Branch II. GNATHOSTOMATA. 



AMONG the many important new characters which distinguish 

 the Gnathostomata from the Cyclostomata, and bear witness to 



the higher grade of development 

 of the former, may first of all be 

 mentioned the possession of a dermal 

 exoskeleton. This superficial skeleton 

 first appears in the form of small tooth- 

 like structures, or denticles, scattered 

 all over the skin. Since the skin 

 grows inwards at the mouth and gill- 

 slits, denticles may be found also in- 

 side the buccal cavity, and on the 

 inner surface of the gill-bars. Along 

 the inner edge of the jaws they become 

 specialised into true teeth (Fig. 39). 



The Gnathostomes as a whole are 

 characterised by the great and diverse 

 development of the supporting tissues, 

 whether endoskeletal or exoskeletal ; 

 and the skeleton is of such great im- 

 portance in their anatomy that we 

 may digress at once to briefly investi- 

 gate the whole question of the relation 

 borne by the original cartilaginous 

 endoskeleton to the dermal denticles 

 and bone. 



mk.'' 



FIG. 39. 

 A, diagram of a transverse section 



Cartilage and bone may be looked 

 of the lower jaw of Seyiiium canicuia, upon as specialisations of the general 



L., showing the development and sue- . t i 



cession of the teeth. B, anterior portion Scaffolding OI Connective tlSSUe, which 



pervades, so to speak, the entire body 

 of a Craniate, affording support to and 



in skin on outer surface ;' mk, Meckel's binding together the various parts of 

 cartilage; o.t, old worn tooth; t, time- ,, r . ,, . 



tionai tooth at wting edge; y.t, young the organism. It is really in connec- 



tooth which will succeed the older teeth t ' n W1 >V. 4-U p rmiopnl'ir e^erem tViaf flio 

 in the direction indicated by the arrow. l ;UIOT S > stem tnat tne 



connective tissues have reached their 

 highest degree of differentiation. The Craniata are elongated 



58 



