42 NATURAL HISTORY OF VERTEBRATES. 



vessels and nerves are distributed in the mucosa proper, and this is clad on the free 

 intestinal surface with the hypoblastic cells, generally disposed in a single layer. 



Tliat portion of the alimentary canal concerned more directly with the elaboration of 

 the food is, as we have seen, divided into three regions by two well-marked valves — the 

 pyloric and ileo-cffical. The former separates the first region comprising the oesophagus 

 and stomach from the small intestine, the latter the small from the large intestine. 



Even in forms which are nearly allied to each other, considerable differences may 

 be observed as to the proportionate length of the intestine. These are attributable to 

 difference in the nature of the food ; as a rule carnivorous forms have a simpler and 

 shorter intestinal tract than those which are dependent entirely on vegetable food. 

 An instructive illustration of this principle may be observed in the proportionate length 

 of the intestine in the tadpole and frog. There are few forms in which the intestinal 

 tube is not considerably longer than the body; from the consequent folding of the tube 

 within the coelom, a corresponding complication of the mesentery which suspends the 

 tube occurs. 



Increase in length of the whole tube is only one way in which a greater secreting 

 and absorbing surface may be brought about. Frequently this is effected by the 

 mucous membrane alone, which may be elevated into villi, or depressed into follicles, 

 or may be raised into straight or spiral folds of various forms as in the Selachians and 

 Ganoids. Again,. blind tubes or cseca may be given off, such as the pyloric appendages 

 of the Teleosts, or the caeca given off by the large intestine in birds and mammals. 

 In different parts of the tract, certain constituents may be specially developed ; thus, 

 in the gizzard of the granivorous birds, the muscular coat is locally thickened, while in 

 the glandular stomach in front of it, the hypoblastic cells are chiefly responsible for 

 the increased thickness of the wall. 



In many lower vertebrates, the limit between oesophagus and stomach is hardly in- 

 dicated, but in most higher forms the stomach is not only distinguished by its being 

 dilated, but also by a sudden change in the nature of the lining cells, and by increased 

 complexity of the muscular coat. The gastric glands are tubular in form ; their func- 

 tion is the secretion of the gastric juice. 



Immediately behind the pyloric valve the small intestine is characterized by the 

 reception of the ducts of two important glands, the liver and pancreas, both of which 

 are developed from this part of the intestinal tract, and are formed, to a large extent, 

 of hypoblastic secreting cells. Of these the liver is the largest, and is further dis- 

 tinguished by its paired development, expressed in the adult by the right and left lobes. 

 Only in some primitive forms do we detect the original tubular disposition of the hepatic 

 cells; as a rule, this is concealed in the course of development. The bile secreted by 

 the cells collects in their interstices, which simultaneously are the roots of the bUe 

 ducts, through which the secretion is conducted, either directly or with the interven- 

 tion of a gall-bladder, into the intestine. 



Frequently in close connection with the bile-duct is the pancreatic duct, the rami- 

 fications of which terminate in a racemose manner, like the ducts of the salivary glands. 

 A considerable resemblance is to be detected between the pancreas and the salivary 

 glands, not only in the cells, but also in their secretion. In certain fishes the pancreas 

 appears to be absent. When such is the case, its function is performed by smaller glands 

 of similar nature in the intestinal wall, but it is sometimes only apparently absent, as 

 it follows the ramifications of the bile-duct throughout the liver, although its cells and 

 ducts are quite independent of those of that organ. 



