A CONTRIBUTION TO THE STUDY OF RELATIVE GROWTH OF PARTS IN INACHUS DORSETTENSIS

Transcription

1 145 A CONTRIBUTION TO THE STUDY OF RELATIVE GROWTH OF PARTS IN INACHUS DORSETTENSIS BY M. E. SHAW, M.Sc. (From the Zoological Laboratory, King's College, University of London.) (With Eleven Text-figures.) {Received July 1928.) 1. INTRODUCTION. THE work which forms the subject of the present paper was carried out under the direction of Professor J. Huxley to supplement work of a similar nature which he himself carried out on Maia squinado (see Huxley 1927) where, inter alia, he found that certain facts could be accounted for by postulating a growth-gradient or graded distribution of what may perhaps be called growth-potential, from a centre in the chelar propus downwards along the chela and then backwards along the body. 2. MATERIAL AND METHODS. The crabs were obtained from Plymouth, as many various sizes as possible being selected, with approximately equal numbers of 33 and $$, and the investigation was carried out by taking linear measurements of the different appendages and parts of the body. The method of measuring as large a number of crabs as possible, taken at random from a population but including a large range in size, was adopted because of the difficulty of keeping crabs in captivity and collecting the successive moults. 162 crabs (75 3 and 87 $) were measured, ranging in size from 6 to 34 mm. in carapace 1. The measurements carried out were as follows (see Text-fig. 1): 1. Carapace (this was the standard measurement with which all others were compared). From a point between the two anterior spines to the median point of the posterior border of the carapace. 2. Carapace breadth. Greatest breadth of the carapace, between the bases of the first two pereiopods. 3. Cheliped. Only the propodite was measured since the folding back of the cheliped in the $ makes a total measurement unreliable. (a) Propus (see diagram). (b) Propus breadth (maximum). 1 All crabs measured were free from Sacculina externa and any showing obvious regeneration of an appendage were rejected. IO BJEB'Vlii

2 146 M. E. SHAW 4. Pereiopods. These were removed from the body at the breaking joint, but the junction of the merus with the ischium was found to be a more convenient point from which to measure, so that the measurement taken was the of the posterior border of the pereiopod from the proximal end of the merus to the tip of the dactylus. The pereiopod is, of course, flexible structure but it is a simple matter to straighten the limb and take the measurement accurately to the nearest \ mm. 5. Third maxilliped. A measurement was taken from the point of junction of the ischium and basis on the inner side to the most distal point of the merus (see diagram). m ' D Carapace E Cara-pwce Text-fig. 1. Diagrams showing measurements taken. A. Carapace; ab, median ; cd, maximum breadth. B. $ Chelar propus; ef, ; gh, breadth. C. 3rd maxilliped; ik, of ischius and merus. D. $ abdomen; Irn, ; no, breadth of 6th segment; rs, of individual segment e.g. 4th; pq, maximum breadth of individual segment e.g. 4th; E. <J abdomen; tu, ; vzv, breadth (6th segment). 6. Abdomen. A?. (1) Length. Greatest of the abdomen in the extended position, from the median point of the posterior border of the carapace to the tip of the sixth abdominal segment. (2) Length of 3rd, 4th, 5th and 6th, abdominal segments. (3) Breadth of 3rd, 4th,^th and 6th, abdominal segments. B <?. (1) Length. Greatest in extended position. (2) Breadth of 6th abdominal segment. Measurements on the pereiopods were carried out by placing the leg in the fully extended condition on a metal mm. rule; on the carapace ( and breadth) and abdomen (, <J and $) with fine accurately adjustable dividers. The third

3 Relative Growth of Parts in Inachus Dorsettensis 147 maxiuiped, chelar propus, and breadth of the abdominal segments were measured by making use of the travelling stage of a microscope with a cross-wire in the eyepiece. The measurements on the pereiopods were probably accurate to the nearest 1 mm. and the abdomen and the carapace to about \ mm., all others to about J^mm. The biometric constants for the measurements have deliberately not been calculated since this piece of work is intended only as a general mapping of the ground which may serve as a basis for more detailed work on certain parts in the future. Some of the smaller differences obtained are doubtless not statistically significant. The original data have been deposited at the British Museum (Natural History). The results were analysed as follows: The S3 were divided into six classes and the $$ into eight classes according to carapace, and the mean absolute sizes and relative sizes of the different appendages and parts of the body calculated for these classes. Graphs were then constructed showing: A. A comparison of the mean relative s of the appendages in the <$ and the? (constructed on data from all classes of crabs together). Graph VII. B. A comparison of the percentage increase in size of the appendages and abdomen in the $ and $ for a given percentage increase in carapace. Graph VIII. C. A comparison of the ratio <?/$ for the absolute size of all the appendages and for the abdomen in large and small crabs, i.e. change in this ratio with change in size. Graph IX. D. Graphs where the relative of the part in question was plotted against the carapace. These showed growth changes in the proportions of the various parts. Graphs III, IV, V. E. Graphs where the logarithms of the mean sizes for the different classes were plotted against the logarithms of the mean carapace for the classes. These were of value in determining k in the heterogony formula y = bx k (y = measurement of part in question, x = standard measurement), and so in comparing the relative growth-rates of the different parts of the body. Graph I. 3. RESULTS. A. GROWTH OF THE PROPODITE OF THE CHELIPED. (1) Male. The points on the log. log. graph showing the relative growth in breadth (Graph I B) form a rough approximation to a straight line whose inclination gives k = 17. However, when we look closer, we find that the relative growth is at first less than this (from 8-15 mm. carapace k = approximately 1-4), followed by an acceleration of relative growth (k = approximately 2-5), with finally a marked falling off for the last size class. The log. log. graph showing the changes in relative growth-rate for the of the chelar propus is exactly similar to that for the breadth, but k calculated for the general slope of the line is only 1-3, and the various changes in relative growth-rate are not so marked. 10-2

4 148 M. E. SHAW The deviation from a simple straight line series of points is in all probability due to the phenomenon of "facultative" high and low dimorphism, first described by G. Smith (1906) for /. scorpio. In this species the normal strong heterogony of the $ chela is replaced by the $ isogonic type of growth in the non-breeding season. This period is usually passed through in the winter months, but not necessarily, so that during the breeding season there may be three classes of <J<J, " high," "low," and "middle" or "female" type. The "high" and "low" <$<J are large bodied and small bodied respectively and most of the "middle" type are of intermediate size. The "middle" <$<$ have the $ type of chela, the others the S type but of different relative size (see Huxley 1927 for analysis). In the summer (breeding season) the number of middle <J<J is small and depends on when the last moult took place. In the winter months there are a few "high" $<$ but no "low" SSj all the small crabs having the flat? type of chela. D Text-fig. 2. A. Dorsal view with abdomen in the extended position showing origin of the 4th pereiopod opposite the ist abdominal segment. B and C. Extreme variation in shape of chelar propusin&jof 18 mm. carapace ; B, "high"c? type; C, "$"typeofcj. D. Chelar propus of? of 18 mm. carapace. The measurements which form the subject of the present paper were carried out on specimens of /. dorsettensis collected at three different times, namely April 1926, October 1926, October Thus the first batch was collected at the very beginning of the breeding season and the other two batches in the non-breeding season, so that one would expect only a small number of " low " SS to be represented. In this species there is evidence that the $ chela does not go over to the $ type of growth, in the non-breeding season, nearly so completely as in /. scorpio. An inspection of the chelae of all the small and medium sized 3 crabs (under 20 mm. carapace ) showed that a small number of crabs had the definite $ type of chela, a small number the full S type (see Text-fig. 2), but that in the majority the chela was intermediate in shape between the < and $ types, and that all gradations between the two extreme conditions existed. A plot of mean chelar breadth against mean carapace on the absolute scale showed no interruption of the

5 Relative Growth of Parts in Inachus Dorsettensis 149 curve at intermediate body sizes, such as was found by Huxley (1927) on analysis of G. Smith's results on /. scorpio. In view of these facts one would expect Graph I B to show only a slight trace of the phenomenon of "facultative" high and low dimorphism, and examination of the graph proves that this is the case. This graph may be interpreted as follows: from 8-15 mm. carapace the <J chela is of the $ type but relatively larger than in $$ of the same size; from mm. carapace the positive heterogony is slightly more marked and this part of the graph probably represents the "low" S$ taking part in the first breeding season; from mm. carapace there is an almost imperceptible decrease in relative growth-rate which may be explained as being due to reversion to the $ type of growth, in a certain number of cases, during the non-breeding ' Carapace in mm. Text-fig. 3 (Graph I). Logarithmic plot of means for $ chelar propus (A), breadth (B); $, chelar propus (c), breadth (D); <J, pereiopod (E); $, pereiopod (F); against carapace. season; from mm. carapace there is strong positive heterogony, this part of the graph probably representing the "high" $$ taking part in the second breeding season; the final falling off may be due to a second non-breeding season in old animals but as it is based on one class only this cannot be pressed in any way. It is more probable that it has no real significance as a similar large decrease in slope is commonly found towards the end of graphs showing the relative growthrate of heterogonic organs, and has a purely mathematical explanation. There is however dimorphism of a sort in the <$ chela as it shows a bimodal frequency curve (Graph II), the two modes occurring at 4 mm. and n mm. chelar breadth, and representing true "high" and "low" forms. Thus there are two phases of <? chelar growth, and the conversion from the "low" type to the "high " type takes place suddenly and presumably at a single moult, which occurs

6 M. E. SHAW most frequently at about 20 mm. carapace. A similar bimodality for chelar propus breadth is shown in the correlation table given by G. Smith (loc. cit. page 97) for /. scorpio, the modes occurring at 3 mm. and 10 mm. chelar breadth. It is, however, not referred to by him in his text. A plot of the modes for the chelar breadth at different carapace s, on the log. log. scale, for /. scorpio, showed HA Relative breadth $ chelar propus IIB a 9it 00 u o Gj I OS T ,L 15Ui T - -r -r - T T. T - T T T 13L 12f f T 91,. T 0 t If. cj Chelar propus breadth in mm. He II S Chelar propus breadth in mm. Text-fig. 4 (Graph II). II A. Frequency of relative <? chelar propus breadth for all specimens. II B. Absolute frequency of <J chelar propus breadth at various carapace s. II c. Absolute frequency of <J chelar propus breadth for all specimens taken together. that there is the same enormous variation in relative growth-rate as in /. dorsettensis. Comparison of G. Smith's normal and infected ^ (no data for the $ available) showed that in /. Scorpio, when there is reversion to the $ type of growth, the <J chela in a certain number of specimens actually goes over to values equal to those for the infected $

7 Relative Growth of Parts in Inachus Dorsettensis 151 (2) Female. The growth of the chelar propus in the $ shows slight positive heterogony; the points on the log. log. graph (Graph I c and D) conforming to approximately straight lines, for which k= 1-16 for and 1-15 for breadth, so that, in contrast to the #, the relative increase in is greater than in breadth. B. GROWTH OF THE PEREIOPODS. (1) Male. The relative growth of the pereiopods is shown in Graph I E and Graph III. Graph III shows that the changes in relative growth-rate are in a general way similar to those in the chelar propus but, owing to the much greater of the pereiopods, all changes are much more marked. The peculiar "back kink" of the curve for all the pereiopods is hard to interpret. It may well be that the relative growth-rate of the pereiopods decreases at the same time as that of Pereiopods Chelar 3rd propus maxilliped 0 20 Mean relative breadth I Mean relative s Text-fig. 5 (Graph III). <?. Mean relative of 3rd maxilliped, chelar propus, pereiopods 1-4, and mean relative abdomen breadth plotted against mean carapace. the chelar propus decreases {i.e. there is a similar slowing down of growth in the non-breeding season) and that after this period when the relative growth-rate of the chela rapidly increases, this acts as a drain on the pereiopods, so that these never again attain to their former relative size. On the other hand it may be entirely due to a "draining" or retarding effect of the chela, the effect of which is most obvious just before the period of strong positive heterogony in "high" &J rather than while this is actually taking place. As the log. log. graph (Graph I E) shows, there is an actual decrease in absolute size (between mm. carapace ) for all the pereiopods except the 3rd which shows a trivial increase, k in the heterogony formula calculated for classes 1-5 gives the following series: P1, 1-22; P2, 1-22; P3, 1-26; P4, 1-27; where P = pereiopod, and after the period of size decrease k does not again reach its previous values for the remaining classes (6-8). (2) Female. The growth of the pereiopods is shown in Graph I F and Graph IV. The graphs are rather irregular in spite of the fact that there are larger numbers 20 40

8 M. E. SHAW of individuals in each class than in the case of the <J, and the meaning of this is not at all clear. Only to an extremely limited extent can the irregularities be said to follow those of the chelar propus. On the other hand the general form of the curve is similar for all the pereiopods. An interesting point brought to light by the log. log. graph (Graph I F) is that, over the whole size range covered, the pereiopods show positive heterogony although this is not so marked as in the case of the <J, the k series being P i, i-n; P 2, I-IO; P 3, 1-06; P4, 1-08 (the last class being neglected for all the pereiopods in the calculation of k). The significance of the rise in k for P 4 will be referred to again later. C. 3RD MAXILLIPED. The maxilliped is very slightly negatively heterogonic in both sexes (see Graphs III and IV), k for the $ being -98 and for the <J -95. This difference between the <? and the $ may not be significant, but it is clear that there is no positive heterogony. Pereiopods K Abdomen 4th. 3rd 2nd 1st Chelar propus 3rd maxilliped!_. I. 1! I I (! I I I lit II Mean relative breadth KQ I Mean relative s Text-fig. 6 (Graph IV). 2. Mean relative of 3rd maxilliped, chelar propus and pereiopods 1-4, and mean relative abdomen breadth plotted against mean carapace. D. ABDOMEN. Graph V is constructed from measurements of the 6th abdominal segment in both <$ and $, and shows that in the $ there is a period of slight positive heterogony, then one of very strong positive heterogony followed by a period of isogony. The first period represents the narrow flat abdomen of the adolescent crab, the last the broad abdomen with convex ventral surface of the adult crab, and there is evidence that these two types are separated by only a single ecdysis as in the case of /. scorpio as mentioned by G. Smith (loc. cit. p. 68). This has not been actually observed in the case of /. dorseitensis but is proved by the existence of a discontinuously bimodal frequency curve for the abdomen breadth (see Graph VI). The case is somewhat similar to that of the <$ forceps in Forficula (see Huxley 1927), but the "low" type (adolescent abdomen) and the "high" type (adult abdomen) are more distinct (see Graph VI). All the small $ crabs are of the "low" type and all the large $ crabs of the "high" type, but crabs of medium size (12-17 mm - carapace ) fall into one or other of the two equilibrium

9 Relative Growth of Parts in Inachus Dorsettensis 153 positions. Within the "low" group there is slight positive heterogony (k = 1-4) and the "high" group is isogonic or even slightly negatively heterogonic (k = approximately -97). The case is different from that of Forficula in that the "high" type follows the "low" type in time, and during the persistence of each type a number of moults may take place. Further the relative abdomen width in the "low" type increases slightly with increasing body size and in the "high" type remains constant, whereas in Forficula the relative forceps taken separately for each type slightly decreases. In Inachus it seems clear that the abdomen growth consists of two long periods, one of slight positive heterogony, the other of isogony, Relative abdomen breadth Text-fig. 7 (Graph V). Mean relative abdomen breadth plotted against mean carapace for o (A-A), $ ( - ). Limits for each carapace class plotted for both <J and?. separated by a short period of violent heterogony, which presumably begins directly after a moult, since its effects are shown completely by the next moult. The range of size over which this may occur is mm. carapace. The linear sizes are as 1 : 1*3, which would correspond closely to a doubling in volume and according to Brook's and Przibram's law and experimental data on Carcinus would imply that there is a range of one whole instar for the particular moult at which the adult abdomen is assumed. Similar considerable variation in the size at which the full relative width or adult abdomen is attained is found in the Fiddler crab Uca (see Huxley 1927). Whereas the $? of both Uca and Inachus acquire a definitive relative abdomen breadth, unpublished data, by Huxley and Richards, on Carcinus moenas, show that this does not occur in Carcinus, the abdomen (like the 3 chela of Uca etc.) becoming relatively larger with increased absolute size throughout the whole of life.

10 154 M. E. SHAW As in the case of /. scorpio, the adolescent abdomen appears to be retained until the first brood of eggs is produced, as (with the exception of four specimens) all the crabs with "adult" abdomen are in berry. The four specimens mentioned above are of carapace s 15*2 mm., 15*7 mm., 16-5 mm., 20-2 mm., and have the full relative width abdomen. There is of course the possibility that they were about to pair or that their previous brood of eggs had hatched. As regards the individual segments of the abdomen in the $, it was found that in the adolescent crabs showing the "low" type of abdomen, k was the same for all the segments, both as regards and breadth (k = 1-3), with the exception of the 5th segment where k for was 2-2. (Only the 3rd, 4th, 5th and 6th segments were measured as the 1st and 2nd segments are too small to be measured with sufficient accuracy.) k for this period was calculated from classes U and V, Table III. For the period when the rapid transition from the "low" a s 22-2lL 18 r - T T IT IT TT. T T o cs a, «a ' Relative abdomen breadth Text-fig. 8 (Graph VI). $. Frequency of relative abdomen breadth at different carapace s. type of abdomen to the "high" type is taking place k is greatest for the 6th segment breadth and is approximately 3-15; for the breadth of the remaining segments it gradually decreases (seg. 5, k = 2-8; seg. 4, k = 2-6; seg. 3, k = 2-5). The greatest increase in relative again takes place in the 5th segment \k = 3-1). The values of k for the s of the other abdominal segments are as follows: seg. 6,k= 2*9; seg. 4, k = 2-9; seg. 3, & = 2-1. From this it will be seen that apart from the high value of k for the 5th segment, there is a gradual decrease in k from the distal to the more proximal segments. On the whole the k values for the breadth are higher than those for the, and this is what one would expect as in the.transition from the "low" to the "high" type the main alteration is an increase in relative breadth.

11 Relative Growth of Parts in Inachus Dorsettensis 155 Graph V shows that the abdomen in the <J is isogonic in young crabs, but becomes slightly negatively heterogonic in older crabs. E. GRAPHS OF RELATIVE GROWTH OF PARTS. A series of graphs were constructed to compare the $ and $ as regards relative growth of parts (list already given, p. 145 etseq.). (1) Graph VII. The mean relative s of the third maxilliped, chelar propus, and pereiopods 1-4, were plotted for the <3 and?, and it was found, as was to be expected, that the chela in the <J attains to a much greater relative size than in the?. The result for the pereiopods was rather unexpected and the exact opposite of the result obtained by Huxley (loc. cit.) for Maia squinado. In Maia the differences in relative of the pereiopods in the and the $ fall regularly from Pi to P4 and, what is more important, the difference expressed as a percentage of the $ value decreased from P1 to P 4 in the same way, indicating that the strongly heterogonic growth of the chela in the 3 was correlated with the growth of the pereiopods, P 1 being affected the most and P 4 the least. The difference in the relative size of the 3rd maxilliped in the <J and $, expressed as a percentage of the $ value was 4*33, indicating a retarding effect of the active growth of the <J chela on the appendage anterior to it. 3rd Chelar 1st and 3rd 4th maxilliped propus "?' I""" (xio) (x6) Pereiopods Text-fig. 9 (Graph VII). Mean relative s of various appendages in $ and $. In Inachus, on the other hand, the differences between the mean relative s of the pereiopods of the <J and $ do not decrease regularly in this way (P 1,15-2; P 2, 16-2; P 3,147; P 4,13-3) and the percentage differences show exactly the opposite result (P 1, 5-0; P2, 7-1; P3, 7-8; P4, 8-2) so that relative to its the 4th pereiopod is affected most. The 3rd maxilliped, however, showed the same result as in Maia, namely a negative percentage (see above) of (2) Graph VIII. This graph was constructed by finding the percentage increase in abdomen and appendage size for <J and $ crabs taken over the same range of increase in carapace. It shows that in none of the $ appendages is the percentage increase in size so great as in the <?, but for the abdomen it is very much greater in the $. In the $ pereiopods the percentage increase is greatest for P 3 and P 4 as was to be expected from the values for k. In the $ pereiopods the percentage increase is practically the same for P 1, P 2 and P 4, but considerably less for P3. This is accounted for by the low value of k for the 3rd pereiopod, but it is difficult to say with certainty what is the meaning of this. It may perhaps be explained by assuming that as the 4th pereiopod lies opposite the 1st abdominal segment it is included within the region of active growth which is responsible for

12 156 M. E. SHAW 3rd maxilliped Chelar propus CO ist O and a O I i J3 3rd - \4th Abdomen SQ Q3 % increase Text-fig. 10 (Graph VIII). Percentage increase in size of various appendages and abdomen in <$ and < for given percentage increase in carapace (shown by vertical lines, 2, ). A Large <?# Large 2 2 Small o* o* (v ' Small 2 2 /o 3rd Chelar 1st and 3rd 4th Abdomen maxilliped propus ' - ~ Pereiopods Text-fig, ii (Graph IX). Ratio <J/2 for various appendages and abdomen in large and small crabs.

13 Relative Growth of Parts in Inachus Dorsettensis 157 the great increase in relative size of the abdomen (see Text-fig. 2 A). If this is the correct explanation then k for the 4th pereiopod is abnormally high ov/ing to its proximity to the abdomen, rather than k for the 3rd pereiopod abnormally low. This would agree with the k series for the 2 pereiopods, as k falls regularly from the 1st to the 3rd pereiopod. For the 3rd maxilliped the percentage increase is greater in the $ than the?. It is worthy of note that in the posterior part of the body the graph is almost the exact reverse in the <J and the $. Graph VIII may be said to give part of the "growth profile" of the two sexes. (3) Graph IX was constructed by plotting the ratio ^ Va ] Ue (as a percentage) Table I. 3. Mean absolute measurements in mm. Reference letter of class Carapace classes Average carapace No. in class Chelar propus breadth 1 Pereiopod rd maxilliped Abdomen breadth A BC D E FG H I-I i3'8 u*7 9*4 76 4' IO * o-8 5O'4 5O-5 43* l8" ' *9 I4' ' O '4 5' 4' ' ' * i-8 Table II. +'. Mean absolute measurements in mm. Reference letter of class Carapace classes Average carapace No. in class Chelar propus breadth Pereiopod rd maxilliped Abdomen breadth Abdomen breadth "low" type "high" type A BC D E FG H J K US ' '5 3'2 3-i i-8 i' ' '5 44'7 43" ' '3 43'3 4 " "3 33'5 32' i 3O' 'O I ' * *7 5' ' i 3-8 3'i 14-4 I3'3 I2'6 12'I S 8" '2 3' i 5"6 II-2 IO-I 9'3 for the 3rd maxilliped, chelar propus, and pereiopods 1-4 in (a) small S3 (classes G and H Table I) and small?? (classes J and K, Table II) and (b) large <?<? (classes A-F, Table I) and large $? (classes A-G, Table II). In the small crabs the chelar propus and 1st and 2nd pereiopods are larger in the S than the $ but the reverse holds for the 3rd maxilliped and the 3rd and 4th pereiopods, and of course for the abdomen. The value for the 4th pereiopod is high because of the high value of k in the Q. In the large crabs the ratio for the chelar propus has increased con-

14 158 M. E. SHAW siderably and the ratios for all the pereiopods have increased to approximately the same value so that pereiopods 3 and 4 have ceased to be longer in the $ than the 3. The ratio for the 3rd maxilliped is slightly nearer 100, showing possibly that the 3rd maxilliped in the <$ starts by being smaller than that of the $ and, although always remaining slightly relatively smaller, more nearly approaches it in size in large crabs. The difference, however, may not be significant. Table III. Mean absolute measurements for abdominal segments in $. Reference of class Carapace classes in mm. Mean carapace in mm. 6th seg. Length 5th seg. in mm. 4th seg. Abdominal segments 3rd seg. 6th seg. Breadth 5th seg. in mm. 4th seg. 3rd seg. U V w X Y Z I i9' o i 5"5 5" O *2 0-7 i-o i-6 2-O o i-5 i '9 5" i II-I ' ii '7 5-o 7* GENERAL RESULTS AND DISCUSSION. 1. Axial Gradients and the Relative Growth of Parts. Analysis of the growth-rates of the different appendages seems to show that it is not possible to interpret the greater relative of the pereiopods and smaller relative of the 3rd maxilliped in the <J as simple stimulating and retarding effects of the enlarged chela of the <$ acting on a pre-existing growth gradient. That there is a graded effect on the pereiopods is obvious but that it is the very reverse of a stimulating effect is equally obvious as the percentage increase in is considerably greater for the 3rd and 4th pereiopods than for the 1st and 2nd pereiopods, and the difference in mean relative between the pereiopods in the <J and the $ expressed as a percentage of the $ value show a steady increase from pereiopods 1-4. The k values for the pereiopods corroborate these results, k in the < being approximately the same (1-22) for the 1st and 2nd pereiopods but greater for the 3rd and 4th pereiopods (1-26 and 1-27). It is possible that both the $ and the $ may be supposed to have the same growth mechanism (a gradient in slight positive heterogony with k greater for the cheliped and least for the 4th pereiopod, but that the draining influence of the very active growth of the cheliped in the <J is such that the gradient is reversed in the <J, k being least for the pereiopods immediately posterior to the cheliped. The original gradient is not affected in the? where the chelar propus is only very slightly positively heterogonic. An explanation of the facts would be provided by the assumption that in the < there is a general growth promoting effect and at the same time a draining effect of the large chela, in antagonism. The 3rd maxilliped in both sexes is slightly negatively heterogonic and more

15 Relative Growth of Parts in Inachus Dorsettensis 159 so in the 3 than the $. This indicates that there is a different growth mechanism in the appendages anterior to the chela, and that possibly the negative heterogony is more pronounced in the 3 because the chela has the same retarding effect on the growth as in the case of the pereiopods. This retarding effect must not be stressed in any way as the difference between the 3 and the $ may well not be significant. 2. Distribution of Growth-Potential. One definite result of this work on Inachus dorsettensis has been to show that there are marked regional differences in the relative growth-rates of the different parts of the body, and that the two sexes differ in this respect, the distribution of what we may call "growth-potential" being in favour of the hinder thoracic appendages, especially the cheliped in the 3, and in favour of the abdomen in the $. Thus greatest heterogonic growth is found associated with the development of the secondary sexual characters, but neighbouring parts, not usually thought of as secondarily sexual, may be involved in this. Somewhat similar results were obtained by Kunkel and Robertson (1928). 3. Growth-Centres. Recent work on heterogonic growth in Crustacea has brought to light the fact that in parts of the body showing positive heterogony there is a growth-centre, which in appendages is usually towards the distal end. In the 3 Inachus dorsettensis it is obvious that the propus is the growth-centre for the cheliped; in the $ the growth-centre for the abdomen is also near the distal end, the greatest increase in breadth occurring in the 6th (last, most distal) segment, and the greatest increase in in the 5th segment. Huxley (1927) showed that in Uca and in Maia the growth-centre for the chela (based on weight measurements) was in the propus. SUMMARY. 1. The relative growth-rate of the various parts of the body was investigated by analysis of linear measurements carried out on the carapace, appendages, and abdomen. 2. The results were as follows: (a) The chelar propus shows strong positive heterogony in the 3 and very slight positive heterogony in the $. The 3 chela shows dimorphism, the 33 being differentiated into "high" and "low" forms and the dimorphism in all probability being due to the fact that the 3 chela assumes the $ type of growth to a greater or less extent in the non-breeding season. (b) The pereiopods are more positively heterogonic in the 3 than the $ and in both 3 and $ there is a graded k series, but whereas in the 3 k increases from P 1-P4, in the $ the series is reversed k being greatest for P 1. In both sexes the heterogony in the pereiopods is not so marked as that of the chelar propus. In the 3 the pereiopods suffer an actual decrease in absolute size at the time when the relative growth-rate is least for the chelar propus and after this period never again attain to their original relative size.

16 160 M. E. SHAW (c) The third maxilliped is negatively heterogonic in both sexes and slightly more so in the <5 than the 2, although this difference may not be significant. (d) The abdomen in the 2 shows marked positive heterogony in young crabs but is isogonic after the attainment of sexual maturity; in the 6* the abdomen is isogonic in young crabs but becomes slightly negatively heterogonic in old animals. The abdomen in the 2 is dimorphic, the "low " type (characteristic of the adolescent crab) being separated from the " high " type (characteristic of adult crab) by a single moult. There is considerable variation in the relative abdominal growth-rate in adolescent crabs and consequently in the time of attainment of sexual maturity and the full relative abdominal width. During the period of adolescence the increase in relative and breadth is the same for all segments except the 5th, which forms a growth-centre for. At the period of rapid conversion of the "low" type of abdomen into the "high" the 5th segment remains the growthcentre for but a growth-centre for breadth is established in the 6th segment, and increase in relative breadth decreases from the distal to the more proximal segments. 3. A comparison of the <$ and 2 as regards relative growth of parts was made and brought to light certain facts. All the pereiopods are relatively longer in the $ than the 2. This is a graded effect the actual difference in relative (in and 2) decreasing from P i-p 4, but the increase in relative relative to the actual size of the pereiopods increasing from P i-p 4. These facts are interpreted as meaning that there is a common stimulating effect in the <, and also a retarding effect of the 3 chela on the appendages posterior to it. It is tentatively suggested that the facts relating to the 3rd maxilliped are explained if we assume that the <$ chela has the same retarding effect on the anterior appendages as on those posterior to it. 4. Certain general conclusions w r ere drawn: (a) There is a different distribution of growth-potential in the two sexes, and strong positive heterogony is found associated with the development of the secondary sexual characters. (b) The results on the relative growth-rates of the appendages in <J and 2 and on the growth-rates of the individual segments of the 2 abdomen, indicate the presence of definite gradients in the body, in relation to which growth takes place. (c) The growth-centre for the heterogonic organs is situated towards the distal end of the organ; this is in agreement with results obtained for other Crustacea. REFERENCES. HUXLEY, J. S. (1927). Biol. Zentralb.47, KUNKEL, B. W. and ROBERTSON, J. A. (1938). jfourn. Marine Biol. Ass. 15, SMITH, G. (1906). Fauna u. Flora Golf Neapel, Monog. 29,