482 



STRUCTURE OF VERTEBRATA. 



the lungs lie ventrally and are double, though connected 



with the gullet by a single tube. 



The beginning of the intestine gives origin to the liver, 



which regulates the composition of the blood and secretes 



bile, and to the pancreas, which secretes digestive juices. 



The pancreas has often a multiple rudiment. 



From the hindmost region of the gut, the allantois 



grows out in all animals from Amphibians onwards. In 



Amphibians it is represented by a cloacal bladder ; in the 



higher Vertebrates it is a vas- 

 cular foetal membrane concerned 

 with the respiration or nutrition 

 of the embryo, or both. 



Cilia are very common on the 

 lining of the intestine in Inverte- 

 brates, but they are much rarer 

 in Vertebrates. Yet as they 

 occur in Amphioxus^ lampreys, 

 many fishes, Protopterus, some 

 Amphibians, and in embryonic 

 Mammals, it seems not unlikely 

 that the alimentary tract was 

 originally a ciliated tube. 



At the posterior end an ectodermic 

 FIG. 232. Origin of lungs, imagination or proctodseum meets 

 liver and pancreas in the the dosed arc h e nteron, and at the 

 chick. After Goette. junction the two epithelial layers give 



The mesoderm is shaded ; the enclo- waVj so that an open tube is formed. 



/^""etnhe lungs ; ., stomach ; The formation of the anus does not 

 /., liver; p., pancreas. take place close to the posterior end 



of the primitive gut, but at a point 



some short distance in front of this. In consequence the so-called 

 post-anal gut is formed. This is continuous with the neurenteric 

 canal, and so communicates with the neural canal. The post-anal gut 

 attains in Elasmobranchs a relatively considerable length. It has been 

 very frequently found in Vertebrates, and is probably of universal 

 occurrence. After a longer or shorter period it becomes completely 

 atrophied, and with it the communication between neural and alimentary 

 canals is completely destroyed 



Speculative. The primitive gut was probably a smooth straight 

 tube, but the rapid multiplication of well-nourished cells would tend 

 to its increase in diameter and in length. But on increase in both 

 directions the slower growth of the general body would impose limita- 

 tions, and in this we may find the immediate growth-condition deter- 

 mining the origin of folds, crypts, creca, and coils, which would be 



