B. SKELETON. 



1. EXOSKELETON. 



The hard exoskeleton, consisting of hone 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 hone, 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 and 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 

 the placoid organ. 



The first bony tissue to be developed 

 is thus formed in connection with 



18. — Dermal Di:>;TicLK 

 OF Gentro]jhoruti caleeux 

 (Slightly magnified. ) 

 (From Gegenbaur's Conqj. An 



itfOllli/. ) 



