THE WATER ELEAS (CLADOCER.A) 68 1 



live chiefly among the weeds, and the hooks and spines of the first 

 foot aid them in cUnging to plants and also may help to pull off 

 attached algae, etc., for food. 



In the more transparent species the digestive tract may be seen 

 throughout its full extent. The narrow csopha«,^us ( Figs. ^050, 105 1 , 

 1096) widens suddenly into the stomach, wliich lies in the head and 

 whose posterior end passes insensibly into the intestine. Attached 

 to the stomach in many species are two sacs, often long and curwd 

 (Figs. 1050, 1053, 1060, 1064, iico). These arc the hepatic ccta. 

 which no doubt function as a digestive gland, l^he stomach and 

 intestine have a muscular wall and a lining of dark-colored, glandu- 

 lar cells. The cavity is ordinarily filled with food. The intestine 

 has a direct course in the first four families. In the Macrothricidae 

 it is sometimes direct (Fig. 1106), and sometimes convoluted (Figs. 

 1 100, 1 103). In the Chydoridae it is always convoluted and there 

 is often a cecimi attached to the ventral side near the [)osterior end 

 (Figs. 1 1 21, 1 141). The terminal part of the intestine, the rectum, 

 is always transparent and the muscles which open and c lose it can 

 easily be seen. The anus Hes either at or near the vnd of the post- 

 abdomen, as is usually the case in the first live families, or in the 

 Chydoridae and in some forms of the other families (Figs. 1089. 

 1091, 1 100, 11C9), on the dorsal side. 



The post-abdomen is ordinarily jointed to the body and is bent 

 forward; hence its dorsal side may come to be the lower one. On 

 the dorsal side it bears two sensory hairs, often very long (Fig. logo), 

 the abdominal setae. At the end of the post-abdomen are two ter- 

 minal claws, which, in turn, may have spines at their base, the basal 

 spines (Figs. 1123, 1144), or, when numerous, the {K-ctcn (Fig. 

 1066), and the concave side may also have a row of very fine spinules 

 (denticulate). The post-abdomen almost always has more or fewer 

 spines, or teeth, the anal spines. In the (liNcioridae there are fre- 

 quently two rows on each side behind the anus, the marginal and 

 lateral denticles (Fig. 1147). These spines and teeth may have the 

 most diverse shape and structure (squamae, fascicles, etc.). and fur 

 nish important systematic characters. Their main use seems to Ik» 

 to comb the legs and keep them clean and free from foreign matters 

 and from parasites which might other^vise readily attach them.selves. 



