CORRELATED GROWTH-CHANGES 123 



This was first discovered in regard to weight-measurements 

 on the spider-crab Maia squinado (Huxley, 1927) ; and next 

 confirmed, this time in regard to linear measurements, on 

 another spider-crab, Inachus dorsettensis (Shaw, 1928). I then 

 felt that the phenomenon might be due to one of two rather 

 different causes. In both these cases, the organ diminished 

 in size in the male was the third maxilliped, the pereiopod 

 which was enlarged as the male chela being the first of the 

 walking-leg series. It was thus possible that the effect had 

 nothing to do with the main growth-gradient, but was in 

 some way due to the nature of the appendages concerned, 

 maxillipeds for some reason responding differently from 

 pereiopods. 



To decide between these two alternatives, it was necessary 

 to find an organism in which some other pereiopod than the 

 first was enlarged to produce a large chela. The true prawns 

 provide a case in which the second pereiopod is so enlarged ; 

 but unfortunately in the common British species there is little 

 difference between the sexes in the size of the large chelae. 

 Eventually, through the kindness of Dr. Seymour Sewell, of 

 the Indian Museum, a number of specimens were obtained of 

 the magnificent Indian prawn, Palaemon carcinus, a species 

 with marked sexual dimorphism in the chelae, and measure- 

 ments on these established the fact that the effect is a true 

 positional effect, since in the male, whereas the relative size 

 of third, fourth and fifth pereiopods were increased, that of 

 the first pereiopod as well as of the third maxilliped were 

 decreased. 



In general, male Crustacea appear to have relatively larger 

 pereiopods than females. But here the difference between rela- 

 tive size of pereiopods in the two sexes is much reduced in the 

 pereiopod anterior to the chela, showing an inhibiting effect 

 of chela-growth. Another method of studying this pheno- 

 menon is to take the relative growth-rate of the male and 

 female appendages. When this is done, it may occur, over 

 a given size-range, that the relative growth-rates of the organs 

 anterior to the chela are actually lower in the male than the 

 female (see Figs. 66, 67). 



The explanation of this curious effect is for the moment 

 completely obscure, though there can be little doubt that it 

 is connected with the fact that any fundamental growth- 

 gradient along the body-axis must be polarized. 



Effects presumably of the same general nature, though not 



