66 Descriptive Zoology. 



the stomach, which is situated in the head. The stomach 

 is very complicated. It has in its walls a set of arms or 

 levers so jointed together as to support the walls of the 

 stomach. Further, these bars, which are composed of 

 chitin, are acted on by sets of muscles on the outside. 



There are teeth on the inner walls of the stomach, some 

 projecting inward from each side and some from the upper 

 surface. Certain muscles attached to the outside of the 

 stomach act in such a manner as to make these teeth work 

 together and masticate food in the stomach. The function 

 of the stomach is wholly masticatory, and it has no digestive 

 function. At the hinder part of the stomach there is a 

 series of stiff hairs which act as a strainer, so' that only 

 very fine particles are allowed to pass on into the intestine. 

 Alongside the stomach on each side is the large digestive 

 gland. Each of these opens by a duct into the intestine just 

 back of the stomach. These glands were formerly called 

 livers, but in function they more closely resemble a pancreas, 

 their secretion digesting proteids and fats, and perhaps 

 also starches. Beyond the stomach the intestine extends 

 in a nearly straight course along the upper part of the 

 abdomen, ending in the anus on the under surface of the 

 telson. 



Respiration in the Crayfish. The crayfish breathes by 

 means of the plumelike gills, which are covered by the 

 sides of the carapace. Each gill has two blood tubes in its 

 stem, through one of which the blood enters, while it returns 

 through the other tube. In the feathery branches of the 

 gill the blood is separated from the water by merely a 

 thin membrane, so that the blood and the water make an 

 exchange, the blood getting oxygen from the water, and 

 giving to the water the waste products such as carbon 

 dioxid which it contains. 



