976 



PISCES. 



In Mylobates, a kind of Ray, in which the 

 teeth are most perfectly adapted for crushing 

 and bruising food, the arrangement of the den- 

 tal apparatus is very curious. In this fish the 

 teeth form broad plates, which are of hexagonal 

 shape, and so closely fitted together by their 

 edges that they form a complete tesselated 

 pavement. 



In the Cestracion Philippii, or Port Jack- 

 son Shark, both the above descriptions of teeth 

 are found united in the same jaws, the anterior 

 resembling the sharp teeth of the common 

 Rays, while the posterior are broad and dis- 

 posed like the paving-stones in a street (Jig. 

 509). The Mylobates affords, moreover, an 

 additional proof of a very convincing descrip- 

 tion that the teeth of Fishes are developed 

 from the lining membrane of the mouth quite 

 independently of the jaws, seeing that here 

 there is a row developed from the mesial line 

 of the jaws, precisely over the symphysis a 

 circumstance never observable in the higher 

 animals, where the jaws and teeth are brought 

 into closer relation with each other. 



The teeth of the Squaloid Fishes or true 

 Sharks are renewed by a very similar mode of 

 growth. In these redoubtable monsters of the 

 deep the teeth consist of numerous rows of 

 broad and trenchant laminae, the anterior row 

 of which (fig. 510, a) stands up perpendicu- 

 larly from the jaws ready for use, while the 

 succeeding layers are recumbent, being covered 

 over by a fold of the mucous lining of the mouth. 

 These teeth, like those of the Rays, are per- 

 petually renovated, new and sharp rows being 

 constantly ready behind to replace the old and 

 worn ones in front as soon as the latter fall out 

 or become useless. 



The situation of these teeth and their mode 

 of growth is represented in the annexed figure. 

 Their only connection with the cartilaginous 

 jaw is evidently through the medium of the 

 interposed fibro-mucous layer (d), which, as 

 it slowly advances forward, carries the teeth 

 with it, and thus brings the successive rows 

 progressively into use. In the Sharks there 

 is no distinct pulp, the dense exterior layer 

 of the tooth being formed by the calcifi- 

 cation of the " membrana propria " of the 

 pulp, so that when divided they are found to 

 be permanently hollow, as represented in the 

 figure (c). 



In the Cyclostomatous Fishes the teeth are 

 still more evidently mere cuticular appendages, 

 seeing that in their case there are no bony 

 jaws to which they can be affixed. In the 

 Myxine Glutinosa, the Hag-fish, one of the 

 most humbly organized but at the same time 

 the most formidable of the finny tribes, this is 

 extremely evident. The Myxine is generally 

 found buried in the substance of some large 

 fish which it has attacked, and by means of the 

 tremendous apparatus we are about to describe 

 succeeds in penetrating, tearing, and devour- 

 ing the soft parts of its unfortunate victim as it 

 pioneers its way into its interior. In order to 

 effect this these fishes are furnished with a 

 single sharp and recurved fang attached 

 to the centre of the roof of their mouth and 

 fixed to the cartilages of the cranium by strong 



Fig. 510. 





fibrous bands. Inferiorly the surface of the 

 tongue is covered with four rows of sharp 

 horny blades, two rows being placed on each 

 side, the whole apparatus being susceptible of 

 being advanced and retracted by means of 

 strong muscles. It is not difficult, there- 

 fore, to imagine the efficiency of a dental appa- 

 ratus such as this in enabling the Myxine to 

 accomplish its murderous purpose. 



The Lampreys, likewise, whose parasitic 

 habits are not very dissimilar from those of the 

 Myxine, have a very singular dental apparatus, 

 which is quite distinct from the cartilaginous 

 skeleton, and evidently purely composed of 

 epidermic structures. The teeth of the Lam- 

 prey are composed of horny plates or tuber- 

 cles of different forms, which are disposed with 

 great regularity over the whole surface of the 

 sucker-shaped mouth. Each of these horny 

 teeth is supported upon and secreted by a pulp 

 derived from the mucous membrane of the 

 mouth, much in the same manner as other 

 corneous epidermic structures, and from their 

 disposition will evidently secure a deadly hold 

 of any victim seized upon. Besides these la- 

 bial teeth there is one fixed to the roof of the 

 mouth which is obviously analogous to the 

 solitary palatine fang of Myxine last described. 

 This (Jig- 511, b) is composed of two horny 

 cones attached by fibrous ligaments to the pala- 

 tine cartilages in the roof of the mouth. The 

 tongue (d), which, like that of the Myxine, 

 is here very moveable and capable of being re- 

 tracted and protruded by means of strong 

 muscles, is likewise armed with serrated 

 teeth, with which, as with a rasp, the Lamprey 

 tears through the flesh of its prey. 



