316 BRANCH CHORDATA 



The mouth is generally characterized by thick and fleshy lips, 

 which serve as organs of prehension to grasp the food, as does 

 also the tongue, the latter being also the seat of the sense of 

 taste. 



Owing also to the varying feeding habits, the different organs 

 of digestion vary in size; for example, the intestine, which is 

 longer in mammals than in any other chordate, is, in the rumi- 

 nants, ten times the length of the animal, while in the carnivora 

 it is only three or four times as long. At the junction of the 

 small with the large intestine there is a blind tube or cecum, 

 which is especially developed in most of the vegetable feeders. 



The Circulatory System. — The heart is composed of four 

 parts: two ventricles and two auricles. The circulation is 

 double, closed, and complete. The left aortic arch is present 

 instead of the right aortic arch, as in birds. "There are two 

 features in the venous .system which distinguish all mammals 

 (except Echidna) from the lower chordates. The hepatic portal 

 system is limited to a \'ein which conveys to the liver blood 

 derived from the alimentary tract; in no mammal (except in 

 Echidna) is there any representative of the anterior abdominal 

 vein of the lower vertebrates. . . In no mammal is there any 

 trace of a renal portal sj'stem. The kidneys derive their blood 

 from the renal arteries only."' 



The red corpuscles of the blood of mammals differ from those 

 of other chordates in being much smaller, non-nucleateil, cir- 

 cular, and biconca\-e, except in the camel, in which they are 

 elliptic. 



The Respiratory System. — No mammals have external gills, 

 but all breathe by means of lungs throughout life. Respiration 

 is aide<l by a diaphragm or muscular partition, which com- 

 pletely divides the l)0(ly cavity, separating the heart and lungs 

 from the abdominal viscera. When this muscular diaphragm 

 contracts its upper surface becomes more conca^(', increasing 

 the lung cavity and allowing the lungs to (■x]:)and under the pres- 

 sure of the air. When the muscles relax tlu' upper surface again 

 becomes convex and the lung ca\it>- is reduced, thus forcing out 

 the air of the lungs. 



The soft palate and the epiglottis are structures peculiar to 

 ' Bcddard, p. SS. 



