COMMENSALISM AND PARASITISM 341 



275. 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 inde- 

 pendent exist- 

 ence, attaches 

 itself to the ab- 

 domen of a crab, 



and there COm- FIG. 204. Sacculina, a crustacean parasite of crabs, a, at- 

 T)leteS its devel- tached to a crab, with root-like processes penetrating the 

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



opment while 



living as a parasite. In its adult condition (Fig. 204) it is 

 simply a great tumor-like sac, bearing many 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. 



276. Parasitic insects. In the order Hymenoptera there 

 are several families, all of whose members live during their 

 larval stage as parasites. We may call all these hymen- 

 opterous parasites ichneumon flies. The ichneumon flies 

 are parasites of other insects, especially of the larvae of 

 beetles and moths and butterflies. In fact, the ichneumon 

 flies do more to keep in check the increase of injurious and 

 destructive caterpillars than do all our artificial remedies 



