ELASMOBRANCHH 431 



The canal begins anteriorly witli the mouth, which leads directly into 

 the pharynx, in whose lateral walls are the gill slits (and spiracle). 

 Following this is the short tubular esophagiis, which leads into the 

 cardiac end of the stomach. This organ, which is somewhat broader 

 than the esophagus, is rather U-shaped. The posterior, or pyloric, 

 portion of the stomach is provided with a sphincter muscle, the 

 pylorus, which controls the passage of food materials into the in- 

 testine. The duodenum is the short anterior portion of the intestine 

 which follows the pyloric portion of the stomach and leads into the 

 valvular portion of the intestine (ileum). This section of the small 

 intestine is of considerably greater diameter than the duodenum and 

 contains internally a spiral valve, which is a spirally arranged in- 

 folding of the mucous lining. This arrangement serves to slow the 

 passage of food and increases the absorption surface. The principal 

 absorption takes place through this part of the intestine. The val- 

 vular portion leads to the short, narrow, large intestine, which emp- 

 ties into the cloaca (Figs. 231 and 232). 



The liver, pancreas, and rectal gland are accessory glands con- 

 nected with this system. The liver is a large, three-lobed organ 

 with the saclike gall bladder located just dorsal to the junction of the 

 right and middle lobes. The gall bladder stores bile produced by the 

 liver and delivers it to the duodenum through the hile duct. The 

 pancreas is divided into two lobes, an oval ventral and a slender dorsal 

 lobe. Ducts lead from it to the duodenum. The rectal gland is a 

 spindle-shaped gland leading into the large intestine directly. The 

 reddish spleen, which is a lymphoid rather than digestive organ, lies 

 around the greater curvature of the stomach. 



The digestive tract and adjacent organs in both species are sus- 

 pended from the body wall by mesenteries, which are extensions from 

 the peritoneum, or membranous lining of the coelom. The mesentery 

 supporting the stomach is the mesogaster, the one extending between 

 the spleen and stomach is the gastrosplenic, and the mesorectum sup- 

 ports the large intestine and rectal gland. 



Circulatory System 



This centers in the heart, which is located ventrally at about the 

 level of the posterior pair of gills, and consists of two principal 

 chambers and two accessory chambers. The pericardium, a mem- 

 branous extension of the peritoneum, encloses the heart. The two 



