358 



EVOLUTION AND ANIMAL LIFE 



Among the more highly organized animals the results of a 

 parasitic life, in degree of structural degeneration, can be more 

 readily seen. A well-known parasite, belonging to the Crus- 

 tacea the class of shrimps, crabs, lobsters, and crayfishes is 

 Sacculina. The young Sacculina (Fig. 215, A) is an active, free- 

 swimming larva much like a young prawn or young crab. But 

 the adult bears absolutely no resemblance to such a typical 

 crustacean as a crayfish or crab. The Sacculina after a short 



B 



FIG. 215. Development of the parasitic crustacean, Sacculina carcinus: A, Naplius 

 stage; B, cypris stage; C, adult attached to its host, the crab, Carcinus mccnas. 

 (After Hertwig.) 



period of independent existence attaches itself to the abdomen 

 of a crab, and there completes its development while living as a 

 parasite. In its adult condition (Fig. 215, C) it is simply a great 

 tumorlike sac, bearing many delicate rootlike suckers which 

 penetrate the body of the crab host and absorb nutriment. 

 The Sacculina has no eyes, no mouth parts, no legs, or other 

 appendages, and hardly any of the usual organs except re- 

 productive organs. Degeneration here is carried very far. 



Other parasitic Crustacea, as the numerous kinds of fish 

 lice (Fig. 216) which live attached to the gills or to other parts 

 of fish, and derive all their nutriment from the body of the 

 fish, show various degrees of degeneration. With some of 

 these fish lice the female, which looks like a puffed-out worm, 

 is attached to the fish or other aquatic animal, while the male, 

 which is perhaps only a tenth of the size of the female, is per- 

 manently attached to the female, living parasitically on her. 



