v. 12 DIGESTION IN ELASMOBRANCHS 159 



now meet for the first time, has probably been formed as a special 

 portion of the oesophagus. Barrington suggests that it evolved with the 

 jaws, serving originally as a receptacle for the large pieces of food, or 

 even whole fishes, which could now be swallowed. The mucous glands 

 became modified to produce acid, since this prevents bacterial decay. 

 Finally, an enzyme, pepsin, was evolved able to digest proteins in 

 acid solution. In the dogfish this condition has been fully established 

 and the stomach has essentially the structure and functions found in 

 all higher vertebrates. However, in the gastric glands only one type 

 of cell is recognizable, there are no separate pepsin-secreting and 

 acid-producing cells. Nevertheless, there is a pepsin-like enzyme 

 present and the contents are acid. The stomach is divided into two 

 parts, a descending cardiac and ascending pyloric limb, the signifi- 

 cance of the divisions being unknown. 



The region where the stomach joins the intestine is guarded by a 

 powerful pyloric sphincter, immediately beyond which open the bile 

 and pancreatic ducts. The liver is a large two-lobed organ, receiving 

 the hepatic portal blood from the gut. It serves as a storage organ 

 containing much glycogen and fat and sometimes the hydrocarbon 

 squalene. It probably also plays a part in the destruction of red blood 

 corpuscles. Bile is carried away to a gall-bladder, from which a bile- 

 duct leads to open at the front end of the spiral intestine. 



The pancreas, hardly recognizable as a distinct organ in the lamprey, 

 forms in the dogfish an elongated body between the stomach and 

 intestine. It contains both exocrine and endocrine cells and its duct 

 enters the intestine shortly below the pylorus. The 'small' intestine 

 of elasmobranchs is of a peculiar form, being short but with its 

 surface greatly increased by the presence of a spiral ridge or 'valve'. 

 The intestinal contents are alkaline and contain trypsin, amylase, 

 and lipase. There is no constant fauna of commensal bacteria. Ab- 

 sorption presumably takes place wholly in this organ, for the remain- 

 ing length of gut consists only of a short rectum, to which is attached 

 an organ of unknown function, the rectal gland, containing branched 

 glands and much lymphoid tissue. 



12. The circulatory system 



The heart develops as a specialization of the subintestinal vessel 

 between the place where it receives the veins from the liver and the 

 body wall and the gills, which are to be supplied under high pressure. 

 It consists of a single series of three main chambers, sinus venosus, 



