PARASITISM AND DEGENERATION 187 



and the worm develops in the upper intestine. It feeds on 

 the blood of the young seal, which finally dies from anaemia. 

 On the beaches of the seal islands in Bering Sea there are 

 sometimes hundreds of dead seal pups which have been 

 killed by this parasite (Fig. 110). 



99. Sacculina. Among the more highly organized ani- 

 mals the results of a parasitic life, in degree of structural 

 degeneration, can be more readily seen. A well-known para- 

 site, belonging to the Crustacea the class of shrimps, crabs, 

 lobsters, and cray-fishes is Sacculina. The young Sac- 

 culina 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 cray-fish or 

 crab. The Sacculina after a short period of independent 

 existence at- 

 taches itself to 

 the abdomen of 

 a crab, and 

 there completes 

 its develop- 

 ment while liv- 

 ing as a para- 

 site. In its 

 adult condition 

 (Fig. Ill) it is 

 simply a great 



i -]-! FIG. 111. Sacculina, & crustacean parasite of crabs, a, at- 



ac tached to a crab, with root-like processes penetrating the 



bearing many crab's body ; b, removed from the crab. 



delicate r o o t- 



like 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 reproductive organs. Degeneration 

 here is carried very far. 



Other parasitic Crustacea, as the numerous kinds of 

 fish-lice (Fig. 112) which live attached to the gills or to 



