Chap. IX. Crustaceans, 271 



tried to distinguish by colour the sexes of the several species 

 which inhabit the Mauritius, but failed, except with one species 

 of Squilla, probably S. stylifera, the male of which is described as 

 being" of a beautiful bluish-green," with some of the appendages 

 cherry-red, whilst the female is clouded with brown and grey, 

 " with the red about her much less vivid than in the male." 15 

 In this case, we may suspect the agency of sexual selection. 

 From M. Bert's observations on Daphnia, when placed in a vessel 

 illuminated by a prism, we have reason to believe that even the 

 lowest crustaceans can distinguish colours. With Saphirina (an 

 oceanic genus of Entomostraca), the males are furnished with 

 minute shields or cell-like bodies, which exhibit beautiful 

 changing colours ; these are absent in the females, and in 

 both sexes of one species. 16 It would, however, be extremely 

 rash to conclude that these curious organs serve to attract the 

 females. I am informed by Fritz Miiller, that in the female of a 

 Brazilian species of Gelasimus, the whole body is of a nearly 

 uniform greyish-brown. In the male the posterior part of the 

 cepbalo-thorax is pure w T hite, with the anterior part of a rich 

 green, shading into dark brown ; and it is remarkable that these 

 colours are liable to change in the course of a few minutes — the 

 white becoming dirty grey or even black, the green " losing much 

 " of its brilliancy." It deserves especial notice that the males do 

 not acquire their bright colours until they become mature. They 

 appear to be much more numerous than the females; they 

 differ also in the larger size of their chelae. In some species of 

 the genus, probably in all, the sexes pair and inhabit the same 

 burrow. They are also, as we have seen, highly intelligent 

 animals. From these various considerations it seems probable 

 that the male in this species has become gaily ornamented in 

 order to attract or excite the female. 



It has just been stated that the male Gelasimus does not 

 acquire his conspicuous colours until mature and nearly ready 

 to breed. This seems a general rule in the whole class in respect 

 to the many remarkable structural differences between the sexes. 

 We shall hereafter find the same law prevailing throughout the 

 great sub-kingdom of the Vertebrata; and in all cases it is 

 eminently distinctive of characters which have been acquired 

 through sexual selection. Fritz Miiller 17 gives some striking 

 instances of this law; thus the male sand-hopper (Orchestia) 

 does not, until nearly full grown, acquire his large claspers, 



15 Mr. Ch. Fraser, in 'Proc. Zoo- 16 Claus, ' Die freilebenden Cope* 



log. Soc.' 1809, p. 3. I am indebted poden,' 1863, s. 35. 



to Mr. Bite for Dr. Power's state- 17 '' Facts and Arguments,' &•:., 



mem. p. 79. 



