282 TEXTBOOK OF ZOOLOGY 



Characterization of Other Crustacea 



Besides crayfish the order Decapoda includes lobster, shrimp, and 

 crab. They all have ten walking legs for which they are named. 

 The crayfish and lobster are verj^ similar except in size. The 

 shrimps and prawns are marine and resemble the crayfish except 

 that they do not have the great pinchers (chela) and the abdomen 

 is bent sharply downward. The crabs are quite different in shape 

 in that the cephalothorax is broader than it is long, the abdomen 

 is poorly developed, and folded sharply beneath the thorax. Crabs 

 of different kinds vary in diameter from a few millimeters to sev- 

 eral inches. There are four species of swimming crabs in the Gulf 

 of Mexico, of which the hlue or edihle crab {Callinectes sapidus, 

 Fig. 408) is the most important and best known. The lady crab and 

 calico crab are also interesting species. When the blue crab is cap- 

 tured at molting time it is called the soft-shelled crab. At other 

 times it is the hard-shelled crab. They maj^ be caught in baited nets 

 or on pieces of meat on a line with which they are brought to the sur- 

 face and lifted out in a dip-net. The hermit crab (genus Pagurus, 

 Fig. 408) is smaller and lives in empty gastropod shells by backing 

 into the shell and carrying it around. Due to the cramping and in- 

 activity the abdomen has become soft and partly degenerate. The 

 fiddler crab (genus Uca, Fig. 408) is another very abundant form 

 found on the coast of the Gulf of Mexico and elsewhere. These are 

 small semiterrestrial crabs which burrow in tunnels, and may thus 

 honeycomb large areas of salt marshes. They can run quite rapidly, 

 often moving sidewise, and thej^ are peculiar in that one pincher 

 of the male, usually the right, is much enlarged. This gives the 

 appearance of a fiddle and the other, reduced pincher resembles 

 the bow. The large pincher is used in a nuptial dance, and occa- 

 sionally a large number of these little crabs will be seen raising and 

 lowering these enormous pinchers in concert. 



Asellus communis is a common fresh-water form found in streams 

 and pools. A salt-water genus, Idotea, is found in the ocean. The pill 

 bug (Armadillidum) and the sow bug (Oniscus asellus or Porcellio sp.) 

 are terrestrial, living in damp places under logs, stones, or heavy 

 vegetation, and in cellars or greenhouses. Their legs are arranged 

 in two groups, which point in opposite directions. Respiration is 

 carried on by gills on the ventral side of the body and for this 



