300 THE DESCENT OF MAN. 



Unfortunately the habits of crustaceans are very imper- 

 fectly known, and we cannot explain the uses of many 

 structures peculiar to one sex. With the lower parasitic 

 species the males are of small size, and tliey alone are fur- 

 nished with perfect swimming legs, atennae and sense- 

 organs; the females being destitute of these organs, with 

 their bodies often consisting of a mere distorted mass. But 

 these extraordinary differences between the two sexes are no 

 doubt related to their widely different habits of life, and 

 consequently do not concern us. In various crustaceans, 

 belonging to distinct families, the anterior attennse are fur- 

 nished 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 seclection, by the better provided 

 males having been the more successful in finding partners 

 and in producing offspring. Fritz Miiller has described a 

 remarkable dimorphic species of Tanais in which the male 

 is represented by two distinct forms which never graduate 

 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 may have originated in certain individuals 

 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, have left the greatest number of progeny 

 to inherit their respective advantages.* 



In some of the lower crustaceans the right anterior 

 antenna of the male differs greatly in structure from the 

 left, the latter resembling in its simple tapering joints the 

 antennae of the female. In the male the modified antenna 

 is either swollen in the middle or angularly bent or con- 



*** 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 Potitoporcia affinis. 



