204 



COLLEGE ZOOLOGY 



and is one of the reasons why crayfish ap- 

 pendages are usually studied in some detail. 



Internal anatomy 



The body of the crayfish (Fig. 114) con- 

 tains all of the important systems of organs 

 characteristic of the higher animals. The 

 coelom is not large but is restricted to the 

 cavities enclosing the gonads and the ex- 

 cretory green glands. Certain organs are 

 metamerically arranged, such as the nervous 

 system; others, like the excretory organs, 

 are concentrated into a small space. The 

 systems of organs and their functions will be 

 presented in the following order: ( 1 ) diges- 

 tive, (2) circulatory, (3) respirator)', (4) 

 excretory, (5) nervous, (6) sense, (7) mus- 

 cular, and (8) reproductive. 



Digestive system 



The digestive tract of Cambarus consists 

 of the following parts: 



1. The mouth opens on the ventral sur- 

 face between the jaws. 



2. The esophagus is a short tube leading 

 from the mouth to the stomach. 



3. The stomach is a large cavity divided 

 by a constriction into an anterior cardiac 

 chamber and a smaller posterior pyloric 

 chamber. In the cardiac chamber are three 

 hard teeth (chitinous ossicles) of use in 

 grinding the food and collectively known 

 as the gastric mill. The teeth are able to 

 move one upon another; and, being con- 

 nected with powerful muscles, are effective 

 in grinding up the food. On either side of 

 the pyloric chamber a duct enters from the 

 digestive glands and above is the opening 

 of the small cecum. 



About 10 to 30 days before molting, two 

 calcareous bodies, known as gastroliths, are 

 present in the lateral walls of the cardiac 

 chamber of the stomach. During the molt 

 these are shed into the stomach where they 

 may be dissolved. When this occurs they 

 are probably used in the calcification of the 

 new exoskeleton. However, a high percent- 



age of the gastroliths are lost in the shedding 

 process, and in these cases there is no pos- 

 sibility of the re-use of the lime which they 

 contain. 



4. A short midgut. 



5. The intestine is a small tube that 

 passes through the abdomen and opens to 

 the outside through the anus on the ventral 

 surface of the telson. 



6. The digestive glands ("liver") are sit- 

 uated in the thorax and abdomen, one on 

 each side. Each consists of 3 lobes com- 

 posed of a great number of small tubules. 

 The glandular epithelium lining these tub- 

 ules produces a pancreaticlike enzyme which 

 may pass into the hepatic ducts and thence 

 into the midgut. 



Nutrition 



Food. The food of the crayfish is made up 

 principally of living animals such as snails, 

 tadpoles, insect lar\'ae, small fish, and frogs, 

 but decaying organic matter is also eaten. 

 Crayfishes also prey upon others of their 

 kind. They feed at night, being more active 

 at dusk and daybreak than at any other 

 time. Their method of feeding may be ob- 

 served in the laboratory if a little fresh meat 

 is offered to them. The maxillipeds and 

 maxillae hold the food while it is being 

 torn and crushed into small pieces by the 

 mandibles. It then passes through the 

 esophagus 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 

 teeth of the gastric mill. When fine enough, 

 it passes through the strainer which lies be- 

 tween the cardiac and pyloric portions of 

 the stomach. This strainer consists of two 

 lateral folds and a median ventral one which 

 bear hairlike processes and allow passage of 

 only liquids or very fine particles. In the 

 midgut, the food is mixed with the secre- 

 tion from the digestive glands brought in 

 by way of the hepatic ducts. From the mid- 

 gut some of the dissolved or partially di- 

 gested food passes into the digestive glands. 



