Chap. IV. TANAIS DüBIUS. 19 



CHAPTEK IV. 



SEXUAL PECULIARITIES AND DIMORPHISM. 



Our Tanais, which in nearly all the particulars of its 

 structure is an extremely remarkable animal, furnished 

 me with a second fact worthy of notice in connection 

 with the theory of the origin of species by natural 

 selection. 



When handlike or cheliform structures occur in the 

 Crustacea, these are usually more strongly developed in 

 the males than in the females, often becoming enlarged 

 in the former to quite a disproportionate size, as we 

 have already seen to be the case in Melita. A better 

 known example of such gigantic chelae is presented 

 by the males of the Calling Crabs (Gelasimus), which 

 are said in running to carry these claws "elevated, 

 as if beckoning with them " — a statement which, how- 

 ever, is not true of all the species, as a small and 

 particularly large-clawed one, which I have seen run- 

 ning about by thousands in the cassava-fields at the 

 mouth of the Cambriu, always holds them closely 

 pressed against its body. 



A second peculiarity of the male Crustacea consists 

 not unfrequently in a more abundant development on 



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