Otto : Beitrage zur Histologie der Echinodermen. Jenaische 
 Zeitschrift fur Naturwissenschaft, Bd. 21, 1887, pp. 124-126. 
 
 2 Romanes, J. G., and Ewart, J. C. : Observations on the Locomotor System of 
 Echinodermata. Philosophical Transactions, Royal Society, London. Pt. Ill, 
 1881, pp. 855-856. 
 
 3 Sarasin , P. B. and C. F. : Ueber einen mit Zusammengesetzten Augen bedeckten 
 Seeigel. Zoologischer Anzeiger, 8 Jahrg. No. 211, 1885. Their main paper I 
 have not seen. 
 
COMPOUND EYES OF ARTHROPODS . 
 
 327 
 
 VII.— Explanation of Figures. 
 
 List of Reference Letters. 
 
 Ap. Appendage. 
 
 Ax. p. Axial process of the ganglion cell. 
 
 B. M. Basement membrane. 
 
 0. Corneal lens. 
 c. Cornea or corneal facet. 
 c. c. Crystalline cone. 
 
 Ch. Chitin. 
 
 c. g. Corneagen. 
 
 d. f Dorsal fold. 
 
 D. 0. Dorsal organ. 
 
 E. Eye (compound). 
 
 Ed. Ectoderm. 
 
 Ent. Entosternite. 
 
 Ep. Epithelial cell. 
 
 F. y. Food-yolk. 
 
 ( x . Ganglion cell. 
 
 H. Hyaline cell. 
 
 L. Lens-cone. 
 
 Ms. Mesoderm. 
 
 N. Nucleus. 
 
 Oc. Ocellus. 
 
 Om. Ommatidium. 
 
 Op. n. Optic nerve fibre. 
 p. c. Packing cell. 
 p. n. Perineural cell. 
 
 Pg. c. Pigment cell. 
 
 Pg. p. Pigment patch. 
 
 Rb. Rhabdomere. 
 
 Rt. Retinula. 
 
 Tr. fb. Transverse striae. 
 
 V. Vitrella. 
 
 V. C. Ventral nerve chain. 
 v.f. Ventral fold. 
 
 x. Axial space or line of the ommatidium, corresponding to the 
 cavity of the invagination. 
 
328 
 
 S. WATASE. 
 
 Plate XXIX. — Serolis , Talorchestia , Cambarus , Callinectes and 
 
 Limulus. 
 
 Fig. 1 . — Serolis. Two ommatidia of Serolis ; the one on the left 
 with the pigment cells enveloping the crystalline cone. The down- 
 ward extensions of the rhabdomeres reach to the retinal side of the 
 basement membrane, i (E X 2) Zeiss. 
 
 Fig. la. — Serolis. Diagram 
 
 a. Corneagen. 
 
 b. Vitrella. 
 
 c. Retinula. 
 
 d. Hyaline cell. 
 
 Fig. lb. — Serolis. Plan of t 
 
 1. Hyaline cell ( d ). 
 
 2. Retinula ( c ). 
 
 3. Vitrella (6). 
 
 the Ommatidium — 
 
 1. Cornea. 
 
 2. Crystalline cone. 
 
 3. Rhabdomere. 
 
 Pg. c. Pigment cell. 
 
 Ommatidium — 
 
 4. Corneagen ( a ). 
 
 5. Pigment cell (pg. c ). 
 
 Fig. 2. — Morphological diagram of the ommatidium of Serolis — 
 
 a. Corneagen. 1. Cornea. 
 
 b. Vitrella. 2. Crystalline cone. 
 
 c. Retinula. 3. Rhabdomere. 
 
 d. Hyaline cell. 
 
 Fig. 3. — Talorchestia. Ommatidium of Talorchestia. — Et f , upward 
 
 extension of the retinula Et; this upward extension sometimes 
 reaches as far as the cornea, p. c. Harrow, non-pigmented packing 
 cells, i (D X 4) Z. 
 
 Fig. 3 a. — Talorchestia. Plan of the Ommatidia. 
 
 Fig. 4. Cambarus. Three ommatidia shown side by side. In c 
 the pigment cells ( [pg.c ) completely envelop the crystalline cone. 
 x is the morphological lumen of the invagination. Ep is the epi- 
 thelial cell charged with light yellow pigment granules. 1(DX2)Z. 
 
 Fig. 5. Callinectes. Two ommatidia shown side by side. Rhab- 
 domere (Eb) shows a fine striation. At the swollen end of the 
 retinula (Et) yellow or brown colored globules are found. 
 
 Fig. 5 a. — Callinectes. Head of the retinulae more highly mag- 
 nified, and the rhabdomeres show slight enlargement at the terminal 
 portion. 
 
 Fig. 6 . — Limulus. Two ommatidia shown side by side, partly 
 schematic. The thick yellow-colored body is the chitinous covering 
 of the eye. L. lens-cone, fitting into the depression of the skin. At 
 the bottom of the depression, the essential part of the ommatidium 
 
COMPOUND EYES OF ARTHROPODS. 
 
 329 
 
 is found, consisting of two parts, Rt (retinulae) and G (central gang- 
 lion cell). Cells forming the walls of the pit are the perineural 
 cells {p.n). Inward prolongations of the neuro-epithelial cells 
 {Rt and G) form the optic fibres {Op. n). Ms. Mesodermic tissue. 
 
 Fig. 6a. — Limulus. Transverse sections of five ommatidia at 
 different levels. In ( a :) the plane of section passes through the 
 upper part of the depression, cutting the lens-cone (Z); in {b) the 
 section plane passes through the retinulae, showing the star-shaped 
 rhabdom and the axial process of the ganglion cell (. Ax.p ) in the 
 axial canal of the rhabdom. A few pigmented perineural cells sur- 
 round the retinulae, while the basement membrane {B. M) forms 
 the complete capsule of the ommatidium, separating the latter from 
 the mesodermic tissue {Ms). In (c) the plane of section passes 
 through the lower part of the ommatidium, cutting the central 
 ganglion cell {G) ; in (d) the ganglion cell occupies an eccentric 
 position ; in {e) the sections of optic nerve fibres alone are seen. 
 
 Fig. 7. — Limulus. Plan of the ommatidium. Ep represent the 
 cells which are found in the walls of the pit; the yellow edge 
 corresponds to the surface where the chitin is secreted. 
 
 Plate XXX. — Limulus . 
 
 Fig. 10. Retinula {Rt) and ganglion cell {G) teased out from 
 the ommatidium after treatment with Haller’s fluid 15 hours. The 
 axial process (Ax.p) of the ganglion cell closely follows the external 
 border of the rhabdomere {Rb). A number of epithelial cells are 
 found closely attached to the surface of the retinula as well as to the 
 ganglion cell. I (E X 2) Z. 
 
 Fig. 11. A bunch of retinulae completely isolated from the 
 ommatidium. Pigmented epithelial cells {Ep) are found between 
 them. The proximal ends of retinulae form the optic nerve fibres. 
 {Op. n). I (E X 2) Z. 
 
 Fig. 12. A retinula cell seen from the side opposite to the rhabdo- 
 mere. A number of extremely attenuated epithelial cells {Ep) are 
 found closely clinging to the external surface of the cell. They do 
 not form any connection with the retinula, and can be completely 
 isolated from it. The cell-body of such an elongated epithelium is 
 reduced to a very slender filament and does not contain any pigment 
 granules, f (E X 2) Z. 
 
 Fig. 13. The central ganglion cell with its axial process {Ax.p) 
 completely isolated. The pigment patch ( Pg.p ), situated between 
 
330 
 
 8. WATASE. 
 
 the nucleus (iV) and the optic nerve fibre ( Op.n ), shows a charac- 
 teristic mark of the cell. A zone around the nucleus (A r ) shows 
 concentric markings. Longitudinal striae run through the axial 
 process and the optic nerve; (Ep) an attenuated epithelial cell. 
 I(EX2)Z. 
 
 Fig. 14. Another type of the central ganglion cell. Pigment 
 patch is greatly extended; nucleus (iV) is eccentric; ( Ep ) attenuated 
 epithelial cells. I (E X 2) Z. 
 
 Fig. 15. The central ganglion cell, with its axial process ( Ax . p) 
 divided into several branches, by some mechanical causes. All these 
 branches are longitudinally striated like the main one and would 
 seem to be made up of a collection of extremely fine longitudinal 
 fibrils. Two pieces of chitinous body (Rb~) are found at the junction 
 of the axial process and the cell-body proper of the ganglion cell. 
 They appear to be fragments of the rhabdomeres. I (E X 2) Z. 
 
 The specimens figured in Figs. 10-14 were treated with Haller's 
 macerating fluid ; that shown in Fig. 15 was treated with sulphuric 
 acid, 8 drops to 30 grams of water. 
 
 Fig. 16. Epithelial cells isolated from the margin of the omma- 
 tidial depression. The part in which there is no pigment is turned 
 outward and comes in direct apposition to the chitin. 
 
 Plate XXXI. — Limulus , Homarus , Cambarus, Squilla , Callinectes, 
 
 Serolis. 
 
 Figs. 17-33. — Limulus . This series of specimens is intended to 
 show the range of modification of the epithelial cells found in the 
 ommatidium and its direct neighborhood. 
 
 Fig. 17. Central ganglion cell with several attenuated, non- 
 pigmented epithelial cells. I (E X 2) Z. 
 
 ‘ Fig. 18 . Retinula cell with a few pigmented epithelial cells at its 
 side, f (E X 2) Z. 
 
 Figs. 19-33. Ordinary epithelial cells isolated from the omma- 
 tidial region. ! (E X 2) Z. 
 
 Fig. 34. — Homarus. Retinulae partly isolated. Each retinula 
 secretes a chitinous body which assumes a serrated appearance 
 ( Rb ). This is the rhabdomere; when the bundle is brought together 
 it assumes a spindle shape. The upper part of each retinula cell is 
 free from pigment, and is connected with the pigmented portion by 
 a long, slender neck. The nucleus of the retinula is situated at the 
 non-pigmented, distal extremity. I (D X 2) Z. 
 
COMPOUND EYES OF ARTHROPODS. 
 
 331 
 
 Fig. 35. — Cambarus. The retinula completely isolated. The 
 rhabdomere shows a coarse wavy outline, f (D X 2) Z. 
 
 Fig. 36. — Squilla, partly diagramatic. 
 
 Fig. 37. — Callinectes , partly diagramatic. 
 
 Fig. 38. — Serolis, partly diagramatic. 
 
 The last three figures are introduced here so as to show the 
 homology of the retinula cells in different groups of Arthropods. 
 
 All the specimens figured in this plate were treated and macerated 
 in Haller’s fluid. 
 
 Plate XXXII. — Limulus. 
 
 Fig. 39. Young embryo of Limulus. D.O. “ Dorsal organ”; 
 * shows the position where the compound eye makes its appearance. 
 
 Fig. 40. Larval Limulus in the so-called “trilobite stage”; imme- 
 diately after hatching. D. 0. “ Dorsal organ ”; E. Compound eye ; 
 Oc. Ocelli. 
 
 Fig. 41. Young Limulus at the first stage in the acquisition of the 
 external Limuloid characters. The whole body is transparent and 
 the dendritic ramifications of the “ liver ” shining through the trans- 
 parent tissues of the body. The “ dorsal organ” has completely 
 disappeared in this stage. E. the compound eye ; and Oc. Ocelli. 
 
 Fig. 42. Transverse section of a young larva, showing the origin 
 of the compound eye {E), the formation of the dorsal and ventral 
 folds of invagination (d.f and v./), and the “ dorsal organ ” D. 0 . 
 1 (A X 2) Z. 
 
 Fig. 43. Transverse section of the compound eye, showing the 
 formation of the optic nerve fibres by the migration of the ectodermal 
 cells. I (D X 4) Z. 
 
 Fig. 44. Outline of the thickening of the nerve plexus beneath 
 the ommatidial layer, f (E X 2) Z. 
 ig. 45. The same, f (D X 4) Z. 
 
 Both are drawn from the gold-chloride preparations. 
 
 Fig. 46. Ganglionic swelling found in the optic nerve fibres of the 
 median eye. !(4 X D) Z. 
 
 Plate XXXIII. — Limulus. 
 
 Fig. 45 a. Longitudinal section of the eye, showing the formation 
 of the optic nerve fibres {Op. n). 
 
 Fig. 4 6a. Oblique longitudinal section of the eye, showing the 
 formation of the optic nerve fibres {Op. n). 
 
332 
 
 S. WATASE. 
 
 Figs. 47-56. Consecutive series of transverse sections of the com- 
 pound eye of Limulus at the beginning of the lateral invagina- 
 tions. Fig. 47, No. 1; Fig. 48, No. 3; (the section No. 2 is essentially 
 the same as No. 3). Fig. 49, No. 4; Fig. 50, No. 5; Fig. 51, No. 6; 
 Fig. 52, No. 7; Fig. 53, No. 8; Fig. 54, No. 9; Fig. 55, No. 10; 
 Fig. 56, No. 11. The lateral invaginations become distinctly marked 
 at Fig. 49, No. 4, and fuse in the median line at Fig. 54, No. 9, 
 forming the median fold ( m.f ). 
 
 Plate XXXIV. — Limulus . 
 
 Figs. 57-64 show the condition of the lateral and median 
 invaginations of the compound eye in the stage shown in Fig. 41, 
 PI. XXXII. 
 
 Fig. 57. In this the plane of section passes through the anterior 
 part of the eye, and the dorsal invagination (d.f) of the compound 
 eye is distinctly shown. The skin is thrown into folds; into the 
 concave lumen of the fold a slightly thickened portion of the cuticle 
 fits, forming the rudiment of the lens-cone ((7); and the cells 
 forming the walls of the invaginated fold form the beginning of the 
 ommatidium. 
 
 Fig. 58 shows the widest portion of the eye in this stage; the 
 beginning of the ventral invagination (v.f) is distinctly recognizable. 
 The dorsal fold {d.f) is very large. Four distinct ommatidia are 
 formed in this section, each depression being accompanied by a 
 slight thickening of the corresponding portion of the cuticle ((7). 
 The basement membrane is very distinctly seen underneath the 
 ocular area and beneath the general ectoderm cells which lie out- 
 side of the ocular area. The action of acid to which the sections 
 were subjected in order to remove the pigment granules has inter- 
 fered with the preservation of the ectodermal cells in those regions, 
 although the thick, refractive basement membrane remains quite 
 distinctly. 
 
 Figs. 59, 60 and 61 show how the lateral invaginations are gradu- 
 ally approaching as we go towards the posterior part of the eye. 
 In Fig. 62 the dorsal and ventral folds ( d.f and v.f) meet in the 
 median line, and in Fig. 63 they form a complete tube, the granular, 
 non-pigmented layer coming to the outside and the pigmented cell 
 layer ( Pg . c) occupying the deeper part of the tube. The basement 
 membrane (#. M) forms a complete capsule around the tube. The 
 basement membrane which underlies the general ectoderm comes to 
 
VOL. IV. PLATE XXXI 
 
 Fi e>9 Fig. 20 
 
 ;o. Ub . 
 
STUDIES FROM’ BIOL.l 
 
 VOL. IV. PLATE XXXII 
 
 Fig A3 
 
 A.HOEN 
 
 
 WAT ASE, del. 
 
 
STUDIES FROM BIOL. LA 
 
 VOL. IV. PLATE XXXI II. 
 
 WATA'SE, del. 
 
 A.HOEN & CO.XITl 
 
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STUDIES FROM BIOL. LAE 
 
 VOL. IV. PLATE XXXIV 
 
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 VOL. IV PLATE XXXV. 
 
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