Chap. IX. CRUSTACEANS. 331 



is gained by their inequality in size on the opposite 

 sides of the body, and by the inequality being much 

 greater in the male than in the female ; and why, when 

 they are of equal size, both are often much larger in 

 the male than in the female, is not known. The chelae 

 are sometimes of such length and size that they cannot 

 possibly be used, as I hear from Mr. Spence Bate, for 

 carrying food to the mouth. In the males of certain fresh- 

 water prawns (Palsemon) the right leg is actually longer 

 than the whole body. 6 It is probable that the great size 

 of one leg with its chelae may aid the male in fighting 

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

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

 In Gelasimus, according to a statement quoted by Milne- 

 Edwards, 7 the male and female live in the same burrow, 

 which is worth notice, as shewing that they pair, and 

 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, 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-crab (Carcinus meenas), 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 moulting, she could then 

 be seized with impunity. 



Fritz Miiller states that certain species of Melita are 



6 See a paper by Mr. C. Spence Bate, with figures, iu ' Proc. Zoolog. 

 Soc.' ]868, p. 3(J3 ; and on the nomenclature of the genus, ibid. p. 585. 

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

 statements with respect to the chelae of the higher crustaceans. 



7 ' Hist. Nat. des Crust.' torn. ii. 1837, p. 50. 



