282 COLLEGE ZOOLOGY 



Table IX gives a brief description of each and the modifications 

 due to differences in function. 



Internal Organs. — Definite systems of organs are present 

 in the crayfish for the performance of the various functions. 

 The cozlom is small, and is restricted to the cavities of the repro- 

 ductive organs and green glands. The cavities around the 

 alimentary canal are blood spaces, and therefore represent a 

 hcemoccel. Some of the organs, like the muscles and nervous 

 ganglia, are segmentally arranged; others like the excretory 

 organs are concentrated in a small space. 



Digestion. — Crayfishes live chiefly on living snails, tadpoles, 

 young insects, and the like, but sometimes eat one another, 

 and may also devour decaying organic matter. They feed at 

 night, being most active at dusk and daybreak. The maxilli- 

 pedes and maxillae hold the food while it is being crushed into 

 small pieces by the mandibles. The food particles pass down 

 the oesophagus (Fig. 202, 20) into the anterior, cardiac chamber 

 of the stomach (21), where they are ground up by a number of 

 chitinous ossicles, called the gastric mill. When fine enough, the 

 food passes through a sieve-like strainer of hair-like setae into the 

 pyloric chamber of the stomach (22); here it is mixed with a 

 secretion from the digestive glands brought in by the hepatic 

 ducts. The dissolved food is absorbed by the walls of the in- 

 testine (24). Undigested particles pass on into the posterior end 

 of the intestine, where they are gathered together into faeces, 

 and egested through the anus (6). 



Circulation. - - The Blood. — The blood into which the 

 absorbed food passes is an almost colorless liquid in which are 

 suspended a number of ameboid cells, the blood corpuscles or 

 amebocytes. The principal functions of the blood are the trans- 

 portation of food materials from one part of the body to another, 

 of oxygen from the gills to the various tissues, of carbon dioxide 

 to the gills, and of urea to the excretory organs. 



Blood-vessels. — The principal blood-vessels are a heart, 

 seven arteries, and a number of spaces called sinuses. Blood 



