THE CRAYFISH AND ARTHROPODS IN GENERAL 2OI 



stomach. Their function is not certain, but is probably for the 

 storage of the calcareous matter used in hardening the exoskeleton. 



(4) The intestine (24) is a small tube passing posteriorly near 

 the dorsal wall of the abdomen, and opening to the outside 

 through the anus (6) on the ventral surface of the telson (7). 



The digestive glands, or " liver," are situated in the thorax, 

 one on either side. Each consists of three lobes composed of a 

 great number of small tubules. The glandular epithelium lining 

 these tubules produces a secretion which passes into the hepatic 

 ducts and thence into the pyloric chamber of the stomach. 



Nutrition. FOOD. The food of the crayfish is made up 

 principally of living animals such as snails, tadpoles, insect larvae, 

 small fish, and frogs; but decaying organic matter is also eaten. 

 Not infrequently crayfishes prey upon others of their kind. They 

 feed at night, being more active at dusk and daybreak than at 

 any other time (167). Their method of feeding may be observed 

 in the laboratory if a little fresh meat is offered to them. The 

 maxillipedes and maxillae hold the food while it is being crushed 

 into small pieces by the mandibles. It then passes through the 

 oesophagus into the stomach. The coarser parts are ejected 

 through the mouth. 



DIGESTION. In the cardiac chamber of the stomach, the food 

 is ground up by the ossicles of the gastric mill. When fine enough, 

 it passes through the strainer which lies between the two divisions 

 of the stomach. This strainer consists of two lateral and a 

 median ventral fold which bear hairlike setae, and allow the pas- 

 sage of only liquids or very fine particles. In the pyloric cham- 

 ber, the food is mixed with the secretion from the digestive glands 

 brought in by way of the hepatic ducts. From the pyloric 

 chamber the dissolved food passes into the intestine by the walls 

 of which the nutritive fluids are absorbed. Undigested particles 

 pass on into the posterior end of the intestine, where they are 

 gathered together into faeces, and egested through the anus. 



Vascular System. THE BLOOD. The blood into which the 

 absorbed food passes is an almost colorless liquid in which are 



