B. SKELETON. 



1. EXOSKELETON. 



THE hard exoskeleton, consisting of bone or other calcified tissues? 

 must be distinguished from the horny exoskeletal parts described 

 in the last chapter, in which, however, the former was also referred 

 to. Thus it will be remembered that the term " scale " is some- 

 times used for a horny epidermal structure, and sometimes for a 

 bony dermal one (pp. 18, 20). 



The first and most primitive hard structures in Vertebrates are 

 met with amongst Elasmobranchs in the form of small, pointed 



denticles (placoid organs') in the skin ; 

 these consist of enamel and dentine, 

 resting on a basal plate of bone, thus 

 resembling in structure ordinary oral 

 teeth, which will be described later. 

 Primitively, as in many Rays, there is 

 a relatively small number of these 

 placoids, which do not touch one 

 another, while in most Sharks arid Dog- 

 fishes they are much more numerous 

 and close-set (Fig. 18). Their shape 

 is rhombic or more or less rounded, 

 each bearing a spine, and new ones 

 being continually formed. The enamel, 

 developed in connection with epidermic 

 cells, is the primary part of the den- 

 ticle (Fig. 19) ; the dentine is developed 

 secondarily that is, later from the 

 phyletically younger mesoderm, and 

 this is also true of the bony portion, 

 The enamel is therefore the first, and 

 originally the only hard substance of 

 t he placoid organ. 



e first bony tissue to be developed 

 atomy.) is thus formed in connection with 



FIG. 18. DERMAL DENTICLES 

 OF Centrophorus calcem, 



An- 



