3 2 



The Dissection of the Fish 



one. In the bony fishes the stomach is an enlarged area, either 

 siphon-shaped, with an opening at either end, or else forming 

 a blind sac with the openings for entrance (cardiac) and exit 

 (pyloric) close together at the anterior end. In the various 

 kinds of mollets (Mugil) and in the hickory shad (Dorosoma), 

 fishes which feed on minute vegetation mixed with mud, the 

 stomach becomes enlarged to a muscular gizzard, like that of a 

 fowl. Attached near the pylorus and pouring their secretions 

 into the duodenum or small intestine are the pyloric caca. 

 These are tubular sacs secreting a pale fluid and often almost as 

 long as the stomach or as wide as the intestine. These may be 

 very numerous as in the salmon, in which case they are likely to 

 become coalescent at base, or they be few or altogether wanting. 



Besides these appendages which are wanting in the higher 

 vertebrates, a pancreas is also found in the sharks and many 

 other fishes. This is a glandular mass behind the stomach, its 

 duct leading into the duodenum and often coalescent with the 

 bile duct from the liver. The liver in the lancelet is a long 

 diverticulum of the intestine. In the true fishes it becomes a 

 large gland of irregular form, and usually but not always pro- 

 vided with a gall-bladder as in the higher vertebrates. Its 

 secretions usually pass through a ductus cholodechus to the 

 duodenum. 



The spleen, a dark-red lymphatic gland, is found attached 

 to the stomach in all fish-like vertebrates except the lancelet. 



The lining membrane of the abdominal cavity is known as the 

 peritoneum, and the membrane sustaining the intestines from 

 the dorsal side, as in the higher vertebrates, is called the mesen- 

 tery. In many species the peritoneum is jet black, while in 

 related forms it may be pale in color. It is more likely to be 

 black in fishes from deep water and in fishes which feed on 

 plants. 



The Spiral Valve. In the sharks or skates the rectum or 

 large intestine is peculiarly modified, being provided with a spiral 

 valve, with sometimes as many as forty gyrations. A spiral 

 valve is also present in the more ancient types of the true fishes 

 as dipnoans, crossopterygians, and ganoids. This valve greatly 

 increases the surface of the intestine, doing away with the neces- 

 sity for length. In the bowfin (Amid} and the garpike (Lepi- 



