Phylum Arthropoda 497 



The Internal Anatomy of Cambarus. — Beneath the exoskeleton 

 is the thin, pigmented epidermis by which the exoskeleton is secreted. 

 The exoskeleton not only covers the body, but in addition has inward pro- 

 jections, one of which lines the hindgut, one the foregut, and others 

 which project inward. These latter constitute an endoskeleton which 

 serves for the attachment of muscles. During molting, the exoskeleton 

 and the extensions within the gut are shed, while the endoskeleton is 

 absorbed. There is a series of powerful muscles which move the various 

 parts of the body and the appendages. These muscles are particularly 

 prominent in the abdomen where they serve for locomotion. 



The digestive tract may be conveniently divided into three main 

 regions : the foregut, the midgut, and the liindgut. The mouth which 

 opens between the jaws on the ventral surface leads into the short 

 esophagus and then into the stomach. These three parts constitute the 

 foregut. The stomach is divided into two main regions : the cardiac 

 and the pyloric. The cardiac portion has a chitinous lining which is 

 calcified in some parts to form one median and two lateral teeth which 

 are worked by powerful muscles. This "gastric mill" helps to grind the 

 food. Small hairs at the entrance to the pyloric portion prevent large 

 particles from entering. The stomach has no true digestive function ; 

 it prepares the food for digestion in the midgut by grinding and strain- 

 ing. During certain periods of the year, large calcareous masses, gas- 

 troliths, may be found in the cardiac portion. These may act to store cal- 

 cium during the molt. After molting, they disappear, and the calcium 

 apparently is used for the rapid calcification of much of the chitin. 



Digestion and absorption of food occur in the midgut. This portion 

 of the digestive tract consists of a short intestine to which are connected 

 large digestive diverticula or caeca. These are complex in both struc- 

 ture and function. Two of these caeca are lateral in position and one is 

 dorsal. The two lateral ones are often termed "livers," but this is not 

 entirely correct for their secretion is both proteolytic and lipolytic. More 

 properly, they should be termed a hepatopancreas, or, more simply, 

 digestive gland. Food is both digested and absorbed in these large lateral 

 caeca. They also function for storage. 



The hindgut consists of the long straight intestine which broadens 

 into the short rectum just before the anus. 



The circulatory system is made up of a heart, arteries, and a series 

 of sinuses which form a hemocoel. The heart is little more than an en- 

 largement of the main dorsal blood vessel, and is located in the dorsal 

 portion of the thoracic region. It is suspended by six ligaments in the 



