3Bi m ^ m w Mm M ^ m ■,(.M-, If 1;5V- nil m m mm mm: X 3 'i"o- «■■ COT „'i ;-:tscJ->!t(>£;^flx3K':^ ;tOoclar.5iio "c'-^a Polyodon Measurements Length of Length of Rostrum as Tote il rostrum to body from per cent of Oo'orces length front of eye front of eye body length mn ' . mm. mm. - — — — - 111. Nat. Hist. Surv . 17. 1 .5 15. ,5 9.7 II 11 II IV. 5 1. .2 16. .3 7.4 II 11 11 18. 5 1 .5 17 8.8 1) It 11 19 1 , .6 17. 4 9.2 II It 11 19 1, .7 17. 3 9.8 II II II 20 1 .7 18. ■z. 9.3 11 11 It 20 1. .8 18 .2 9.9 Borodin 37 8 29 27.6 II 57 15 42 35.7 II 60 19 41 46.3 Danforth 74 22 52 42.3 Barbo ur 80 24 56 42.9 Danforth 89 26 63 41.3 II 104 34 70 48.6 11 107 34 73 46.6 Barbo ur 130 46 84 54.8 Danforth 140 48 92 52.2 II 144 49 95 51.6 II 170 58 112 51.8 11 175 58 117 49.6 II 200 78 122 63.9 111, Nat. Hist. Surv. 200 69 131 52.7 II It It It 212 71 141' 50.4 II 11 11 It 215 69 146 47.3 II It It It 220 71 149 47.7 11 II It It 225 78 147 53.1 11 11 11 11 226 80 146 54.8 II ti It 11 227 82 145 56.6 11 11 11 11 229 75 154 48.7 It 11 It It 230 79 151 52.3 II 11 It 11 O-Z 'Z 78 155 50.3 II It 11 It 233 83 150 55.3 11 11 II 11 235 82 153 53.6 II 11 II 11 240 89 151 58.9 11 It It ti 241 77 164 47.0 11 11 11 It 248 83 165 50.3 II 11 11 ti 250 92 158 58.2 11 11 11 II 252 88 164 53.7 II 11 11 11 255 87 168 51.8 11 11 It 11 261 89 172 51.7 n 11 II II 263 92 171 53.8 11 11 It 11 266 85 181 47.0 II 11 11 It 267 89 178 50.0 11 11 It 11 270 92 178 51.7 II 11 It 11 277 102 175 58.3 11 11 II 11 309 113 196 57.7 zr^^-^.-^'TrT ^. 0'^ H tl l\ Polyodon Measurements — continued Length of Length of Rostrum as Total rostrum to body from per cent of Sources length front of eye front of eye body length mm. mm. mm. 111. Nat. Hist. Surv. 410 131 279 47 »0 ti II II II 453 155 298 52.0 n II 11 II 514 152 362 42.0 II II It II 515 173 342 50.6 II II 11 It 526 177 349 50.7 It II 11 11 528 177 351 50.4 It 11 II 11 531 180 351 51.3 II 11 II It 540 175 365 47.9 11 11 11 II 543 160 383 41.8 II II 11 11 558 185 373 49.6 II 1! 11 II 577 188 389 48.3 Ctockard 610 203 407 49.9 111. Nat. Hist, Surv. 621 199 422 47.2 11 11 II 11 632 197 435 45.3 11 11 II 11 752 236 516 45.7 Stockard 762 229 533 43.0 11 914 259 655 39.5 Barbour 914 245 669 36.6 Danforth 1,000 260 740 35.1 11 1,030 290 740 39.2 II 1,050 310 740 41.9 II 1,070 260 810 32.1 n , 1,090 300 790 38.0 Stockard 1,118 305 813 37.5 Danforth 1,180 320 860 37.2 II 1,200 340 860 39.5 11 1,210 330 880 37.5 II 1,210 310 900 34.4 Stockard 1,245 333 912 36.5 II 1,295 343 952 36.0 Danforth 1,300 330 970 34.0 Stockard 1,346 343 1,003 34.2 11 1,499 356 1,143 31.1 11 1,524 404 1,120 36.1 11 1,575 406 1,169 34,7 11 1,600 400 1,200 33.3 11 • 1,600 419 1,181 35.5 II 1,676 400 1,276 31.3 II 1,702 432 1,270 34.0 It 1,753 454 1,299 34.9 Nichols 2,159 432 1,727 25.0 ••>>■' i* « mliti ■ v\* •J. ■ c. were measured from the front edge of the eye in order to be able to include published measurements. The eye is a well-defined base of reference for measurements in fishes and is particularly useful in growth studies since it is differentiated very early and is firmly anchored in the chondrocranium. Differential growt h. — In the accompanying figure, p. 5, the measurements of the lengDi of rostrum are plotted against those of the length of body using the same logarithmic scale. An examination of the distribution of these points suggests a curve rather than a com.bination of two or three straight lines. Hux- ley (1932) has described a number of relative growth curves in which one straight line breaks abruptly into another straight line with a different slope. He also shows instances in v;hich one straight line breaks into another through a short curve. May it not be that all such relative growth curves are hyperbolas with asymptotes crossing at varying angles and approaching these asymptotes more or less closely? It appears from inspection that the f olyodon measurements lie on an hyperbola whose asymptotes cross at an angle of about 135 degrees. A smoothed curve has been drawn by selecting the point where curvature is greatest and by drav;ing a symmetrical curve to the rather straight limbs at either end. The upper end of the curve, representing measure- ments of the larger specimens, is almost straight with a slope of 0,68, The lower end has a slope of about 4.1. In order to learn something about the distribution of this differential growth within the rostrum of Folyodon , the length from the barbel to the eye was plotted again.^^t body length. This region was found to grovir at the same rate as the body. Therefore, from the standpoint of relative grov;th, this part between the barbel and the eye will be considered as part of the body, since all of the heterogenic growth occurs in the part distal to the barbels. The distance from the posterior edge of the eye to the tip of the opercular flap was measured on the specimens in the collections of the Illinois Natural History bvTvej and combined with other measurements made in the same way by Danforth (1911). Wr.en the logarithm of this length is plotted against the logarithm of body length one obtains a curve of heterogonic growth similar to that for the rostrum except that it shov/s smaller departures from isogenic growth. The slope of the lower limb of the operculum curve is about 1.9, It reaches a value of 1 at about the same body size as does the rostrum curve, and then declines slightly. Barbel length was also measured throughout a wide range of sizes of spoonbill. When the logarithm of the length of barbel is plotted against the logarithm of length of body it may be seen that its relative rate of growth is at all times slower than that of the body within the size range of fi.-^hes used. The barbel curve begins with a slope of 0.7 but, in body lengths exceeding 100 millimeters, it has a slope of only 0,3. It should be mention- ed that the barbel, like the rostrum and the opercular flap, is 'Ui 'yO O' •f-n.- . :nz ; - t - ■ . ia;;n^-*:<>i:4li*3: 'Oj-J .^'S.A ?.iri' -.0 iv.'I" '"'.?■' ::*\ ■J"""" ^i;;= uo.r^oaq--;; " •"- : -■r:■l:-T-^ -" ••-•■,-.,■,-- -.- • ■ ^- nr 4 ,-, «• •' - • • ■ -■ ■ - ■'/••.t V ■ "5.=-/. ,,.•.• , -fc .» • ^&^ 7feiy :,:v;- ,/vc;f)Ov;.!:c" o ■: •J^?iO• 0:K : 0* 'e-^ / •■ jVX;'?)3r?( f -S.i.i5<*>iO O'Oft ^|),iVf -J >iQvijgi;y>j}:; ;•i3'• ■■-br Si^C ; '.);• o o Q Q..i Len^soon lO LENGTH OF BODY IN MM. K)0 lOOO 6 made up essentially of cartilage and skin. I should like to emphasize that the rostrum, the opercular flap, and the barbels each show declining rates of grov/th relative to the rest of the body. Discussion .-~.A3suming an earlier stage in the development of Polyodon than has yet been discovered, a stage in v;hich recog- nizable rudiments of the rostrum, opercular flaps, and barbels have not been differentiated, and keeping the relative growth curves of these organs in mind, an examination of the 17 millimeter specimen suggests that the barbels are differentiated first and go through an initial period of rapid development followed by declining growth rates relative to the rest of the body. Subsequently, the rostrum and opercular flaps are differentiated and grow at high initial rates which decline to the rate of the rest of the body or lower. Since barbels, opercula, and the rostrum each originate from cells, or groups of cells, with the capacity for synthesizing cartilage, it seems likely that the growth histories of these organs may be expressed as an equilibrium reaction betv/een the total amount of growing cartilage in these organs and the concen- tration of cartilage-forming substances in the blood stream which feeds this cartilage. I-olyodon feecs on the same food tliroughout its life. Hence, it may be supposed that the digestive and assimilative pro- ces^^es remain the same throughout its life and, under comparable environmental conditions, that the amount of cartilage-forming substances put into the blood stream is closely proportional to the size of the body. Thus, we may expect that the actual concen- tration of these hypothetical cartilage-forming substances in the blood stream is greater in early stages of development, v/hen there is little or no cartilage, than in later stages when there is relatively much cartilage to use them up by its mere subsistence. The rate of growth of those cartilaginous parts may then be expect- ed to decline to the rate of the rest of the body, Willie the assumption of an unutilized excess of cartil- age-forming substances may account satisfactorily for the high- initial growth rates of rostrum, barbels, and opercular flaps, and for the subsequent decline in the relative growth rate of these parts, it must be modified to account for the fact that the barbel curve has a slope of 0.3 in the same fishes which shov/ rostrum, and operculum slopes of approximately 1, In order to account for such differences in relative growth rates ve may suppose that there are intrinsic differences in their component cells as regards their rate of multiplication, and since the barbels are attached to the rostrum, we may suppose that these Intrinsic differences arise by some process akin to starvation. The same sort of explanation may be used to account for the decline in the rate of growth of the rostrum below that of the rest of the body. Polyodon shows an increasing, proportion of rostrum until the fish has a total length of about 250 millimeters after v/hich the proportion of rostrum declines. It has already been mentioned that the- hcterogonlc -O' . ■ t.>'' .-«,^oi^ ■:'u'K'T'.■'^^cp•,x •^rt «'>41*'*; £j.;r', ,fe.'!!?« :',>r ■■.'©'I >^ •• growth of the rostrum is limited to its distal part. One may look on this distal part as a growing tip — that there is isolated in the tip of the snout of a spoonbill larva a cell, or group of cells, which grow more rapidly than do those of the remainder of the animal; furthermore, that this cell, or group of cells, is carried forv.-ard by its own growth leaving behind it cell descen- dants which grow at the same rate as the rest of the body, i.e., more slowly. Since this growing tip apparently has the capacity to produce cartilage at a greater rate than its cell descendants in the same concentrations of cartilage-form.ing stuffs, it v/ill tend to build up a surplus which cannot be maintained at an isogon- ic growth level once this growing tip is destroyed or is changed in its growth potentialities. As the gross amount of the cell descendants of this growing tip increases relative to the rest of the body, the blood supply reaching the tip must traverse m.ore and more cartilaginous tissue and hence may become poorer and poorer in cartilage-forming p-ubstances. It seems reasonable that this starvation will reach a point where the growing tip is either destroyed altogether or is so altered that its cells no longer multiply more rapidly than thope of other parts of the rostrum. Ap one inspects the outstanding anatomical features of Polyodon in the material which we have available, it is apparent that, the rostrum and the opercular flaps are the last organs to be differentiated. All other organs and major parts already have made their appearance and have paR<=ed through the initial stages of their growth history. We should look upon the heterogenic growth of the rostrum and the operculum as the normal growth beha- vior of parts which have been delayed in their differentiation while other large organs of the body have already been differentiat- ed and have come into growth equilibrium with each other. The rostrum is in a variable and l^j- ve in; oxuii . J-ia uiic uuiiijjcii-Ba a ffCJ. j-co ux a^cuj-iiicuB ux r OlyOdon of the same body length one finds that such anatomical features as body contour, length, width, and insertion of fins, size of eye, and expanse of mouth are very uniform and fixed in their proportions. In contrast with the fixity of these. morphological features the length of rostrxim varies widely. It is apparent, however, that individuals vi;ith short rostra show a compensating increase in width. The lengths of opercular flaps not only vary from fish to fish but are usually different on the right ard left sides. The rostrum of the spoonbill, as compared with the snouts of other fip'hes, may be considered as an organ which has undergone great changes since the ancestors of Polyodon split off from, other fishes, and is twice as large as that of the Eocene spoonbill, Crospopholis . 3A ra^j "^» »r ''l^ I- f^s)'>ir.i3~**'i' 8 The sturgeons are the Folyodontidae , and, since sturgeons have snouts about nearest living relatives of the the same size as that portion of the Polyodon snout between the barbel and the eye (which we have already shown grows at the same rate a?' the rest of the body) we may look on the remainder of the Polyodon rostrum as something added to the essential organization of a sturgeon-like fish. Since this added part appears late in develop- ment and is recognizable only insofar as it grows faster than the rest of the body, it may be considered as a sort of benign tumor which has become partially fixed in its characteristics and is in some. degree useful ap a bearer of sense organs for the detection of fbod. Like a tumor it may be removed without seriously hampering the welfare of the individual since we have found a number of specimens with various fractions of the rostrum cut off and healed over, as far back as the barbels. BIBLIOGRAPHY Barbour, Thom.as , 1911 The smallest Polyodon. Biol. Bull. Vol. 21, pp, 207-208. Cope, E. D. 1883 A new Cbondrostean from the Eocene. Amer. Nat. Vol. 17, pp. 1152-1153. 1885 Eocene paddle-fish and Gonorhynchidae . Amer. Nat. Vol. 19, pp. 1090-1091. 1886 On two new forms of Polyodon and Gonorhynchid fishes, from the Eocene of the Rocky Mountains. Mem, Nat. Acad. Sci , Vol, 3, pp. 161-165. Danforth, C, H. 1911 A 74 mm. Polyodon. Biol. Bull. Vol. 20, pp, 201-204. Dean, Bashford. 1895 Fishes, living and fossil. Columbia Univ. Biol. Series. Ill Macinillan and Co. Huxley, J. S. 1932 problems of relative growth. Methuen & Co., Ltd., London. Nichols, J. T. 1916 A large Polyodon from Iowa. Copeia. No. 34, p. 35. Stockard, C. R, 1907 Observations on the natural histor.y of Polyodon spathula. Amer. Nat. Vol. 41, pp. 753-766. Thompson, David H. 1933 The finding of very young Polyodon, Copeia No, 1, pp. 31-33, a 61 ;J \f4\i-^' ,- . •■ -"-i^ \J ic- .:jM: '■