556 



COMPAKATIVE ANATOMY. 



connection between the fcetus and the uterus. A rudiment of the 

 yolk-sac is retained as the " umbilical vesicle," which is not taken 

 into the body-cavity, as it is not of any use in the nutrition of the 

 embryo, but is after birth separated from the young animal with 

 the coverings of the egg. 



The divisions of the alimentary canal are the same as those in 

 the Invertebrata ; fore-, mid-, and hind -gut. 



J 



Fore-gut. 



§ 418. 



The first portion of the alimentary canal proper is an exceedingly 

 short piece in Amphioxus, and is placed directly in front of a 

 diverticulum which is directed forwards, and is regarded as the liver. 

 If we consider that the liver is always derived from the portion 

 which is to be regarded as the mid-gut, and that it forms its 

 anterior boundary, it follows that the fore-gut is exceedingly small 

 in many of the Craniota also. This character is seen in the Cyclosto- 

 mata, Chimserae, and vai-ious Teleostei. The rest of the Craniota differ 

 in this point, for their fore-gut forms a large piece, which can be 

 divided into oesophagus, and stomach. In any case these parts 

 appear to have been acquired by the Gnathostomata only, among the 



Vertebrata. The distribution of the 

 vagus on their walls is of importance 

 as bearing on their origin ; and there 

 is in consequence of this distribu- 

 tion reason for supposing that the 

 tract in question has been developed 

 from a portion which primitively 

 belonged to the cephalic enteron. 

 The reduction of a larger number 

 of branchial clefts, and the conver- 

 sion of a portion of the respiratory 

 antechamber to a purely nutrient 

 function, is in agreement with 

 this distribution. On the other 

 hand, the extension of this tract, 

 and especially of the portion which 

 represents the stomach, as well as 

 its position in the coelom, are due 

 to the great quantity of food in- 

 gested. The stomach is almost 

 always separated from the mid-gut 

 by a fold of the enteric wall (pyloric 

 valve). 



In Fishes, the oesophagus, which is very wide and provided with 

 longitudinal folds of the mucous membrane, generally passes at 



Fig. 313. Digestive canal of Fishes. 

 A Of Gobius melanostomus. B Of 

 Salmo. Oesophagus, v Stomach. 

 i Mid-gut. ap Appendices pyloricse. 

 r Hind-gut. 



