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 anemia. 
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, freeswimming 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. 111) it is 
simply a great 
tumor-like sac, 
Fie. 111.—Sacculina, a crustacean parasite of crabs. a, at- 
tached to a crab, with root-like processes penetrating the 
bearing many crab’s body ; 5, removed from the crab. 
delicate root- 
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 
