TEETH OP FISHES. 



13 



FIG. 256. HEAD OF SHARK. 



strosity coincides with a defect of symmetry in other parts of 

 the body. 



532. Fishes are very voracious ; there are only a few which 

 live principally on vegetable matter ; and generally they swallow 

 without any selection all the small animals which come within 



their reach. Some 

 species are destitute 

 of teeth, but amongst 

 the greatest part, 

 they exist in several 

 rows, as in the mouth 

 of the Shark, for 

 example ; and they 

 are more commonly 

 found, not only in 

 the two jaws, but also on the palate, implanted on the vomer 

 and palatine bones, on the tongue, upon the interior edge of 

 the branchial arches, and even in the back of the mouth upon 

 the pharyngeal bones, which surround the entrance of the oeso- 

 phagus. They have never any roots, but are fixed to the bones 

 which support them ; they fall off nevertheless probably by a 

 mechanism analogous to that of the fall of the horns of the stag 

 and are replaced by new teeth, which arise sometimes beneath, 

 sometimes by the side of the old ones. The teeth with which 

 the jaws are armed, only serve, in general, to hold or to crush 

 the prey ; those situated at the bottom of the mouth are rarely 

 (in existing fishes at least) disposed in such a manner as to 

 reduce it. Their form varies very much ; sometimes they are so 

 fine and closely set, that they present the appearance of velvet ; 

 whilst in other instances they constitute strong hooks, plates 

 with cutting edges, or rounded tubercles. 



533. There are some Fish which are not supported on solid 

 matter, and which live only by sucking the liquids which they 

 draw from the bodies of other animals ; the Lampreys are an 

 example of this. Their mouth, instead of having the usual 

 arrangement, presents a very singular structure, but one which 

 is in complete accordance with its functions. The cartilages, 



