OF THE CLASS MAMMALIA. 233 



Finally, the organs of sense are always five in number, and 

 their arrangement is much as in man. 



385. The digestive apparatus presents hut few im- 

 portant differences in this great division of the animal king- 

 dom : the orifices of the digestive tube are always remote 

 from each other; the jaws move vertically or in the axis of 

 the body, and not laterally, as in the annulata ; the intestine 

 is secured in the abdomen by a mesentery ( 45) : and the 

 chyle, the product of digestion, is absorbed by peculiar ves- 

 sels, which convey it into the veins, and thus into the mass 

 of blood. 



386. The blood, much richer in globules and redder 

 than in other animals, reaches the heart by the veins. It 

 enters an auricle, then passes into a ventricle, and by its 

 means is wholly or in part transmitted to the organs of res- 

 piration, the lungs. In general, the blood returns to the 

 heart before its transmission to other parts of the body ; but 

 sometimes it proceeds directly to these, and its circulatory 

 movement is determined in some cases by an auricle and 

 ventricle only ; in others, by two auricles assisted by one 

 ventricle ; and in others, by a heart composed of two auricles 

 and two ventricles ( 107, 108, 109). The respiration in this 

 class of animals takes place in a cavity of the body internally, 

 but it is not always aerian,sis in man, being performed some- 

 times by lungs, sometimes by gills. 



Of the secreting organs found in man, there are two which 

 are always present in the class vertebrata : these are the liver 

 and kidneys. The presence of a pancreas and spleen is also 

 very uniform. 



387. Nature thus seems to have followed one plan in the 

 construction of the vertebrata, yet they differ much from each 

 other, and hence the necessity for subdividing this great 

 primary division into five classes : namely, mammals ; birds ; 

 reptiles ; batrachia ; fishes. 



OF THE CLASS MAMMALIA. 



388. This class is composed of man, and of all the ani- 

 mals which most nearly resemble him in the most important 

 points of their organization. By their proximity and utility 

 to man, their higher intelligence and organization, they natu- 

 rally place themselves at the head of all that lives. 



