ALIMENTARY CANAL OF FISHES. 409 



its full size ; and, owing to the rapidity of the succession,, the 



cavity at the base of the fully formed tooth is never consolidated. 



The fossil jaws of the extinct Crocodilians demonstrate that the 



same law regulated the succession of the teeth at the ancient 



o 



epochs when those highly organised Reptiles prevailed in greatest 

 numbers, and under the most varied generic and specific modifica- 

 tions. Of these the most remarkable, in reference to the dental 

 system, is the Galeosaurus, in which the well-marked differences 

 of size and shape permit the division of the teeth, in both upper 

 and lower jaws, into incisors, canines, and molars. This is the 

 nearest approach to a mammalian type of dentition hitherto ob- 

 served in the Reptilian class. 1 



72. Alimentary canal of Fishes. The cavity, commonly 

 termed ' abdominal,' which lodges the main part of the alimentary 

 canal and its appendages, seems to occupy a smaller proportion of 

 the trunk in Fishes, fig. 276, I, h, i, than in Reptiles, fig. 292, 

 d> iv, by reason of the slight and gradual contraction of the body 

 beyond the vent to form the muscular organ of the tail-fin : the 

 greater and more abrupt contraction of the answerable part in 

 Reptiles distinguishes it more plainly as the ' tail ' ; the f trunk ' 

 is usually a longer segment of the body than in Fishes. In these, 

 however, the abdominal cavity commences immediately behind the 

 head : in most Reptiles a ( neck ' intervenes. In Fishes a thoracic 

 or pericardial cavity, fig. 276, o, is partitioned off from the fore- 

 part of the proper abdominal one : and there are in this class 

 exceptional examples of the shortest abdominal cavity in pro- 

 portion to the length of the body known in the Vertebrate 

 province, as e. g. in Gymnotus, fig. 232, in which the abdomen does 

 not extend into the compartment, b, much beyond the vent, 

 which is seen near the angle of the cut integument, beneath the 

 mandible. 



The cavity containing the beginning of the alimentary canal is 

 called the ' mouth.' This, in Fishes, is the common entry and 

 vestibule to both the digestive, fig. 276, d to g, and the respi- 

 ratory, ib. t, u, organs ; it is, therefore, of great capacity : and, as 

 the transmission of the food to the stomach and of the respiratory 

 currents to the gills is performed by similar acts of deglutition, 

 the bony arches which surround the mouth are not only large, 

 but are complicated by a mechanism for regulating the transit 

 of the nutritious and oxygenating media, each to its respective 

 locality. The branchial slits, in most Fishes, are provided with 



! ccxxm. p. 58, pi. u. 



