IV. VERTEBRATA. 



515 



tissue. Both of these constituents of the skin, aside from their 

 own firmness, can give rise to protective structures. The horny 

 layer of the epidermis in places becomes greatly developed and 

 thus forms the tortoise shell of the turtles, the scales, shields, and 

 scutes of the snakes and lizards, the feathers of the birds, the hair 

 and horns of the mammals. Other epidermal products are the 

 claws, nails, and hoofs of the terrestrial vertebrates. The derma 

 is often the seat of ossifications which, in contrast to the deeper 

 bones, are called the dermal skeleton. 



First of the dermal skeletal parts are the scales of the fishes,, 

 which, in spite of similarity of name, 

 are different from the epidermal scales 

 of the reptiles. They may be traced 

 back to the primitive form, the pla- 

 coid scales of the Elasmobranchs. 

 These are rhombic plates, bearing in 

 the middle pointed spines, which are 

 called dermal teeth from similarity in 

 structure and development to the teeth 

 of the mouth cavity (fig. 554). They 

 consist of dentine (d) and have a large 

 pulp cavity (p), with numerous blood- 

 vessels in the interior. Whether the 

 thin layer (.sr/<) covering the tip can 

 be called enamel is disputed. Der- 

 mal teeth and true teeth are identical 

 structures which, in consequence of 

 different position and consequent dif- 

 1'erence of function, have developed differently. 



The scales of fishes have a wider anatomical interest, since from 

 them have arisen, besides the bony plates which form the resistant 

 armor of the turtles, alligators, and many mammals (Armadillos), 

 important parts of the axial skeleton, the secondary or membrane 

 bones. A membrane bone is a bony plate which has arisen from 

 a fusion of dermal ossifications, becomes transferred to a deeper 

 position, and contributes to the completion of the axial skeleton. 

 After what was said above about the relations of dermal and true 

 teeth it is readily seen that a further source of formation of mem- 

 brane bones lies in the lining of the mouth cavity. 



In describing the axial skeleton, the notochord comes first. 

 This has already been mentioned in connexion with the lower 

 Chordates. It persists in the cyclostomes, but from them upwards 



