In all fishes the teeth are constantly shed or renewed during the whole course of their life. In fishes which have compound teeth, as the Dipnoi, Chimæroids, Scari,[14] Gymnodonts, as well as in those which have apparently permanent teeth, as in the saw of _Pristis_, the detrition of the surface is made up by a constant growth of the tooth from its base. When the teeth are implanted in alveoli, they are generally succeeded by others in the vertical direction, but in others they succeed one another, side by side. In the majority of 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 surface of the buccal membrane. Generally there are more than one tooth growing, which are in various stages of development, 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 rotatory progress, over the alveolar border of the jaw, the teeth being successively cast off after having reached the outer margin, and fulfilled for a longer and shorter period their special function. [The richest materials for our knowledge of the teeth of fishes are contained in _Owen’s_ “Odontography.” Lond. 1840. 8vo.] [Illustration: Fig. 54.--Cardlike teeth of Plectropoma dentex, with canines.] The _intestinal tract_ is divided into four portions: œsophagus, stomach, small and large intestine; two or more of these divisions may coalesce in fishes and become indistinguishable. But it is characteristic of the class that the urinary apertures are constantly situated behind the termination 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 œsophagus, 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 cœcal diverticulum of the stomachic dilatation. A mesenterium is absent. In the _Cyclostomi_ the intestinal tract is likewise straight, and without clearly defined divisions; however, in _Petromyzon_ the œsophagus shows numerous longitudinal folds, and the intestine proper is provided with a single longitudinal fold. A mesentery, which is present in the Myxinoids, is represented by a short median fold only, by means of which the hindmost part of the intestine is fixed. The _Palæichthyes_ show differences in the structure of their intestinal tract as considerable as are found among the _Teleostei_, but they have that in common that the absorbent surface of their intestine is enlarged by the development of a spiral valve, evidence of the presence of which in extinct Palæichthyes is still preserved in the fossilised fæces or _coproliths_, so abundant in some of the older strata.