4i ELEMENTARY ZOOLOGY 



essentially like a young prawn or crab. After a short 

 period of independent existence it attaches itself to the 

 abdomen of a crab, and lives as a parasite. It completes 

 its development under the influence of this parasitic life, 

 and when adult bears absolutely no resemblance to such 



FIG. 159. Sacculina. a parasitic crustacean; A, attached to a crab, the 

 root-like processes of the parasite penetrating the body of the host; B, 

 the active larval condition; C, the adult removed from its host. (After 

 Haeckel.) 



a typical crustacean as a crab or crayfish. Its body ex- 

 ternal to the host crab is simply a pulsating tumor-like 

 sac, with no mouth-parts, no legs, and internally hardly 

 any well-developed organs except those of reproduction. 

 Degeneration here is carried very far. 



