Glass 2. Pisces. 



371 



pterygoid, the branchial bars (especially tlie superior and inferior 

 pharyngeal bones), the basibranchials of the visceral skeleton, and 

 the vomer; they may, however, be wanting from one or other of 

 these bones. The teeth are of somewhat diverse form: most often 

 pointed, conical, slightly curved, and more or less powerful ; in other 

 cases they are low, rounded, grinding teeth (Rajidse, certain Teleostei) ; 

 or compressed and triangular (Squalidse) ; or chisel-shaped, resem- 

 bling the incisors of Man (teeth on the premaxilla and mandible 

 of certain Teleosteans) . Very often they are extremely numerous, 

 covering the bones like a mosaic ; on the jaw there is frequently 

 only a single row of teeth, or a row of larger, outside a row of 

 smaller, denticles. They are either attached to the subjacent bone 

 by connective tissue, and then often partly movable, or they are 

 implanted in bony sockets. They are renewed throughout the whole 

 life, the old teeth fall out as the connection between them and the 

 cartilage or bone gives way, or if a socket is present this is absorbed. 

 The usual conical, piscine teeth are chiefly prehensile, and the points 

 are therefore turned backwards and inwards; they are movable so 

 that the point may assume another position. Teeth of other forms 

 are used for biting or masticating the food. 



The CBSophagus is so short and wide that the mouth passes almost 

 directly into the stomach. In most Teleosteans a varying number of 

 short, blind, glandular sacs (1 — 100), the appendices pyloricce, open 

 into the anterior part of the small intestine, close to the stomach. In 

 Cyclostomes, Selachians, and Ganoids, there is a spiral valve in 

 the small intestine, a large projecting fold, attached to the inner 

 side of the gut, and almost filling up its cavity ;* it is absent from the 

 Teleosteans. The large intestine is quite a short tube. 



1^'- - V '.■'4S!6^; 



Fig. 306. A fish with pectoral anus (Siemarchus curvir 

 ventral view of head end. a anus, o mouth. — After Bonlenger. 



A lateral view, B 



In some Fish (e.g., the Plaice), the anus does not lie at the boundary of trunk 

 and tail, as in all higher Vertehrata, but has moved forwards, sometimes even far on 

 to the trunk. The anal fin in such cases follows the anus, and usually takes up 

 its position close behind it. 



* Exceptionally, in some Squalidse, the fold springs from the gut-wall in an almost 

 straight line, it is then broad and rolled like a piece of paper. 



B B 2 



