DEGENERATION IN PARASITES 



401 



nervous system dwindles away to a mere ganglion, from which a 

 few nerves come off, and the entire animal is reduced to the 

 condition of a bag, with two openings through which the 

 remaining organs obtain their food supply and communicate 

 with the outside world by means of a stream of water maintained 

 by ciliary action. 



Still more conspicuous is the degeneration undergone by 

 the great majority of parasites, whether animals or plants. 

 Sacculina, for example, in the earlier stages of its existence, is 

 an active crustacean which swims vigorously about by means of 

 well developed appen- 

 dages. It belongs to a 

 group, the barnacles or 

 cirripedes, which are 

 notorious for sedentary 

 habits and consequent 

 degeneration in the adult 

 condition. Sacculina, 

 however, not content with 

 a sedentary life, goes 

 further down hill and 

 becomes parasitic. It 

 attacks crabs, and in the 

 adult state is reduced to 

 the condition of a large, 

 irregularly shaped bag 

 (Fig. 183) fixed to the 

 under surface of the crab's 

 abdomen by root -like pro- 

 cesses which penetrate the body of the host and extract nutri- 

 ment therefrom. With the exception of these root-like processes, 

 which are a special, caenogenetic development, adapted for nutri- 

 tion under new conditions of life, the only organs which have 

 not undergone degeneration are those of reproduction, for upon 

 these depends the perpetuation of the race and upon these, 

 therefore, natural selection is still able to retain her hold. 



It is, indeed, a general rule amongst parasitic animals that the 

 reproductive organs are largely developed and very complicated, 

 for the conditions which have become necessary for the existence 

 of these animals are so complex and highly specialized, while 

 the chances of mating between different individuals for purposes 



FIG. 183. Lower surface of a Swimming 

 Crab (Portunus depurator) with a Sac- 

 culina (Sac.) attached to it. (From a 

 photograph.) 



B. 



D P 



