38 



THE ELASMOBRANCH FISHES 



tinal canals appear which are essentially like those of the scale. Superficially 

 the filament is surrounded by a more compact layer, but structurally it is 

 similar to the base. The central canal of the filament is surrounded by a "non- 

 vascular dentine" in which there is a network of dentinal canals. 



Stomodeal Denticles 



The stomodeal denticles are also modified scales, although it may 

 be more correct to speak of them as atrophied rather than hyper- 

 trophied. These denticles may be abundant over the larger part 

 of the buccal and pharyngeal walls as well as over the branchial 

 arches {Heterodontus, fig. 34d), or they may be present in the 

 buccal cavity and restricted to the hyoid and first branchial 

 arches (Squatina), or they may be confined to the pharyngeal 

 margins of the branchial arches (Alopias vulpes). In certain 

 types they are rudimentary (Squalus sucklii) and in others they 

 have ceased to be developed altogether {Scyllium canicula). 



It has been suggested that the denticles may serve to hold and 

 to a slight extent to grind food, but it seems more probable that 

 they are structures which, l)ecause of their location and origin, 

 are without pronounced function, and hence are usually ves- 

 tigial (see Chapter V). This is indicated by the fact, as Imms 

 (1905) has suggested, that whether the food be hard or soft they 

 develop equally well (as in Galeus, and Mustelus). Further- 

 more, like vestigial structures they often appear much later 

 than do the scales of the body {Carcharias). 



Teeth as ^Modified Scales 



The most important structures, however, with which placoid 

 scales are associated are the teeth. In a type like Accndhias, in 

 which these are sharp pointed, superficial resemblance between 

 the two is more or less pronounced. In a continuous section cut- 

 ting through both the scales and the teeth it has been observed 

 that the scales outside of the buccal region have their spines pro- 

 jecting backward, while those within the mouth may have their 

 spines in a reverse direction, and still be pointing backward. 

 Some of the transitionals lietween the two regions, however, 

 possess both an anterior and a posterior spine, and hence are so 

 generalized that the retention of the latter projection would re- 

 sult in a scale, its loss would result in the formation of a tooth. 



In other Elasmobranchs, little outward similarity between 

 tooth and scale is seen. Such is in part true of the immense tooth 

 of Carcharodon (see p. 129, fig. 127); especially is it true of 

 manj' of the plate-like crushing or pavement teeth like those of Myliobatis 

 (p. 128, fig. 126b). Invariably, however, the two are essentially identical in 

 nature. For a further consideration of the teeth, see Chapter V. 



.Fig. 44. 



Gill raker of 



Cetorhinus 



maximus. 



(From 



Hendricks.) 



