68o FRESH-WATER BIOLOGY 



delicate than the outer, and between the walls the blood circulates 

 and the inner surface serves as a respiratory organ. 



Just back of the head, on the dorsal side, lies the heart, an oval 

 or elongated sac (Figs. 1050, 1051, 1089), whose rapid pulsations are 

 easily seen in the living animal. It receives the colorless or yellow 

 blood by one opening on each side and expels it in front. There are 

 no blood vessels, but the circulation passes along definite courses 

 through a complex series of passages all over the body. The 

 movements of the blood corpuscles may be readily seen in trans- 

 parent Cladocera. 



Respiration is not served by any single organ. The legs and 

 the inside of the valves are the main surfaces for the exchange of 

 gases. 



In the anterior part of the valves lies an organ whose structure 

 is not readily made out. This is the shell gland (Figs. 1050, 1051, 

 1056), a flattened glandular tube in several loops, which probably 

 serves the function of a kidney. 



The body lies free within the valves and is divided into the main 

 portion, bearing the feet, which is not plainly segmented, and a 

 single unjointed portion, the post-abdomen. Through it runs the 

 intestine, and along the sides of the body lie the simple reproduc- 

 tive organs. To the ventral side are attached the feet, ordinarily 

 five pairs, sometimes six. These are mainly leaf-like structures, 

 each with several parts, bearing numerous hairs and long setae (Figs. 

 1050, 1142). Their structure is too complex to describe here. In 

 the first two families all the feet are similar and foliaceous. Their 

 use is to create a current of water through the valves, bringing 

 in oxygen for respiration and particles of food. The latter consists 

 chiefly of algae, though nothing edible is rejected that the current 

 brings in. The food particles collect below the body between the 

 bases of the feet and are fed forward into the mouth. The maxillae 

 push them between the jaws as the labrum opens, the mandibles 

 grind them up, and they pass on into the esophagus. Cladocera 

 are normally eating all of the time. 



In the Daphnidae and remaining families the feet differ in struc- 

 ture; the first pairs being more or less prehensile and having other 

 functions besides the main one of drawing in water. These animals 



