PKELIMINAKY OBSEKVATIONS. 55 



cavity containing the gills themselves. In the 

 shark tribe, the gills are fixed, there are no 

 true gill-covers, the water escaping through 

 fine branchial apertures on each side. In the 

 flat skates, the mouth, nostrils, and branchial 

 orifices are on the under surface, and the eyes on 

 the upper surface, near which are two temporal 

 orifices, which, while the fish reposes supine on 

 the mvid, admit the entrance of the water into 

 the respiratory or branchial cavity, and also 

 its exit. In the lamprey, seven orifices on 

 each side of the neck lead to so many cells, 

 performing the office of gills, and through these 

 orifices the w^ater is received and expelled. 

 The teeth in fishes vary exceedingly ; in some 

 species they are disposed upon the jaws, the 

 palate, the tongue, and the throat ; in others, 

 the mouth is destitute of teeth, while the throat 

 or pharynx is provided with them. They 

 differ also very greatly in form ; in some fishes 

 they are constructed for crushing ; in others, for 

 cutting ; in others, for prehension ; but in every 

 instance their character accords with that of 

 the food upon which the species habitually 

 subsist. 



Fishes are divided into two great primary 

 sections, namely, the osseous and the cartila- 

 ginous. In the osseous fishes, the bones of the 



