96 ANIMAL BIOLOGY 



bers of the barnacle group have become parasitic and have 

 degenerated to such a degree that the adults would never 

 be taken for crustaceans at all were it not for our knowl- 

 edge of their life history. One of the most extreme cases 

 of degeneration through parasitism that is known is fur- 

 nished by Sacculina, a parasite on crabs. This parasite 

 appears as a fleshy mass attached to the body of the crab 

 commonly under the abdomen. The Sacculina sends 

 rootlets into its host which penetrate and draw nourish- 



FIG. 81. Sacculina carcini attached to Carcinus moenas, whose abdo- 

 men is extended, m, shell opening; r, network of roots ramifying the 

 crab; s, stalk; a, o, d, antennula, eye and anus of the crab. 



ment from nearly all parts of the body. The parasite 

 has no appendages, segments, or any external structures 

 characteristic of crustaceans, and the alimentary canal 

 and most other organ systems, except the reproductive 

 organs, have disappeared. The Sacculina has been re- 

 duced to an apparatus for absorbing food from its host 

 and producing eggs and sperms. The eggs hatch into 

 nauplius larvae which pass through early stages of meta- 

 morphosis closely resembling those of typical barnacles. 



