The Evidence from Sexual Characters. 177 



able that no one, on first examining one of the more 

 modified parasitic forms, such as the one shown in Fig. 

 15, would detect any resemblance to the free or non- 

 parasitic members of the group, or would even suspect 

 that the animals are crustaceans. 



The females, which are known as "fish-lice" are 

 parasites upon fishes and other aquatic animals, while 

 the males are parasites upon the bodies of the females, 

 and are usually of minute size as compared with the 

 females. 



The adaptation to a parasitic life has not only pro- 

 duced the most profound changes in the general struc- 

 ture, but it has also brought about an almost unparal- 

 leled difference between the sexes. It is true that this 

 is not due to the modification of males alone, for the 

 females as well as the males exhibit the most extreme 

 departures from the organization which is characteristic 

 of typical or non-parasitic Crustacea, and it is difficult 

 to decide from structure alone whether the male or the 

 female is most modified. The fact that the male has 

 been adapted to a life as a parasite upon the body of the 

 female, while the female has simply become adapted to 

 a parasitic life on other animals, seem to show that the 

 male organism is somewhat more plastic than the female. 

 Simple parasitism may be brought about by indefinite 

 variability, but parasitism upon a parasite demands 

 definite variation to meet the definite changes which 

 have taken place in the host. 



The highly specialized parasitic Copepods are joined 

 to the non-parasitic forms by a long series of intermedi- 

 ate species, in which the parasitic habit is only slightly 

 developed, and I give a few figures to illustrate some of 

 the steps in this most interesting series. The female 

 Notodelphys (Fig. 10), which lives in the body cavities 



