32^ SEXUAL SELECTION. [Part IL 



than the whole body." It is probable that the great size 

 of one leg with its chela; may aid the male in fighting 

 with his rivals ; but this use will not account for their in- 

 equality in the female on the opposite sides of the body. 

 In Gelasimus, according to a statement quoted by Milne- 

 Edwards,' the male and female live in the same burrow, 

 which is worth notice, as showing that they pair, and the 

 male closes the mouth of the burrow with one of its chela?, 

 which is enormously developed ; so that here it indirectly 

 serves as a means of defence. Their main use, however, 

 probably is to seize and to secure the female, and this in 

 some instances, as with Gammarus, is known to be the 

 case. The sexes, however, of the common shore-ci-ab 

 ( Carcinus nicenas), as Mr. Spence Bate informs me, unite 

 directly after the female has moulted her hard shell, and 

 when she is so soft that she would be injured if seized by 

 the strong pincers of the male ; but as she is caught and 

 carried about by the male previously to the act of moult- 

 ing, she could then be seized with impunity. 



Fritz Miiller states that certain species of ]\[elita are 

 distinguished from all other amphipods by the females 

 having " the coxal lamellae of the penultimate pair of feet 

 produced into hook-like processes, of which the males lay 

 hold with the hands of the first pair." The development 

 of these hook-like processes probably resulted from those 

 females which were the most securely held during the act 

 of reproduction having left the largest number of off- 

 spring. Another Brazilian amphipod {Orchestia Dar- 

 winii, fig. 1) is described, by Fritz Miiller, as presenting a 

 case of dimorphism, like that of Tanais ; for there are two 



* See a paper by Mr. C. Spence Bate, witli figures, in ' Proc. Zoolog. 

 Soc' 1868, p. 363; and on the nomenclature of the genus, ibid. p. 585. 

 I am greatly indebted to Mr. Spence Bate for nearly all the above state- 

 ments with respect to the chela; of the higher crustaceans. 



' ' Hist. Xat. des Crust.' torn. ii. 1837, p. 50. 



