THE ELASMOBRANCH PISHES 



131 



tubes end in enlarged islands continuous with the enamel (Galeus) . In others 

 the tubules ])enetrate far outward into the enamel and either divide into 

 branches {Carcharias) or are lost as fine single tubules (Lamna). 



EEPLACEMENT OF TEETH 



Teeth which have been injured or lost are replaced by new ones. To gain a 

 notion of the provision made for repairing injury or loss it is only necessary 

 to examine the mouth of a form like Carcharias, or Heterdontus (fig. 128) . In 



B 



Fig. 129. Finer structure of a tooth, Myliohatis. (From Eose.) A. Sagittal section of whole 

 tooth. B. Transverse section cutting a few canals ; highly magnified. 



CO., central canals ; d., dentine ; d.c, dentinal canals ; e., enamel layer ; vd., vasodentine. 



these forms the teeth are arranged horizontally in crescentic rows, several of 

 the outer rows functioning at the same time; upon injury or loss the outermost 

 teeth are replaced by others which migrate outward over the jaw to take their 

 places. It may thus happen that the teeth in several rows may be lost in en- 

 counter or in capture of food. In such an event, provision has been made for 

 their replacement, for at the base of the innermost row are other deeply buried 

 and less mature teeth ranging in development down to flabby tooth-buds. 



Pharynx 



The pharynx is that part of the tract which leads from the buccal cavity and 

 which is characterized by branchial perforations or clefts opening through 

 its walls. In addition to the spiracle seven such apertures perforate the 

 pharynx in Heptanchus, six in Hexanchus and Chlamydoselachus, and five in 

 pentanchid Elasmobranchs. In the adult the spiracle opens into the hyoidean 

 pocket and is of small size. The remaining perforations are large but de- 



