CRAYFISH 411 



and carrying it around. Dae to the cramping and inactivity the ab- 

 domen has become soft and partly degenerate. The fiddler crab 

 (genus Uca) 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 side- 

 wise, and they 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 occasionally a large number 

 of these little crabs will be seen raising and lowering these enormous 

 pinchers in concert. 



Asellus commu7iio is a common fresh-water form found in streams 

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



Fig-, 166. — Asellus, a common fresh-water crustacean. (Courtesy of General 



Biological Supply House.) 



bug (Armadillidiun) and the sow bug (Oniscus asellus or Porccllio) 

 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 

 reason they must live in moist places. They are a garden pest in 

 that they eat leaves of delicate plants. There are a number of 

 aquatic forms which are parasitic on fish and others, such as the 

 gribble (Limnoria), which tunnel in submerged wood. 



The amphipods are sand and beach dwellers which may be found 

 burrowing or jumping around on the seashore or walking around 

 on the bottom in fresh water. Gammarus is the best knoAvn fresh- 

 water form. The legs of representatives of this order are divided 



