12 



Vcrtebrata. 



tremity of the vertebral column is appended a flattened 

 tail, which, by moving like a scull or screw-propeller, 

 can drive the body forward. The limbs are also, in 

 fishes, developed into fanlike bars, the fins. 



9. Scales. The surface of the body in fishes has 

 only a scanty epidermis^ or outer layer of skin, which is 

 generally of a mucous or slimy consistence; beneath this 

 is the demits^ or inner skin, whose surface consists of 

 numerous thin, flattened scales. These structures, so 

 characteristic of fishes, are composed of bony plates, 

 which are ossifications of flat dermal processes, often 

 containing or bearing little tooth-like points, composed 

 of the same material as true teeth. In some fishes, like 

 sharks, the entire scale consists of this dentine or tooth 



FIG. 3. 



Placoid scale of dog-fish (vertical section magnified). 

 a, enamel layer ; b, dentine of spine on scale. 



structure (fig. 3) ; in others the bony element, which 

 forms around the tooth, covers or entirely supersedes 

 the dentinal, but in its essential nature the coating of 

 scales or dermal exoskeleton of fishes may be regarded 

 as consisting of or containing ossified papillae, which 

 in their structure are identical with the tissue of ordi- 



