566 COMPARATIVE ANATOMY. 



the loops of the mid-gut, as in most Mammals, or partly conceals the 

 stomacli (Ruminantia) . 



The mesentery of the hind-gut retains its primitive characters in 

 Vertebrata with a short hind-gut. When that portion of the hind- 

 gut which is known as the colon increases in length, as it does in 

 the Mammalia, the mesentery or mesocolon accompanies it, and is 

 at the same time raised by one portion as far as the root of the 

 mesogastrium so that the two arise close together. This gives rise 

 to that gradual connection between the mesocolon and the posterior 

 fold of the mesogastrium, which is seen in the Primates, and which 

 ends with the condition seen in Man, where a portion of the colon 

 (0. transversum) is enclosed in the hinder wall of the omental sac. 

 At the same time the anterior and posterior walls of the omental 

 sac grow out, so that the great omentum is formed, which consists 

 of four lamellae of the peritoneum. 



Pneumatic Organs of the Enteric Tube. 

 § 423. 



As the enteric tube is the road for everything that is taken in 

 from the exterior, the water which serves in respiration as well as 

 the substances that will be converted into food in the organism, so, 

 too, the enteric tract may take in air, which is collected in special 

 spaces which are differentiated from it, and which, therefore, repre- 

 sent portions of the primitive enteric tube. This ingestion of air 

 leads us to suppose that sometimes, at any rate, the animal came to 

 the surface of the water ; indicating a not unimportant stage 

 between those in which life was passed in water exclusively, and 

 those in which life was also possible out of this medium. 



The organs which are formed when air is taken in are known as 

 air-bladders. It is still uncertain what is the jDractical use of 

 these organs to the whole organism, but, as they are found in so 

 many forms, they must be regarded as important parts. The arrange- 

 ment of air-spaces in the body of aquatic animals must have some 

 influence on the specific gravity of their bodies, so that there is 

 good reason for supposing that these organs have a hydrostatic 

 function. 



A great change occurs in this character when the relations of 

 the circulatory system are changed. The organs have a respiratory 

 function, for the air in them exchanges its gases with those in the 

 blood which is brought to the wall of the organ, so that the blood 

 which passes away from it is richer in oxygen. The organ, there- 

 fore, becomes one of the respiratory organs, and is called the lung. 

 The first point in this metamorphosis is not the change which has 

 occurred in the blood-vascular system, but rather the commence- 

 ment of an exchange of the gases in the blood of the walls of the 



