+4] PLANT-ANIMALS | SS SSS St 2 aS SaaS Ses84 r Ns Reis pepe reat e ts beoeiees a ‘ “ ee a Si. Dy IN SYMBIOSIS: HF REALE oe I J en aac, Aaah iis ete oan Latter > 8 By 23S Franti 5 ah 1 : Gi) 4 ae a Hf Pe i. EP 1° 1928 ‘C7 & “Loaigai gw Division (Qi 34 | Section ‘ Re | A 2. 6 We SIAn ie Ph The Cambridge Manuals of Science and Literature Ay — PLANT-ANIMALS CAMBRIDGE UNIVERSITY PRESS London: FETTER LANE, E.C, C. F. CLAY, MANAGER €vinhurgh: 100, PRINCES STREET Dondon: H. K. LEWIS, 136, GOWER STRERT, W.O. WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND Berlin: A. ASHER AND CO. Leipsig: F. A. BROCKHAUS few Bork: G. P. PUTNAM’S SONS Bombay and Calcutta; MACMILLAN AND CO., Lrp, All rights reserved Digitized by the Internet Archive in 2022 with funding from Princeton Theological Seminary Library https://archive.org/details/plantanimalsstud00keeb_0 1 Eyes i ROTCer sis =a9,8 SoS eres te ie) ma » Eyes Otocys ie A A. The Green Plant-animal (Convoluta roscoffensis). ~“ B. The Yellow-brown Plant-animal (Convoluta paradoxva) | 40 times natural size. PLANT-ANIMALS A STUDY IN SYMBIOSIS BY, VA FREDERICK KEEBLE, Sc.D. PROFESSOR OF BOTANY IN UNIVERSITY COLLEGE, READING ; AUTHOR OF ‘**PRACTICAL—P SA eS ZA = F} Me ROA, ARN ~ SS %, M} First Edition 1910 Reprinted 1912 With the exception of the coat ofarms at the foot, the design on the title page is a reproduction of one used by the earliest known Cambridge printer John Siberch 1521 PREFACE URING some ten years’ work in a small marine laboratory in Brittany it has fallen to me not infrequently to attempt to explain to curious visitors what were my objects in going to and fro upon the shore, in wading among the sea-weeds and in bringing into the laboratory minute, worm-like animals which represented often my sole “catch.” I discovered that many of the visitors to the laboratory became interested in the work that was going on, and that, though they disclaimed a knowledge of biology, they followed with under- standing and interest the story of the behaviour and life histories of “the worms” :—indeed, they succeeded generally in putting to me pertinent and unanswerable questions with respect to these “ plant- animals,” The pleasant recollection of hours spent in this way is responsible primarily for my undertaking to vi PREFACE contribute this volume to The Cambridge Manuals of Science and Interature. If it succeeds in interesting the layman, success will be due to the severe educational régime to which my visitors submitted me in their cross- questionings as to the bearings and objectives of my biological work. If it fails, they must bear the blame: for had they not exhibited a fondness for “Convoluta” I should scarcely have ventured to publish its doings to the world at large. Of these friends | would mention particularly Mr Alfred Dutens, whose interest in “Convoluta roscoffensis” has been a source of constant encouragement to me. The biological facts recorded in this volume are the outcome of researches carried on for some years by Professor Gamble and myself, and, subsequently, without Professor Gamble’s co-operation. Throughout the whole time during which the work has been in progress, it has benefited more than may be stated explicitly by the unremitting assistance rendered by my wife. ‘To her, are due the long and patient records of the periodic changes of behaviour of the plant-animals—Convoluta roscoffensis and C. paradoxa:—records which entailed visits to the Convoluta colonies at all phases of the tide and at all hours of the day and night. Though an adequate PREFACE Vii ~ expression of my thanks to my wife were out of place here, I beg leave to give myself the pleasure of acknowledging how great has been her share in this work. The original memoirs, giving detailed accounts of the life histories of the plant-animals, have appeared in the Quarterly Journal of Microscopic Science. A list of these memoirs and of other researches to which reference is made in the text is included in the short bibliography appended to this volume. .The dates, enclosed within brackets in sundry places in the text, refer each, to the year of publication of the research which is cited and indicate that the title of the re- search in question may be found in the bibliography under that date. The black and white illustrations have been pre- pared specially for this volume by Mrs Seward from the original drawings made by Miss Dorothea Richardson in the laboratory at Trégastel. I am deeply indebted to Mrs Seward and Miss Richardson for their kind assistance, and to the skill and patience which they have bestowed on the drawings I offer a sincere and admiring tribute. Should the reader find that the main arguments exposed in the course of the volume are intelligible, he may, perhaps, be inclined to forgive the use, which I hope is as occasional as inadvertent, of un- Vill PREFACE familiar biological terms. I have endeavoured to — avoid this pit-fall, but have doubts as to the com- pleteness of my success. I shall be obliged therefore if readers will point out passages which require elucidation, so that, in the event of another edition being published, the defects may be remedied. FREDERICK KEEBLE. TREGASTEL, Corrs-pu-Norp, FRANCE. September, 1910. CHAP. CONTENTS 15a rub al Tur BEHAVIOUR OF THE PLANT-ANIMALS Introductory: the worms, Convoluta roscoffensis and Convoluta paradoxa: their habits and habitats II, The origin and significance of the habits of Con- voluta roscoffensis and Convoluta paradoxa : PART IT THE Nature OF THE PLANT-ANIMALS III. The green cells of Convoluta roscoftensis and the part they play in the economy of the plant-animal IV, The origin and nature of the green cells of Convoluta roscoffensis V. The significance of the relation between coloured cell- and animal-constituents of the plant-animals BIBLIOGRAPHY . ; ; : ° : ° : : INDEX . . s : : : - . : . WirH Text-FicurgEs, 1—22. CoLovuRED FRontiIsPigcE. The Green Plant-Animal, Con- voluta roscoffensis and the Brown Plant-Animal, Convoluta paradoxa. PAGE 3 37 75 100 130 159 161 PART I THE BEHAVIOUR OF THE PLANT-ANIMALS bi ( . a ale 3 4 CHAPTER I INTRODUCTORY: THE WORMS: CONVOLUTA ROS- COFFENSIS AND CONVOLUTA PARADOXA: THEIR HABITS AND HABITATS. BIoLOGIstTs who devote themselves to the investi- gation of the life histories and life processes of the lower animals are apt to encounter the criticism: why expend pain and labour on insignificant creatures when so much remains to discover with respect to the higher animals, including man himself? This perfectly legitimate criticism admits of a con- clusive reply and, since it is possible that a question of the kind may arise in the mind of anyone taking up this book, it shall be answered forthwith. The reply may take one of three forms. In the first place, it may be urged that the most important modern biological discoveries have resulted from researches into the life histories of the lower organisms. Modern surgery relies for much of its technique on the results of investigations into the physiology of the bacteria. Yet more recently, -the experimental elucidation of the life-histories of the protozoa—the lowest group of 1—2 4 PLANT-ANIMALS | CH. animals—has laid the foundation of a great and increasing body of knowledge with respect to the cause of malaria, sleeping sickness, and other tropical diseases. In the second place, it may be urged that, the more complex the organism, the more difficult it is to use the results of observations upon it for the purpose of generalising on important biological problems such as those of the origin of instinct and habit, or of the meaning of heredity and the course of evolution. The higher the organism, the more it has covered up the tracks along which the species to which it belongs has travelled. For this reason alone, the study of the lower organisms is not only to be justified but also urged on zoologists as one bound to lead to results of the greatest value. In the third place, it has yet to be proved that the higher animals differ in any fundamental respect from more lowly forms of life. Hence, if, as a physiologist must hold, such differences as exist between higher and lower forms are differences of degree and not of kind, it follows that an increased knowledge of the nature of the lower organisms connotes also an increase in knowledge with respect to the higher organisms. On these grounds, the patient and exhaustive _ study of the lower organisms ‘is to be justified. Nay more, if the reasons for this study are valid they 1] INTRODUCTORY 5 should serve to induce some of the younger genera- tion of physiologists to devote their attention to a field of research both rich in promise and too little cultivated by the men of science of this country. Though the results recorded in this volume are but modest, throwing here and there only a faint light on the problems which they raise, nevertheless they suffice to demonstrate that more skilful observers would, by taking up similar subjects of investigation, make notable contributions to the science of com- parative physiology. Having vindicated the importance of research on the lower organisms, let us proceed to our task. The plant-animals whose life histories and habits form the subject of this volume are two simple, marine worms, Convoluta roscoffensis and Convoluta paradoxa(Frontispiece). Both are small, though large enough to be seen easily by the unaided eye, and both are conspicuous by reason of their colours. C. ros- coffensis is dark, spinach green, and C. paradoxa yellow-brown. Even among worms they occupy a lowly place. Unlike the higher members of this group, C. ros- coffensis and C. paradoxa are unsegmented. Instead of consisting, like garden worms, of a series of ring- like pieces, the bodies of our plant-animals are in one piece and, consequently, bear no ring-like markings PLANT-ANIMALS |RSS SR nse oa Rr escnr Fig. 1. The distribution of the colonies of Convoluta roscoffensis on the sea-shore. I. at spring-tidal periods (low water): II. at neap-tidal periods (low water). Though a colony remains fixed in position, its size waxes with the spring tides and wanes with the neap tides. C,C=the colonies, S.=sea 1| INTRODUCTORY 7 on their surfaces (Frontispiece). Imagine a minute, elongated fragment of a most delicate leaf, some % in. long by ~;in. broad, and you have a picture of C. roscoffensis. Imagine, further, myriads of such green, filmy fragments lying motionless on moist, glistening patches of a sunny beach between tide-marks and you see the species in its native habitat (Fig. 1). To find C. paradoxa at home it is necessary to follow the receding tide, to gather handfuls of the brown seaweeds (Fig. 2) which are exposed towards the low- water limit of the larger tides and to allow the tips of the weeds to dip into water in a white dish. Singly from their hiding-places chubby, brown C. paradoxa come gliding down with rounded “head” and pointed “tail” to swim uneasily in the water of the dish. C.roscoffensis is pre-eminently gregarious, C. paradoxa by comparison is solitary. Sand from a Convoluta patch scooped up in a cup contains many thousands of C. roscoffensis ; a patient fishing throughout the time of low tide may result in a catch of fifty, or at most a hundred, specimens of C. paradoxa. The surface of the bodies of the plant-animals is somewhat slimy ; particularly in C. roscoffensis, and is covered by fine cilia (Fig. 3) which, during the life of the animals, are in constant motion. The cilia, which are protoplasmic projections from the super- ficial cells, serve, by their unceasing movements, to row the animal through the water. 8 PLANT-ANIMALS [CH. C. paradoxa possesses, in addition to cilia, occa- sional, stouter, bristle-like structures which stick out from its body, chiefly in the “tail” region (Fig. 16, p. 84). These structures serve, when put in action by the animal, to pin it down and thus enable it to stop and stick in any position. Fig. 2. Convoluta paradoxa (C) attached to sea-weeds of the paradoxa zone. (Magnified eight times.) In both animals, the sides of the body are flexed beneath the under surface, and together form a groove which, in C. paradoxa, serves to fit the animal saddle- wise to the fine sea-weeds over which it glides (Fig. 2). This animal, in its general progress, appears almost to flow over the substratum on which it is moving. 1| THE STRUCTURE OF CONVOLUTA 9 Occasionally, however, on meeting with an obstacle it rears Its head-end, caterpillar-wise, relaxes the grip of its flexed sides, readjusts them to the surface and glides on with stealthy motion. Though we have called C. roscoffensis and C. paradoxa simple worms, it is not to be inferred that the structure of their bodies is really simple. Both species possess a well-defined nervous system and efficient sense-organs. At the front or “head” end of the body, on the upper surface, a little way behind the anterior end, lie two eyes right and left of the median line (Frontispiece and Fig. 3). Though of the simplest construction, each consisting of a minute spot of orange pigment lying over nervous tissue, the eyes are efficient for distinguishing light of different intensities. Numerous orange-pigmented glands, scattered over the surface of the body, function probably as accessory eyes. Between the two eyes, in the median line on the dorsal (upper) side of the body of either species, lies the otocyst (Frontispiece and Fig. 3, OZ). It consists of a hollow sphere of nervous tissue enclosing a space within which lies a small lump of chalk. Like a pea in a thimble, the heavy, chalky mass, or otolith, lies freely in the otocyst, and, if the position of the animal change with respect to the line of action of gravity,—the vertical—the otolith falls or rolls on a new part of the otocyst-wall. Pressing on 10 PLANT-ANIMALS | CH. this area it acts as a stimulus to the nervous tissues beneath. As the result of stimulation of this tissue, nervous impulses may be despatched to the muscles of the body, and, causing them to contract, give rise to movements of the body which are definite in direction. Thus the otocyst serves as an indicator of the line of gravity; in other words it acts as the organ Fig. 8. Young Convoluta paradoxa. C=cilia covering the surface of the body. OT-=otocyst. OC=eyes. V=empty digestive vacuoles. for gravi-perception. By its means, the animal is able to orientate itself with respect to the vertical, and so to find its way downward or upward. That the otocyst does indeed serve this end has been established by experiments with other animals, and may be inferred in the case of C. ros- 1] THE STRUCTURE OF CONVOLUTA Il coffensis from the following facts. . Occasionally, among just-hatched larvee specimens occur which fail to respond like their fellows to gravitational stimulus. Such specimens are found, on microscopic examination, to lack properly developed otocysts. For example, if numbers of C. roscoffensis larvee are taken up with water into a glass tube and the tube is shaken slightly, the animals come down, some tumbling, some curvetting. These animals in general respond to vibration by a geotactic movement—that is, one having reference to the line of action of * gravity—but the one or two, devoid of otocysts, fail’ to descend, remain glued to the side of the tube and are dislodged with the greatest difficulty. As indicated already, the bodies of Convoluta possess a well-developed system of muscles by the ordered contractions of which the movements of the animals are effected. The digestive system is of a primitive order. A well-developed mouth, capable of a wide gape, occurs on the under side of the body rather nearer the “head” than the “tail” end. The mouth communi- cates by a short gullet, not with a distinct digestive tube, but with a loose, central tissue. Hence food which is ingested passes through the mouth to the gullet whence it is distributed to improvised spaces or vacuoles in the tissues (Fig. 3). In these vacuoles the food is digested. The undigested residue is discharged 12 PLANT-ANIMALS (cH. at any point of the body, generally, however, toward the hinder end. Neither species of Convoluta possesses a circulatory system. In the absence of heart and blood-vessels, the distribution of the nutritive substances derived from the food is effected in a primitive manner, the materials being passed from cell to cell. Fig. 4. Convoluta paradoxa. a. Seen from ventral surface, showing the folds of the sides of the body. 6. An animal with nearly ripe eggs (E). There is, moreover, no excretory apparatus, and the waste products are not discharged from the body but remain and accumulate in the tissues. Both C. roscoffensis and C. paradoxa are herma- phrodite, each animal possessing male and female reproductive organs, the essentials of which are, re- spectively, spermatozoa and egg-cells. The eggs are numerous and attain to so considerable a size that 1] » THE STRUCTURE OF CONVOLUTA § 13. they may be seen lying in rows in the _ bodies of “ripe females,’ that is, animals in the female stage (Fig. 4, >, #). The eggs are fertilized in the body, though the spermatozoa which effect fertilization are derived from another individual of the same species. After fertilization, the eggs are discharged in groups or clutches of from about eight to fifteen or more. As it is extruded from the body the egg-clutch becomes surrounded by a transparent, mucilaginous, sticky capsule secreted by the glands on the surface of the skin. A clutch of eggs of Fig. 5. Egg-capsule of Convoluta paradoxa, Each egg is contained in an egg-membrane and the group of eggs is enclosed by a common capsule. (Magnified twenty times.) C. roscoffensis is recognisable to the trained eye as a minute, more or less transparent sphere of about the size of a small pin’s head. The egg-clutch of C. paradoxa is of a similar size; but, owing to the presence of pigmented granules, it is of a rufous colour (Fig. 5). C. roscoffensis lays its eggs on the beach just beneath the surface of the sand : C. paradoxa deposits them on the fine sea-weed lower down the shore. 14 PLANT-ANIMALS (CH. The habitat of C. roscoffensis is restricted and localised (Fig. 1). This gregarious species occurs within a well-defined zone of the foreshore of sandy beaches of Normandy, Brittany, and the Channel Islands. Elsewhere it is unknown. An observer, walking at low tide seaward across a golden beach in Brittany, passes scattered granite rocks scantily clad with yellow-brown patches of seaweed adventuring landward and before he reaches the main belt of brown seaweeds, some yards land- ward of the thin line of green Cladophora which lies bleaching in the sun, he may see dark, spinach-green glistening patches—the colonies of C. roscoffensis. He must tread softly lest the patches melt away at his approach. The colonies may extend for many yards as dark green, irregular strips running more or less parallel with the shore-line, or they may consist of apparently disconnected patches varying in size from an inch or so to a yard or more across. From the intervals between the colonies, the animals are not absent. Though they are not to be seen, they may be smelt. Sand from a part of this roscoffensis zone where no animals are visible, when squeezed between the fingers, emits from the crushed, occasional Convolutas contained in it a pungent and evil smell. The odour, which is like that of decaying fish, is due to the volatile trimethyl amine which is produced by the animal. 1] THE HABITAT OF CONVOLUTA 15 On a peaceful beach, in quiet times, when storms and tourists are absent, the colonial patches of C. roscoffensis keep their respective outlines with sur- prising constancy. Day after day the several patches may be recognised, waxing in size with the spring tides, waning with the neap or slack tides (Fig. 1): larger, also, on any day soon after the tide has receded from their borders; smaller, just before the rising tide invades them. At certain times, the multitude of individuals which make up a patch may be seen lying lethargic and motionless, bathed in the sunlight and the film-like stream of drainage sea-water which oozes from the sand and flows over them seaward. On days of bright sunshine, in particular, the animals lie very still; on duller days, a constant gliding too and fro of these minute films of living matter is to be observed within the confines of a colony. It is on such occasions that the observer must tread softly, for C. roscofiensis is so sensitive to vibration that his heavy, approach- ing tread may send it to earth with lightning speed. How quickly the animals may make their descent from the surface may be judged from the illustration (Fig. 6) which depicts two photographs of a colony, the second taken at an interval of five minutes after the first. Three gentle taps on the sand, after the first photograph was taken, served as the signal for retreat. At that signal, the army, many millions strong, vanished with amazing swiftness and took cover underground. Lest the words “many millions” should seem to 16 PLANT-ANIMALS (cH. savour of exaggeration, it may be said that one colony of moderate size—extending over some two square yards—was found by estimation to contain 5,600 million individuals. Of such flaky thinness are these animals that as many as 28,000 may be packed in a If, Fig. 6. Response of C. roscoffensis to vibration. Reproductions of photographs of a colony. I. before, II. five minutes after the sand had been tapped lightly with the foot. The dark patches in I. represent vast numbers of the animals which in II. have disappeared almost entirely below the surface of the sand. space measuring one cubic centimetre. A search on dark nights at low tide in the roscoffensis zone fails to reveal any of the animals upon the surface. In such circumstances they remain just beneath the sand. On moonlight nights, some, but not many, may be 1] THE HABITAT OF CONVOLUTA Vi seen, by the light of a lantern, lying in the river-films of their diurnal stations. [xcept for a rich micro-flora and -fauna of diatoms, bacteria and infusoria, except for a rare, solitary enemy—another worm,a species of Plagiostoma, which shovels live Convolutas by the hundred into its capa- cious body—except for an occasional, small shore-crab, picking its way with rolling but deliberate gait over the patches, C. roscoffensis enjoys undisputed posses- sion of its tract of foreshore. Though the wastage from each colony must be prodigious, every incoming tide taking toll, yet the species, fecund and resource- ful, rises superior to the circumstances of its environ- ment and maintains itself in the strange situation which fate has chosen for it. The roscoffensis zone (Figs. 1 and 7) is as localised as the range of distribution of the species is restricted. The upper limit of the zone is marked by the level reached by high water at the slackest of the neap tides: for, further landward, C. roscoffensis could not obtain at all tidal periods the diurnal plunge-bath without which it does not thrive. Risk of desiccation bars its more landward advance. The lower limit of the zone is but a few yards seaward, for C. roscoffensis loves the light and ensues it. At every making tide, this zone is submerged and C. roscoffensis becomes a submarine plant-animal sheltering beneath the surface of the sand out of ‘Se 2 18 PLANT-ANIMALS (cH. reach of the shock of the waves. At every falling tide, as the receding waters lay bare the zone, C. ros- coffensis rises to the surface of the sand and becomes a land plant-animal, or rather, a sedentary denizen of the filmy rivers which have their sources in the sand flooded by water when the tide is full. Where the springs of drainage-water reach the sur- face and become rivulets cutting seaward courses, is the upper limit of the C. roscoffensis zone. Thus the colonies are so situated on the beach that they are bathed continuously in running water and receive the maximun of light-exposure during low water at all tidal periods. Records kept during a lunar month show that the time of exposure during low tides is very fairly constant. The time during which C. roscoffensis lies on the surface is, on the average, five and a half hours, and ranges from four and a half to six hours. ‘Twice during twenty-four hours the ros- coffensis zone is submerged and the animals live a life of darkness underground: twice the zone is uncovered and the animals are free to rise to the surface of the sand (Fig. 7). By fixing its station and adjusting its habits, C. roscoffensis succeeds to a remarkable degree in simplifying its environmental conditions. In that station, periods of inundation succeed periods of ex- posure at fairly regular intervals, and, by synchronising its rhythmic movements up to the surface and down below the surface with the movements of the tides, I| THE HABITAT OF CONVOLUTA 19 C. roscoffensis adjusts its working days to the rhyth- mic changes of its environment. How remarkable is the rhythmic movement up and down we shall pre- sently discover. 50 SmgS ee aS A SOLIS, MCE OM BOE IE. Goss SAI ALLDS VBL ALLEL PRE EMR YE be 50 Fig. 7. Habitats of C. roscoffensis and C. paradoxa shown in relation with the rise and fall of the tides during a lunar period. S=spring tides. N=neap tides. Rose. zone=habitat of C. roscoffensis. Parad. zone=habitat of C. paradoxa. The position of colonies of C. roscoffensis is just below the high-water level of the slackest tides. The habitat of C. paradoxa is un- covered at low water except during the slacker neap tides. Leaving the C. roscofiensis zone and passing the rank, brown sea-weeds left high and dry by the tides, the observer paddles into the shallow water, or, if the tide is a big one, walks almost dry shod and sees the long, yellow bands of another sea-weed (Ascophyllum) swaying beneath the water of the pools or lying 2—2 20 PLANT-ANIMALS | CH. prone on the soft, grey ooze of the sea-floor. The extremities of the Ascophyllum are clothed with tufts of fine, epiphytic brown and red sea-weeds. Further out, as the tide continues to fall, the browner weeds are becoming uncovered; first, the dichotomous straps of Himanthalia which spring from button or saucer- like stalks attached to the rocks, and then the finger- like Pyenophycus (Fig. 2) which extends beyond the seaward limits of even the biggest spring tides. It is among the fine weeds attached to Pycnophycus that C. paradoxa is to be found. On dangling these weeds in water, the animals come out, but as single spies not in battalions like C. roscoffensis which lies in swarms thirty yards further up the beach. The abode of C. paradoxa is less circumscribed than that of C. roscoffensis and shifts with the tides. At the onset of the spring tides, minute specimens may be taken from among the epiphytic weeds attached to the most landward of the brown sea-weeds (Fucus). During subsequent spring tides, the animals must be sought lower down the beach in the zone occupied by Himanthalia and Ascophyllum ; whilst, yet later in the same series of spring tides, C. paradoxa is to be found only in the Pycnophycus zone. Just where that dark brown weed ceases to be exposed at low water of the largest spring tides is the further limit of the paradoxa zone (Fig. 7). Like the Greek sailors described in Hothen C. paradoxa hugs the shore. Ex- 1] THE HABITAT OF CONVOLUTA 21 posed now to the violence of the sea and now to the hot sun striking on the drying, emerged rocks and weeds, C. paradoxa has chosen its abiding place. But, unlike C. roscoffensis, C. paradoxa fails to finds in its station a regular recurrence of change, and hence it is constrained to shift its station during the lunar periods. At times of slack tide, the seaward part of the C. paradoxa zone is submerged continuously and the light which reaches the animals clinging to weed some feet below the surface is too feeble for their requirements. Hence, during such tides, C. paradoxa edges up landwards to the shallower water and reaches so far asthe Fucus zone. During the spring tides, this latter zone is left high and dry for hours and hapless C. paradoxa stranded there would suffer from the intense insolation and also run the risk of desiccation. So, as the tides increase, it works its way down the beach, reaching, at the median spring tides, to the more seaward weeds, and at the largest springs, when these weeds may no longer harbour it in submerged peace, it treks again yet further toward the sea and takes up its station among the tangle of fine weeds which hang in tassels from the finger-like, dark brown Pycnophycus. During the slack periods, at low water, when the landward part of the Pycnophycus zone is uncovered, C. paradoxa creeps into the deepest re- cesses of the matted, emerged weeds. Soon, the making tide covers the Pycnophycus with an in- 29 PLANT-ANIMALS [cH. creasing load of water and C. paradoxa, clinging painfully to the floating, swaying weed, finds itself exposed to a light intensity none too high for its requirements. Unlike C. paradoxa which, as we see, migrates periodically, its flittings coinciding with the phases of the moon, C. roscoffensis, having selected its station on the beach, maintains it in spite of time and tide. Small wonder therefore that the latter organism has learned to respond so swiftly to vibrations that it sinks below the sand at the approach of heavy feet. How sure and swift are the uprisings and downlyings of C. roscoffensis may be learned by standing at the water’s edge near by the situations known to be occupied by C. roscoffensis colonies. Scarcely has the tide run off them when a faint green discolouration of the sand marks the contours of each colony, and before the water has receded more than a few yards the dark greenness of the patch indicates that all the animals have risen to the surface. Or if, when the sea is smooth, we watch the incoming tide making its way with gentlest approach toward the | patches, we see the animals inert and lying massed together, bound into scum-like lumps by the muci- laginous excretion of their bodies. They lie motion- less, oblivious of the lapping waves a yard or so away. Then, as the latest wave washes over the patch, © lethargy gives place to action and, in an instant, r| THE HABITAT OF CONVOLUTA 23 C. roscoffensis is gone. On stormy days, when the making tide announces its landward progress angrily —thundering like ramping clouds of warrior horse— the reverberations of the sand send signals to the colonies which make their dispositions underground long before the breaking waves can reach or damage them. All these ordered goings and comings may the observer see on any day on any beach in Brittany. But, to discover more precisely the physiological methods of these purposeful movements, the labora- tory must take the place of the beach, and simple scientific methods must supplement bare observation. In this way, it is possible to refer movements, so pur- poseful as to suggest volition, to simple, non-conscious, nervous responses to one or more of several stimuli, the chief of which are gravity and light. Before, however, we investigate the living animals in the laboratory we may note yet another example of rhythmic behaviour in our plant-animals. However carefully the observer seeks at low water among the exposed weeds of the paradoxa zone, he will find no animals bearing ripe eggs. As the tides be- come large enough to permit of approach on foot to that zone, the animals which he obtains are, for the most part, minute,immature specimens. Onsucceeding days, the catch consists of larger animals, till, during the latest spring tides, it is composed chiefly of adults, many of which may contain unripe eggs. Then comes a period O4 PLANT-ANIMALS [cH. of slack tides when the paradoxa zone is constantly submerged beneath ten feet or more of water. At the succeeding spring tides, the same sequence of immature, young and adult animals is obtained by the collector. The absence of mature females and of deposited egg-capsules is not to be explained by a migration of gravid females to some other place more convenient for the purpose of egg-laying ; for, now and again, a solitary capsule may be found during the latest spring tides glued to the weed of the paradoxa zone. By hatching experiments carried out in the laboratory, it may be demonstrated that the time of maturing of the animals coincides with a definite tidal period. It takes either a month or a fortnight for the animals to become mature. They reach maturity at neap-tidal periods. At the beginning of these periods, or soon after, when the zone is submerged continuously for some seven or eight days, C. paradoxa lays its eggs. No matter how the conditions are altered in artificial hatching experiments, C. paradoxa is faithful to its habit. As indicated by the diagram (Fig. 8), which records the results of such experiments, the females lay their egos only during the neap tides. Nor is it without significance that the large yellow-brown eggs of C. paradoxa, rich in food- yolk, hatch with extraordinary rapidity. Within twenty-four to forty-eight hours of the time of lay- bubs 1] PERIODICITY IN CONVOLUTA —~— 25 ing, the larvee, after circling actively within the capsule, burst the walls thereof and escape. Thus they have, during the remainder of the neap-tidal period, some days of comparative tranquillity and uniformity of conditions. Not for some days yet will they be exposed to the full to the chances and changes which must beset their adult lives. They are born _ as submarine animals, and in their earliest days are spared to some extent the buffetings which shall be theirs when, with the advent of the spring tides, they are, now, clinging to fragile weed dashed against duly. ane ‘ 827 2e 1 466 9 10 111218 1415 16 718192 92. 25 262 29303 6 8 26 8293081 OEE CTT Ba seb ST anRaNSRETTIecee afte PTE rere PENT sli A TAERAMEUAMATAHATATATONUAGOMOMATOUGTOLE Too O@OIMETERS a Fig. 8. Periodicity of egg-laying and hatching of C. paradoxa. The shaded band shows the position of the Paradoxa zone with respect to low water-marks of spring and neap tides. The undulating line, joining up the low water-marks of successive day-tides, is obtained by marking off, along the verticals indi- cating successive days, from a zero line above, the amount of vertical descent (in decimeters) of each day’s tide. On those days when the undulating line falls below the shaded band, the Paradoxa zone is uncovered during low water; on those days when the low water-line lies above the shaded band, the zone is continuously submerged. The dots represent egg-capsules, the crosses signify larve hatched; the positions of dots and crosses give the dates on which the capsules were laid and larve emerged, 26 PLANT-ANIMALS [CH. the rocks and, now, still clinging to the weed, left stranded, prone upon the ooze beneath the glare of an August sun. With ©. roscoffensis, also, egg-laying is a periodic phenomenon, though, in this species, the times at which it occurs coincide not with the beginnings of neap tides but with the onset of the springs. A colony of C. roscoffensis is indeed a well-drilled army. , Not only do all its members take cover as one unit at a given signal, not only do the individuals keep their ranks when the order comes to climb to the surface once again, but they are born together, grow up to- gether, mature at the same time and lay their eggs simultaneously. As a consequence, it is easy to obtain large numbers of egg-capsules, though only at definite tidal periods. To secure them, all that is necessary is to visit a fertile colony at low water during one of the earlier spring tides, tap with the foot and thus drive C. roscoffensis below the surface, scoop up a little sand, shake it with sea-water in a glass tube, and isolate the slow-sinking, transparent capsules. It is still easier, however, to rear them in the labora-_ tory. By collecting a cupful of sand and C. roscoffen- sis just before the onset of a series of spring tides, bringing the cup into the laboratory, adding a little sea-water, leaving it till the plant-animals have col- lected on the surface, scooping them off with a watch-glass and putting them with sea-water in a {| PERIODICITY IN CONVOLUTA 27 large glass vessel, hundreds of egg-capsules may be obtained within a few days. The laying continues for a week or more and then, when the time of the slack tides arrives, it ceases, even though some of the animals are yet carrying mature eggs. After a barren fortnight, egg-laying begins again. Both the animals which failed to bear and those which pro- duced eggs contribute to the fortnightly crop. The mode of egg-laying of C. roscoffensis is in some respects peculiar. Occasionally, the eggs are dis- charged separately one or two at a time; but more often they are contained, as has been stated, in a common, gelatinous capsule. It happens frequently that oviposition results in a rupture of the tissues of the parent. The body becomes torn and may even break across the middle. The anterior end crawls away and, behaving like an intact animal, heals its wounds, regenerates its lost parts and recovers completely. The tail end remains near the egg- capsule, and exhibits ceaseless, revolving, “circus ” movements, swimming in devious spirals; then it comes to rest and finally disintegrates. Unlike C. paradoxa, C. roscoffensis is slow in hatching. After about four days, the larvee begin to revolve actively within the capsule-membrane, then at the fifth to the seventh day after the eggs were laid, the ege-membranes split equatorially and the society of larvee is set free to creep and swim within the common 28 PLANT-ANIMALS [cH. capsule. Suddenly they leave it, passing with ease through the mucilaginous wall, though they not infrequently return now and again to the capsule after enjoying a short spell of activity—a fact the significance of which we shall have occasion to comment upon later. In seeking an explanation of the significance of - the fortnightly periodicity in egg-laying, it is easy to conclude that the periods chosen by C. roscoffensis are the most convenient for this purpose. For in summer, to which period of the year these observa- tions apply, low water of spring tides occurs about midday and midnight. Now, as we have learned, when the roscoffensis zone is uncovered during the darkness of night the animals do not remain on the surface. Hence, during the spring tides, C. roscoffensis has an uninterrupted periad of some eighteen hours in which to lay its eggs. At other tidal periods, its leisure would be less, for, as the tide runs off the patch, the animal must come up to the light and it must re- main up till the returning tide gives the signal that its vigil is at an end. In short, whereas, during the neap-tides, at which periods low water occurs in early morning and late afternoon, C. roscoffensis has two up- and two down-periods, during the springs it has only one period of compulsory “upness” in each twenty-four hours. The weak point, however, of all such teleological 1| PERIODICITY IN CONVOLUTA 29 explanations is that they tend to exercise the in- genuity of men of science rather than to advance our knowledge of physiology. To which it may be answered that adaptation is as much a property of protoplasm as weight is a property of matter, and that the biologist is performing a service in showing how deep-bitten into the organism are the adaptations whereby it adjusts itself to its environment. The critic replies: That is very true, but to rest content with a teleological explanation, to say that this animal does such and such a thing because it is convenient or useful for it to do that thing is to renounce profound investigation. Before this can be regarded as the proper philosophical attitude toward life, the resources of chemistry and physics must be exhausted, and the behaviour under con- sideration must at least be proved not to be due to a chemical or physical change induced by some factor or factors of the environment. In other words, the least the physiologist can do is to attempt to discover how the adaptive trick is performed by the animal which exhibits it. An admirable example of an apparently adaptive character, which is capable of a simple physical explanation, is given by Loeb (1909) in his brilliant essay on the influence of environment on animals. The two species of Salamander, Salamandra atra and §S. maculosa occupy distinct stations. ‘The former 30 PLANT-ANIMALS (cH. species occurs in dry alpine regions of relatively low temperature ; the latter, in lower regions with plenty of water and of higher temperature. In the dry, alpine regions 8. atra deposits eggs which hatch out as land-animals; in the wet lowlands, the eggs laid by 8. maculosa contain embryos in a less advanced stage of development. The young, when born, are gill-bearing and complete their de- velopment whilst leading an aquatic life. Thus each species is adapted to the physical conditions of its environment. But it has been shown that if 8. atra is exposed to lowland conditions, that is, to a moist atmosphere and a relatively high temperature, it lays its eggs earlier, the young hatch out in the gill-bearing stage and development is completed during their life as independent, aquatic animals. Conversely, if S.maculosa is exposed to alpine conditions, oviposition does not take place till the embryos have passed beyond the aquatic, gill-bearing phase. Therefore, in these circumstances, they are born as_land- animals. Hence the adjustment of each species to its environment is due to the direct effect of certain of the physical conditions of that environment on the course of development of the embryos. The fact of adaptation is not denied, but the mechanism whereby it is effected is discovered, and the way made clear 1] PERIODICITY IN CONVOLUTA 31 for a fuller physiological analysis of the mode of reaction of protoplasm to physical stimuli. The problem with respect to periodicity of egg- laying by Convoluta requires us to ascertain whether it is possible to refer the periodicity to any definite, recurrent physical condition of the natural environ- ment of the animal. The facts about to be related appear to indicate that this is possible. It may be premised that if adult C. roscoffensis are kept in darkness for some time previous to the full development of their eggs, no egg-capsules are laid. The lack of egg-production on the part of dark-kept animals is due to the fact that animals kept under such conditions become starved and, as a consequence, incapable of supplying the eggs with food-materials. But if a similar experiment is made with animals containing eggs in an advanced stage of development and already supplied with plenty of food-materials, it is found that the number of egg- capsules produced by animals kept in darkness is actually greater than that produced by animals which are exposed throughout the day to the light. Hence we may infer that exposure to long spells of twelve or more hours of light is unfavourable to the maturing or deposition of eggs. Further experiments on similar lines show that egg-laying reaches its maximum when the animals are subjected daily to one short spell of 32 PLANT-ANIMALS [CH. six hours’ light-exposure followed by a long spell of eighteen hours’ dark-exposure. But—and the fact is remarkable—these conditions of light and darkness are precisely those to which C. roscoffensis is exposed during the spring tidal periods at which its eggs are laid habitually. At such periods, low water of suc- cessive tides occurs about the middle of the day and of the night, and hence, in twenty-four hours, the C. roscoffensis zone is uncovered once during day- time and once during night-time. ‘So it comes about that, during the spring tides, C. roscoffensis is exposed for about six hours to the light and for the rest of the twenty-four hours it is in darkness. Therefore, as the laboratory experiments show, of all the daily changing light conditions to which it is subjected throughout a lunar period, those which obtain at spring tide are most favourable to the deposition of egg-capsules. In ascribing to light a leading véle in determining the periodicity of egg-laying we have the support of not a few well-established biological facts. Thus the profound influence which light exerts on plants, both on their development in general and on their flower- production in particular has long been recognised. Perhaps the best-known example of this influence is afforded by the common ivy. It is a fact of general observation that ivy growing on a wall rarely if ever flowers, though when climbing over 1] PERIODICITY IN- CONVOLUTA 33 an arch exposed on all sides to the light it blooms freely. These effects of illumination on _ flower- formation have been investigated by Voéchting, whose researches are summarised by Goebel (1900). In order that plants may form flowers in a normal way, the illumination must not sink below a certain amount which is very unequal in different species. If illumination is allowed to fall below the required amount, the size of the whole flower or of its individual parts is diminished and, with decreasing illumination, a stage is reached at which the formation of flowers ceases. Similar phenomena are doubtless common among animals though they have not been investigated systematically. Thus, though the phenomenon is not one of reproduction in the strict sense, we may cite Loeb’s account (doc. cit.) of the effect of light in inducing regeneration of the polyps of the Hydroid Kudendrium racemosum. If a stem of this Hydroid, covered with polyps, is put into an aquarium, the polyps fall off very soon. [If the aquarium is in darkness, no regeneration of the polyps takes place even after several weeks; but, when they are exposed to the light, new polyps form in the course of several days. We may suppose that light favours the forma- tion of definite substances which are the pre-requisites for polyp formation. Similarly, we are bound to conclude from our K. 3 34 PLANT-ANIMALS [cn. experiments on C. roscoffensis that a spell of illumi- nation of brief duration favours one or other of the series of processes which results in egg-laying, that a longer or shorter spell of illumination is un- favourable to this process, and that, when animals are subjected to these unfavourable conditions, many of them, though they are carrying eggs in an advanced stage of development, remain sterile. With respect to the periodicity of egg-laying by C. paradoxa it is not so easy to refer the periodic character of this event to the influence of light. It is noteworthy that other littoral, marine organisms, (certain brown sea-weeds) living in almost identical habitats, exhibit an identical periodicity with respect to their reproductive processes. This, according to Williams (1898), is the case with the brown sea-weed Dictyota dichotoma, and subsequent observers have shown it to be true of other marine algze. Not only does Dictyota liberate successive crops of fertile eggs at fortnightly periods but it sheds them at the same point in the tidal period as that chosen by C. paradoxa for the dis- charge of its egg-capsules. In either case, the eggs are liberated some three to five tides after the greatest springs. During the subsequent tides of smaller amplitude, the zone which forms the habitat of both sea-weed and plant-animal is continously sub- merged. Hence we can scarcely escape the conclusion 1| PERIODICITY IN CONVOLUTA 35 that the period selected for egg-laying has reference to the greater security which is offered during the first days of larval life. Born into the world at this period, the animals and the plantlets have some days of submerged grace before they become subjected twice daily to such extreme environmental changes as occur at other phases of the tidal sequence. Thus, driven back provisionally on a teleological explanation, we may interpret the significance and origin of periodicity of egg-laying in the following way. One condition for survival of the species C. paradoxa and of Dictyota is that the just-liberated young shall be for some days after birth continually submerged: that, for one reason or another ultimately connected with nutrition, the maturing of these marine organisms and the development of their sexual cells requires a period of fourteen days, and that the organisms fit their fortnightly periods into the tidal periods in such a way that they reach their climaxes at the most convenient moments. As the waving flag of the guard gives the signal for the train to start, so change of light intensity appears to give the signal for the maturing of the sexual organs and thus secures their liberation at the proper moment. Whether such bi-lunar periods of fertility ex- hibited by littoral, marine organisms have any bearing 3—-2 36 PLANT-ANIMALS [cH 1 on similar periodic phenomena exhibited by the higher land-animals it is impossible to say; though it is tempting to think with “The Lady from the Sea,” “that we all are descended from sea-animals, and that if we had only accustomed ourselves to live our lives in the sea we should by this time have been far more perfect than we are.” CHAPTER II THE ORIGIN AND SIGNIFICANCE OF THE HABITS OF CONVOLUTA ROSCOFFENSIS AND CONVOLUTA PARADOXA. THE fact which stands out most prominently from open-air observations of C. roscoffensis and C. para- doxa is that the behaviour of these animals is complex and purposeful. By some means or other they create for themselves an ordered life, in spite of the welter of change in their environment. Through the ever-varying conditions of the world in which they live, they thread their consistent way as surely as we, with conscious self-control and agility, pick our ways safely through the crowded traffic of the street. We have now to endeavour to ascertain the nervous components of the complex behaviour of our plant-animals; to learn, by the method of ex- perimental analysis, whether it is possible to refer the ordered complexity of this behaviour to some few, simple, nervous acts. 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