i84 



COMPARATIVE ANATOMY 



In elasmobranchs, on the edges of the jaws, these minute scales become 

 enlarged into formidable biting teeth or sometimes inside the mouth 

 into bony pavements that are used for grinding the food. 



That the biting tooth of elasmobranchs is a modified placoid scale is 

 obvious from inspection, since the two look aUke, and there is a transition 

 in size and form between them. This identity is further borne out by 



OLFACTORY PIT- 



TASTE PAPILLA 



PLACOID SCALE 



ESOPHAGEAL VILLUS 



Fig. 140. — The pharynx of an elasmobranch (Squalus) laid open to show the double 

 row of teeth in both upper and lower jaws. Such teeth differ only in size from the 

 placoid scales of the pharynx and skin. Elasmobranch teeth, like scales, are fastened 

 in the skin and are not attached to the jaw cartilages. (After Cook.) 



other evidence. Like true teeth, placoid scales have a base of dentine, 

 which contains a pulp-cavity filled with connective tissue. Both scales 

 and teeth have a spinous process, covered wath enamel, which protrudes 

 through the skin. Moreover, their development is similar, in that, in 

 both, the enamel is secreted by the ectoderm and the dentine by mes- 

 enchyme, and both arise in that portion of the mouth where the ectoderm 

 has invaginated to line the digestive tube. See Fig. 141. 



