PISCES FISHES. 515 



vessels and nerves. The particles become arranged into linear 

 series or fibres ; an appearance which is first apparent at the super- 

 ficies of the pulp, to which the fibres are vertical. At this period 

 ossification commences in the dense and smooth membrana propria 

 of the pulp, and is thence continued centripetally in the course of 

 the above-mentioned lines towards the base of the pulp. Lastly, 

 around the capillaries of the pulp the granules become condensed 

 into concentric layers, which then form the walls of minute tubes, 

 visible on a microscopic examination of the substance of the tooth. 



In some genera, as Balistes and CAryaopry*, an enamel-pulp is 

 developed from the inner surface of the capsule which surrounds 

 the bone-pulp, and by this organ the surface of the teeth of such 

 fishes is coated with enamel in a manner to be described more at 

 large hereafter. 



In most osseous fishes, in addition to the lips, which even when 

 fleshy, being destitute of proper muscles, would be unable to retain 

 food in the mouth, 'there is generally behind the front teeth in 

 each jaw a valve formed by a fold of the lining membrane of the 

 mouth, and directed backwards so as efficiently to prevent the 

 alimerjt, and more especially the water swallowed for the purpose 

 of respiration, to escape again from the oral orifice.* 



(545.) Fishes have no salivary glands, as saliva to them would 

 be entirely useless : their esophagus (Jig. 227, g ; Jig- 236, d) is 

 capacious ; and, from the circumstance of their having neither neck 

 nor thorax, extremely short, so that the food when seized is con- 

 veyed at once into the stomach. 



(546.) The stomach itself is generally a wide cul-de-sac 

 (fig- 227, A), the shape and proportionate size of which varies 

 of course in different species. Its walls are most frequently thin, 

 and the lining membrane gathered into large longitudinal folds 

 (Jig. 286, e), so as to admit of considerable distension ; but occa- 

 sionally, as for example in the Mullets, its muscular walls are 

 so thick that it might almost deserve the name of gizzard, and 

 in such fishes its power of crushing the food is no doubt consi- 

 derable. 



(547.) The intestinal canal in the osseous fishes is a simple tube 

 (Jig. 227, ) folded in sundry gyrations proportioned to its length ; 

 but in the cartilaginous families, such as the Sharks, the Rays, and 

 the Sturgeons, it presents internally a very remarkable arrange- 

 ment, evidently intended to increase the extent of surface over 



* Cuv. et Valenciennes, op. cit. p. 367. 



