TEETH. 



127 



fishes the new tooth is not developed (as in reptiles and 

 mammals) in a diverticulum 

 of the sack of its predecessor, 

 but like this from the free sur- 

 face of the buccal membrane. 

 Generally there are more than 

 one tooth growing, which are 

 in various stages of develop- 

 ment, and destined to replace 

 the one in function. This is 

 very conspicuous in Sharks, in 

 which the whole phalanx of 

 their numerous teeth is ever 

 marching slowly forwards (or 

 in some backwards), in rota- 

 tory progress, over the alveolar 



border of the jaw, the teeth ^g. 54._Cardlike teeth of Plectropoma 

 being successively cast off after dentex, with canines. 



having reached the outer margiu, and fulfilled for a longer 

 and shorter period their special function. 



[The richest materials for our knowledge of the teeth of fishes are con- 

 tained in OwetCs "Odontography." Lond. 1840. 8to.] 



The intestinal tract is divided into four portions : oeso- 

 phagus, stomach, small and large intestine ; two or more of 

 these divisions may coalesce in fishes and become indistin- 

 guishable. But it is characteristic of the class that the 

 urinary apertures are constantly situated behind the termina- 

 tion of the intestinal tract. 



In Branchiostoma the whole intestinal tract is straight, 

 and coated with a ciliated mucous membrane. The wide 

 pharynx passes into a narrow oesophagus, this into a gastric 

 cavity, the remainder being again narrower and terminating 

 in the anal aperture, which lies somewhat to the left of the 

 median line. The liver is represented by a green coloured 



