164 



ESSENTIALS OF ZOOLOGY 



to form dentine. During their development the teeth are 

 gradually rotated along the inner side of the dental band, the 

 dentine gaining in thickness and the whole tooth in size. 



Epidermis Tooth Epidermis Dental land Epidermis 



Pulp cav. Dermis Margin of dental band 



FIG. 82. Development of teeth of Myliobatis. After Rose. 



Finally it reaches the jaw epithelium, which it pierces. In 



structure it is exactly the same as the scale, presenting a strong 



surface of hard dentine graduating internally into a more 



alveolar condition invaded by canals 



m ^ e P U *P cav ity- The canals 

 embrane) have the structure and relationship of 

 Haversian canals of bone. It has been 

 said that the ectoderm is able to form 

 on the tooth and on the scale an enamel 

 crown, but it is more probable, as Eose 

 said, that the teeth of the Elasmobranchs 

 are, like the teeth of the Edentate 

 mammals, devoid of enamel. 



The mouth cavity expands into a 

 spacious pharynx in which may be ob- 

 served the inner openings of the spiracle 

 and the branchial clefts. Behind the 

 last branchial opening the pharynx 

 narrows to form the oesophagus or 

 gullet. The canal expands slightly to 



become the stomach, which is bent gradually into a curve, 

 and is thus resolved into a proximal cardiac and a distal 

 pyloric region. The mucous epithelium of the stomach 



Submucous 

 layer 



FIG. 83. Raia. Gastric 

 gland in longitudinal 

 section. 



