252 FISHES CHAP. 



on the jaws." ^ Pharyngeal teeth, differ greatly in size and 

 structure in different Teleosts, and, like the jaw-teeth, they are 

 capable of replacement by vertical succession. The teeth are 

 sometimes restricted to the inferior pharyngeal bones (cerato- 

 branchials of the last branchial arch), and then, as in the Carp 

 (Cyprimis), they may bite against a callous pad on the under 

 surface of the basioccipital bone ; or, as in some of the Wrasses 

 (Labrus), the inferior teeth are opposed to superior teeth on the 

 upper pharyngeal bones (pharyngo-branchials of more or fewer 

 of the branchial arches). When pharyngeal teeth are present 

 it is probable that they are the principal masticatory organs, the 

 jaw-teeth being used for seizing or holding the prey. 



Alimentary Canal. 



A protrusible tongue is never developed in Fishes. A rudi- 

 ment of that organ is present in the Elasmobranchs (Fig. 153) 

 and Dipnoi, and also in the Crossopterygii, and usually consists 

 of an elevated area of mucous membrane provided with free 

 lateral edges and a forwardly projecting apex ; it is supported by 

 the basi-hyal element of the hyoid arch. In the Crossopterygii 

 (e.g. Polypter^us) the tongue contains muscle fibres, and in the 

 Dipnoi, where the organ is better developed than in any other 

 Fishes, special lingual muscles are present. 



The pharynx succeeds the oral cavity, and is perforated on 

 each side by the branchial clefts (Figs. 153, 154). The rest of 

 the alimentary canal differs considerably in various Fishes in 

 the degree of distinctness of its several regions, and in the 

 extent to which it is convoluted. As a rule the pharynx is 

 followed in succession by an oesophagus, a stomach, and an 

 intestine (Fig. 153), the latter terminating in a portion usually 

 termed the " rectum.'' The boundaries of these regions are not 

 always very obvious, but are indicated by variations in calibre, 

 by changes in the character of the lining epithelium, by special 

 valves or sphincter muscles, or by the entrance of the ducts of 

 certain glands like the pancreas and liver. The oesophagus is 

 occasionally separated from the stomach by a slight constriction, 

 but more frequently the replacement of the squamous epithelium 

 of the oesophagus by the columnar epithelium of the stomach 



■■ Ridewood, op. cit. p. 390. 



