Chap. IX. Crustaceans. 265 



Dr. Mcintosh 6 cannot discover that these colours are of any 

 service. The sedentary annelids become duller-coloured, ac- 

 cording to M. Quatrefages, 7 after the period of reproduction ; and 

 this I presume may be attributed to their less vigorous condition 

 at that time. All these worm-like animals apparently stand too 

 low in the scale for the individuals of either sex to exert any 

 choice in selecting a partner, or for the individuals of the same 

 sex to struggle together in rivalry. 



Sub-kingdom of the Arthropoda: Class, Crustacea. — In this great 

 class we first meet with undoubted secondary sexual characters, 

 often developed in a remarkable maimer. Unfortunately the 

 habits of crustaceans are very imperfectly 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 they 

 alone are furnished with perfect swimming-legs, antennae 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 con- 

 sequently 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 more successful in 

 finding partners and in producing offspring. Fritz M tiller 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 Midler suggests 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, whilst other individuals varied in the 

 thape 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 



* See his beautiful monograph on : See M. Perrier, 'l'Origine de 



' British Annelids,' part i. 187o, l'llomrae d'apres Darwin,' ' Kevu< 

 p. 3. Scientifique,' Feb. 1873, p. 866. 



