444 



THE DOGFISH 



It will be noticed that while the skeleton of the crayfish is a series of 

 articulated tubes, with the muscles inside 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 rotatory 

 movement. 



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

 transverse aperture bounded by the upper and lower jaws. 



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



&> Kg- 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.} 



In the mucous membrane covering the jaws are im- 

 bedded large numbers of teeth conical, calcined bodies, 

 with enamelled tips, arranged in transverse rows. They 

 are to be looked upon as special developments of the 

 placoid scales or dermal teeth (p. 433) enlarged for the pur- 



