650 



PISCES. 



granules become condensed into concentric layers, which then form the 

 walls of minute tubes, visible on a microscopic examination of the sub- 

 stance of the tooth. 



(1775.) In some genera, as Batistes and Chrysophrys, 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. 



(1776.) 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 aliment, and more 

 especially the water swallowed for the purpose of respiration, escaping 

 again from the oral orifice*. 



(1777.) Fishes have no salivary glands, as saliva to them would be 

 entirely useless: their oesophagus (fig. 31 7, g\ fig. 327, d) is capacious and, 

 from the circumstance of their having neither neck nor thorax, extremely 

 short ; so that the food when seized is conveyed at once into the stomach. 



(1778.) The stomach itself is generally a wide cul-de-sac (fig. 317, h), 



Fig. 317. 



Plan of the general arrangement of the viscera in a Fish. 



the shape and proportionate size of which vary of course in different 



species. Its walls are most frequently thin, and the lining membrane 



gathered into large longitudinal folds (fig. 327, e), so as to admit of con- 



* Cuvier et Valenciennes, Histoire Naturelle des Poissons, p. 367. 



