444 



THE DOGFISH 



CHAP. 



them, that of the dogfish and of the frog is a series of 

 articulated rods with the muscles outside. The joints, 

 formed by two rods applied at their ends and bound 

 together by ligament, are not all confined to movement 

 in one plane, like the hinge- joints of the crayfish, but may 

 be capable of more or less rotary movement. 



Digestive Organs. The mouth, as we have seen, is a 

 transverse aperture bounded by the upper and lower 



FIG. 116. Diagram of the development of a tooth. 

 Bg, Bg. mesoderm ; DS. dentine ; EM. epithelium of mouth ; Ma. epithelium of 

 enamel-organ; O. odontoblasts ; SK. dental lamina; ZK. dental papilla. 

 (From Wiedersheim's Comparative Anatomy.) 



jaws. In the mucous membrane covering the jaws are 

 embedded large numbers of teeth conical, calcified 

 bodies, with enamelled tips, arranged in transverse 

 rows. They are to be looked upon as special develop- 

 ments of the placoid scales or dermal teeth (p. 433) 

 enlarged for the purpose of seizing prey, and are con- 

 tinually renewed on the inner sides of the jaws as they 

 are worn away on the outer sides. 



