THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 165 



pull.) At the posterior end of the intestine is a small cylindrical body, the 

 rectal gland, attached to the intestine by a duct. It is possibly excretory in 

 function. The rectal gland marks the division between large and small intestine, 

 that part of the intestine anterior to its attachment being the small intestine, 

 that posterior to it, the large intestine. The latter is so short as to be almost 

 absent and opens at once into a terminal chamber, the cloaca, which opens to 

 the exterior through the anus. 



Cut open the stomach and wash out its contents. Partly disintegrated 

 fish and squids are commonly found in the stomach. Observe the folds or 

 rugae in the walls of the posterior part of the stomach and the papillae project- 

 ing from the walls of the anterior part. Cut open the small intestine along one 

 side midway between the large blood vessels which traverse its walls longitudi- 

 nally. Observe the spiral valve in its interior. It consists of a fold of the intesti- 

 nal wall spirally coiled so as to make a series of overlapping cones. The purpose of 

 the spiral valve is to increase the digestive and absorptive surface of the intestine. 



The reproductive organs and their ducts in part may also be identified at 

 this time. In the spiny dogfish and the skate the gonads are a pair of soft bodies 

 located dorsal to the anterior part of the stomach. The lobes of the liver must 

 be raised to see them. In the smooth dogfish they are long slender bodies extend- 

 ing the entire length of the cavity dorsal to the digestive tract and terminating 

 at the rectal gland. In mature females the oviducts are noticeable as stout 

 white tubes, one on each side, in contact with the dorsal walls of the coelom. 



3. The mesenteries. — The viscera are held in place by delicate membranes, 

 the mesenteries, whose mode of origin was explained in the introduction to this 

 section. In studying them lift. and spread each organ as it is mentioned. The 

 dorsal mesentery extends from the median dorsal line of the coelom to the diges- 

 tive tract but is not complete in the animals under consideration, a gap being 

 present in the region of the small intestine. That part of the dorsal mesentery 

 supporting the stomach is called the mesogaster; in the skate it is limited to the 

 anterior part of the stomach. The mesogaster incloses the spleen between its 

 two walls, and that portion of the mesogaster from the spleen to the stomach 

 is the gastrosplenic ligament. That portion of the dorsal mesentery which sup- 

 ports the small intestine is called the mesentery, in the limited sense. This 

 is absent in the skate. In the spiny dogfish there is a fusion between the mesen- 

 tery and the mesogaster so that a sort of pocket is formed dorsal to the bend of 

 the stomach. In the dorsal wall of this pocket is located the greater part of the 

 pancreas which is thus in the dorsal mesentery. The dorsal mesentery begins 

 again in the region of the rectal gland, this portion of the mesentery being named 

 the mesorectum. 



The ventral mesentery is represented in these animals, as in all vertebrates, 

 by remnants only. Such a remnant is the gastro-hepato-duodenal ligament ex- 

 tending from the right side of the stomach to the liver and duodenum. It is 



