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 rule the epithelium of the rectum is also columnar, 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. Neverthe- 
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. Perca and Esoz), 
and even in the oesophagus (e.g. Perea). 
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, Mitt. zool. Stat. zw Neapel, viii. 1888, p. 307. 
