268 The Descent of Man. Part II. 



inequality in the female on the opposite sides of of the body. In 

 Gelasimus, according to a statement quoted by Milne-Edwards, K 

 the male and the female live in the same burrow, and this 

 shews that they pair; the male closes the mouth of the burrow 

 with one of its chelae which is enormously developed ; so that 

 here it indirectly serves as a means of defence. Their main use 

 however, is probably to seize and to secure the female, and thi*- 

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

 The male of the hermit or soldier crab (Pagurus) for weeks 

 together, carries about the shell inhabited by the female. 12 The 

 sexes, however, of the common shore-crab (Carcirius mamas), as 

 Mr. Bate informs mo, unite directly after the female has moulted 

 her hard shell, 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 before moulting, she could then be 

 seized with impunity. 



Fritz Muller states that certain species of Melita are distin- 

 guished from all other amphipods by the females having " the 

 " coxal lamella? 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 has probably followed from those females which were 

 the most securely held during the act of reproduction, having 

 left the largest number of offspring. Another Brazilian amphi- 

 pod (Orchedia Darwinii, fig. 8) presents a case of dimorphism, 

 like that of Tanais ; for there are two male forms, which differ 

 in the structure of their chela?. 13 As either chela would certainly 

 suffice to hold the female, — for both are now used for this purpose, 

 — the two male forms probably originated by some having varied 

 in one manner and some in another ; both forms having derived 

 certain special, but nearly equal advantages, from their differently 

 shaped organs. 



It is not known that male crustaceans fight together for the 

 possession of the females, but it is probably the case ; for with 

 most animals when the male is larger than the female, he seems 

 to owe his greater size to his ancestors having fought 

 with other males during many generations. In most of the 

 orders, especially in the highest or the Brachyura, the male is 

 larger than the female ; the parasitic genera, however, in which 

 the sexes follow different habits of life, and most of the Ento- 

 mostraca must be excepted. The chelae of many crustaceans are 



11 * Hist. Nat. des Crz.ot.' torn. ii. of S. Devon.' 



1837, p. 50. 13 Fritz MuLer, ' Facts and Argu« 



12 Mr. C. Srence Bate, Brit, ments for Darwin,' 18G9, pp. 25-28 

 Assoc, Fourth Report *n the Fauna 



