EXOSKELETON 



31 



these denticles, the basal-plate representing an accessory portion 

 of the denticle, and serving to fix it within the skin. In the 

 further course of evolution the denticle itself undergoes reduc- 

 tion, the basal-plate remaining as an independent structure. 

 This is illustrated by a study of the exoskeleton in other 

 Vertebrates. 



In the Holocephali dermal denticles are only present on 

 certain appendages (the claspers), and the first dorsal-fin is 

 strengthened by a large bony spine. 



In most Ganoids thick plates, usually rhombic in form, are 

 present in the skin ; in bony Ganoids these cover the entire 

 body, their margins being in apposition. 1 These ganoid-scales- 

 correspond to the main (deeper) part of the placoid basal-plates, 



FIG. 19. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYO SHARK. 

 (From Gegenbaur's Comp. Anatomy.) 



C, derails ; c, c, c, d, layers of the dermis ; p, papilla ; E, epidermis ; e, its layer 

 of columnar cells ; o, enamel layer. 



the spine having become rudimentary. Their surface is dense 

 and smooth (ganoin-layer), and was formerly erroneously supposed 

 to consist of enamel. In Lepidosteus they bear numerous small 

 denticles ; but from what has been said above, this fact does 

 not indicate that each ganoid-scale corresponds to a multiple 

 of placoids. The exoskeleton was largely developed amongst fossil 

 Ganoids. 



The scales of Teleosts, the first indication of which, as in 

 the case of placoid scales, is seen in the form of small papillae of 

 the dermis extending into the epidermis, correspond to the super- 

 ficial portions of the basal-plates. In the further course of develop- 

 ment they are seen to consist of bony plates arranged in oblique 

 rows and lying directly beneath the epidermis, the individual scales 

 not touching one another, and their surfaces lying parallel to the 



1 In Amia, the scales have a " cycloid" form. (See note on p. 32.) 



