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. 



Cartilage and bone may be looked 

 upon as specialisations of the general 

 scaffolding of connective tissue, which 

 pervades, so to speak, the entire body 

 of a Craniate, affording support to and 

 binding together the various parts of 

 the organism. It is really in connec- 



tooth which willsucc I the older teeth t j Qn w j tn t j ie muscu ] ar system that the 



in the direction indicated by the arrow. , J , 



connective tissues have reached their 

 highest degree of differentiation. The Craniata are elongated 



58 



A, diagram of a transverse section 

 of the lower jaw of Scyllium canicula, 

 L., showing the development and suc- 

 cession of the teeth. B, anterior portion 

 of the right mandible of Acanthias vul- 

 garis, Bisso, cut through at the sym- 

 physis (enlarged), b, mucous membrane 

 lining the buccal cavity ; d, ilenticles 

 in skin on outer surface ; mk, .Meckel's 

 cartilage; o.t, old worn tooth; t, func- 

 tional tooth at biting edge; ?/•'. .vim 



