THE WATER FLEAS (CLADOCERA) 68 1 



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

 foot aid them in clinging to plants and also may help to pull ofif 

 attached algae, etc., for food. 



In the more transparent species the digestive tract may be seen 

 throughout its full extent. The narrow esophagus (Figs. 1050, 1051, 

 1096) widens suddenly into the stomach, which 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 curved 

 (Figs. 1050, 1053, 1060, 1064, iico). These are the hepatic ceca, 

 which no doubt function as a digestive gland. The 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 cecum attached to the ventral side near the posterior end 

 (Figs. 1121, 1141). The terminal part of the intestine, the rectum, 

 is always transparent and the muscles which open and close it can 

 easily be seen. The anus lies either at or near the end of the post- 

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

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

 1091, iioo, 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. 1090), 

 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. 11 23, 1144), or, when numerous, the pecten (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 Chydoridae 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 be 

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

 and from parasites which might otherwise readily attach themselves. 



