Chap. IX.] CRUSTACEANS. 319 



torted mass. But these extraordinary differences between 

 the two sexes are no doubt related to their widely-differ- 

 ent habits of life, and consequently do not concern us. 

 In various crustaceans, belonging to distinct families, the 

 anterior antennae are furnished with peculiar thread-like 

 bodies, which are believed to act as smelling-organs, and 

 these are much more numerous in the males than in the" 

 females. As the males, without any unusual development 

 of their olfactory organs, would almost certainly be able 

 sooner or later to find the females, the increased number 

 of the smelling - threads has probably been acquired 

 through sexual selection, by the better provided males 

 having been the most successful in finding partners and 

 in leaving offspring. Fritz Miiller has described a re- 

 markable dimorphic species of Tanais, in which the male 

 is represented by two distinct forms, never graduating 

 into each other. In the one form the male is furnished 

 with more numerous smelling-threads, and in the other 

 form with more powerful and more elongated chelae or 

 pincers which serve to hold the female. Fritz Miiller sug- 

 gests that these differences between the two male forms 

 of the same species must have originated in certain indi- 

 viduals having varied in the number of the smelling- 

 threads, while other individuals varied in the shape and 

 size of their chelae ; so that of the former, those which 

 were best able to find the female, and of the latter, those 

 which were best able to hold her when found, have left 

 the greater number of progeny fo inherit their respective 

 advantages. 4 



In some of the lower crustaceans, the right-hand an- 

 terior antenna of the male differs greatly in structure 



4 ' Facts and Arguments for Darwin,' English translat. 1869, p. 20. 

 See the previous discussion on the olfactory threads. Sars has described 

 a somewhat analogous case (as quoted in 'Nature,' 1870, p. 455) in a 

 Norwegian crustacean, the Pontoporeia affinis. 



