250 



FISHES 



CHAP. 



like in many other respects, also has teeth implanted basally in 

 the gums, and quite free from any special connexion with the 

 jaw -hones. In some Teleosts with movable teeth, the latter 

 are merely attached to the jaws by iibrous, and often elastic, 

 ligaments, as in the Pike (Usox) and the Angler-Fish (Zophius). 

 As a rule, however, the teeth are directly ankylosed to the 

 bones developed in relation with the jaws. Very rarely, as, for 

 example, in some Characinidae, are the teeth implanted in 

 sockets. 



Xearly all Fishes are polyphyodont, that is, the old teeth are 

 constantly replaced by new teeth as fast as they become worn 

 down or fall out. In the Sharks and Dog- Fishes^ for example, 

 where the teeth are arranged in rows parallel to the axis of each 

 jaw, the functional teeth along the upper edge of the jaw are 



usually erect, while those iji the 

 rows more internally situated point 

 inwards towards the oral cavity ; 

 and behind these again there are 

 rows of developing teeth in different 

 stages of growth, and partially hidden 

 beneath a projecting fold of the oral 

 mucous membrane (Fig. 152). As 

 the teeth in use become lost they 

 are successively replaced by the 

 inner rows, which, with the mucous 

 Fig. 152.— Transverse section through membrane in which they are em- 



the jaw of a Shark [Carchdrias), i , , -, j, 



showing how the teeth are re- Dedaea, move lorwards to the edge 

 placed, c. Cartilage of the jaw ; of the jaw, where they become erect 



t, fiinetiODal tooth ; t \ its ira- j i> • 



mediate succes.sor ; t", t", still and functional The teeth of the 



younger teeth, covered by the Holocephali and of the Dipnoi are 

 fold of mucous membrane, m. m. '^ jj^^wi ^io 



(From Eidewood.) not slied, but the loss which they 



sustain through wear and tear is 

 made good by persistent growth at their bases. In the Teleo- 

 stomi the succession is less regular, new teeth being formed between 

 or at the bases of the old teeth. In the case of socketed teeth 

 the succession is usually vertical, the new teeth being formed at the 

 sides of the old ones ; and by the absorption of the bases of the 

 latter, -the former come to lie directly below them, and eventually 

 they occupy the same sockets. 



As might be expected from the remarkable diversity in the 



^^i -rn.m. 



