597. 3 8 ° 



591.49 



II. 



The Teeth of Fishes. 



THE calcified teeth of vertebrated animals bear no morphological 

 relation to any of the heterogeneous structures called teeth that 

 occur in invertebrates, but have been evolved within the confines of 

 the vertebrate group. The low position which fishes occupy in the 

 vertebrate series renders the study of their dentition of the greatest 

 interest, because it is among them that the first evolution of such 

 teeth originated. Vertebrate teeth are not new structures, sui generis, 

 and independently developed, but have arisen by a modification of 

 tegumentary or dermal organs ; and nowhere is this fact more clearly 

 demonstrable than in fishes. A comparison, under the microscope, 

 of vertical sections of the teeth of- the jaw with the placoid spines of 

 the skin of the skate or dogfish, shows that the teeth are formed on 

 the same fundamental plan as the spines, and are made up of the 

 same essential tissues. In the young dogfish, shortly before hatching, 

 there is no lip, and the spines which clothe the skin form a continuous 

 series with those embedded in the mucous membrane covering the 

 jaw. It is only when the lips develop that the continuity is inter- 

 rupted, and the differences between the teeth and spines become more 

 marked. The teeth then increase in size and solidity, and acquire 

 their specific characters, whereas the spines on the exterior of the 

 body remain practically unchanged. 



Structure. — The three tooth-tissues, dentine, enamel, and cement, 

 are represented in the teeth of fishes, the first, as usual, occurring in 

 greatest bulk. The dentine is usually deposited uniformly around a 

 conical pulp by the activity of a superficial layer of " odontoblast " 

 cells ; and the dentinal tubules, occupied by delicate fibrils proceeding 

 from these cells, are arranged at right angles to the surface of the cone. 

 Blood-vessels do not penetrate into the dentine, and its texture is con- 

 sequently dense. In a few cases, e.g., Lepidostens, the surface of the 

 pulp cone is fluted at the base, and the form of the dentinal covering 

 is modified accordingly, so that a transverse section shows the 

 dentine thrown into radiating plications or folds. 1 



1 The complex pattern of the section of the tooth of some of the extinct 

 Labyrinthodont Amphibia is hardly more than a further elaboration of this form of 

 plication. 



