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 



MT v.\ x ', endoskeleton to the dermal denticles 



mK. \). / \ 



and bone. 

 .j+. 



A, diagram ofTtmns verse section Cartilage and bone may be looked 



of the lower jaw of Scyiiium caniwia, U p n as specialisations of the general 



L., showing the development and sue- .,,.,. . . & , 



cession of the teeth. B, anterior portion Scaffolding 01 Connective tlSSUC, which 



: pervades, so to speak, the entire body 



yes f a Craniate ' affOTdil 'g SU PP rt t0 and 



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

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



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



tooth which will succeed the older teeth tl ' An W : i .i 1 f u A !].,.. Trott>m fkof fho 

 in the direction indicated by the arrow. tl0n Wlt / 1 t " e > HlUSCUlar System that the 



connective tissues have reached their 

 highest degree of differentiation. The Craniata are elongated 



58 



