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268 



SEXUAL DIMORPHISM 



acquire his conspicuous colours until he is sexually mature. 

 The chelse of the male are larger than those of the female, 

 and in some species of the genus, if not in all, the sexes pair 

 and inhabit the same burrow. The colours of the male are 

 liable to change in the course of a few minutes, the white 

 becoming dirty gray, the green losing much of its brilliancy. 

 In this case the elements of coloration are probably the same 

 in kind in the two sexes, though quantitatively they may differ. 

 It is known that the colours of Crustacea are largely due 

 to " chromatophores " which expand and contract under the 

 influence of the nervous system, as in fishes. Thus the 

 only reasonable conclusion concerning this difference of colour 

 is that in principle it is due to the nervous excitement of 

 the male under the influence of the sexual instinct, both in 

 courting the female and defending his possession of her 

 against other males. I believe that the more the phenomena 

 are investigated, the more important will be found the influence 

 of the nervous system in relation to the excitement resulting 

 from sexual relations. 



Darwin states that it seems to be a general rule in Crustacea 

 that the unisexual characters of the males are only developed 

 towards the approach of sexual maturity. As in other cases, 

 this is one of the strongest proofs that the characters are 

 the direct result of stimulations which are only set up by the 

 sexual activities. 



Sexual dimorphism also occurs in the Macrura. In the 

 fresh -water crayfishes of North America belonging to the 

 genus Cambarus it has long been known that two dis- 

 tinct forms of males occur. These were first described by 

 Hagen in his Monograph of the North American Astacidse. 1 

 One of the forms is much differentiated from the female, 

 while the other is less divergent and evidently less specialised. 

 These forms differ in the first place in the structure of the 



1 III. Cat. Mus. Harv. iii., Cambridge, Mass. 1870. 



