262 FISHES CHAP. 



occasionally encircle both the artery and the corresponding 

 vein.^ 



The lining epithelium differs considerably in character in 

 different portions of the alimentary canal. The epithelium of the 

 mouth, pharynx, and anterior section of the oesophagus is often 

 squamous and is succeeded in the hinder part of the oesophagus, 

 and in the stomach and intestine, by a columnar epithelium. 

 As a rale the epithelium of the rectum is also colunmar, but in 

 Elasmobranchs it may become squamous. Goblet cells are of very 

 frequent occurrence throughout the whole length of the alimentary 

 canal, from the mouth to the rectum inclusive, interspersed 

 between the superficial epithelial cells ; in the same position 

 in the intestine migratory leucocytes have been found. The 

 primitive ciliation of the Vertebrate alimentary canal is retained 

 to a greater or less extent in many Fishes, and is sometimes, but 

 not always, associated with a feeble development of the muscula- 

 ture. In the larval form of Petromyzon (Ammocoetes), the whole 

 canal is ciliated except the pharynx and rectum ; but in the 

 adult ciliation is retained only in places which gradually become 

 fewer as the rectum is approached. In the Myxinoids, however, 

 cilia are said to be absent. 



In the Dipnoi (e.g. Protopterus) the epithelium of the stomach 

 and intestine is largely ciliated, but in Elasmobranchs, ciliation 

 is usually restricted to the posterior portion of the oesophagus 

 and the edge of the spiral valve. Among the more generalised 

 Teleostomi (e.g. Acipenser, Lepidosteus, Amia), the oesophagus, 

 stomach, and intestine may be ciliated, but to an extent which 

 varies in different genera. The pyloric appendages, when present, 

 are also more or less extensively ciliated. In Teleosts, however, 

 the recorded instances of ciliation are relatively rare. ISTeverthe- 

 less, ciliated epithelium has been found in the intestine of a few 

 species (e.g. Rhombus aculeatus and Syngnathus acus), and also in 

 the pyloric appendages ; in the stomach (e.g. Pe7'ca and Usox), 

 and even in the oesophagus (e.g. Perca). 



The mucous membrane, including the submucosa, is frequently 

 developed into variously arranged ingrowths projecting into the 

 lumen of the alimentary canal ; these are generally of the nature 

 of longitudinal or transverse ridges, or a combination of the 

 two, giving rise to retiform structures. The simple longitudinal 



1 Paul Mayer, 3RU. zool. Stat, zu Nmpel, viii. 1888, p. 307. 



