ix PLACOID SCALES, TEETH 297 



upon as it usually is in the higher animals as being produced by the 

 ectoderm cells of the enamel-organ or on the other hand as being simply 

 the outer layer of dentine specialized and modified, possibly under the 

 influence of the enamel-organ. The present writer considers the latter 

 view to be on the whole the more probable. 



Placoid scales of the type just described are scattered over the whole 

 surface of the body, the spines projecting backwards. Shark's skin 

 specially prepared (shagreen) is used for rasping, and the corresponding 

 adjective " chagrined " is used in a figurative sense ! 



In the case of Acanthias there exists in front of each dorsal fin a 

 powerful defensive spine which is capable of inflicting severe wounds : 

 this is simply a placoid scale with its spine much enlarged. 



The alimentary canal is a longitudinal tube leading from the mouth 

 to the anal or cloacal opening. Apart from greater complexity in detail 

 the wall of the alimentary canal is constructed on the same fundamental 

 plan as that of the earthworm. Its inner surface is lined by a layer of 

 endodermal epithelium : its outer surface is covered by the coelomic or 

 peritoneal epithelium : while interposed between these layers is a mus- 

 cular coat by which the peristaltic movements are brought about. 

 Immediately external to the endoderm is a rich network of blood- 

 vessels concerned with the absorption of the food. 



The crescentic mouth leads into the buccal cavity which is in great 

 part stomodaeal in its nature, representing a space on the ventral side 

 of the head which has been floored in by a forward growth of the lower 

 jaw. The cavity is consequently in part lined by enclosed portions of 

 the outer skin. The roof of the buccal cavity for the most part is simply 

 the skin of the ventral surface of the head : while on the floor of the 

 cavity the outer skin extends inwards all round over the lower jaw. 

 These enclosed portions of outer skin carry the usual armature of placoid 

 scales. Over the greater part they are small or absent (Scyllium) but 

 along a line just within the margin of the mouth opening there is a band 

 of enlarged scales with prominent recurved spines. These are the teeth 

 and here we come into touch with a fascinating conclusion of vertebrate 

 morphology that the teeth are simply specialized placoid scales. In 

 the teeth of one of the higher vertebrates, e.g. a Man, we see the last 

 persisting traces of the coating of placoid scales which covered the 

 surface of the body in a far back ancestral stage. 



That the teeth are in truth placoid scales can be demonstrated readily 

 enough in Scyllium by making sections through the lower jaw when the 

 typical tooth and the typical placoid scale outside are seen to be members 



