428 



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 

 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 confined to movement in one plane, like the hinge- 

 joints of the crayfish, but are capable of more or less rotatory move- 

 ment. 



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

 transverse aperture bounded by the upper and lower jaws. 



Fig. 107. — 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.) 



In the mucous membrane covering the jaws are im- 

 bedded large numbers of feeik — conical, calcified bodies, 

 with enamelled tips, arranged in transverse rows. They 

 are to be looked upon as special developments of the 



