CHAP. XI .MAMMALIA 483 



almost uniformly within a few degrees of 100 Fahr., 

 and does not vary to any appreciable extent with the 

 temperature of the air ; the absence of nuclei in the red 

 corpuscles of the blood ; the presence of mammary glands 

 beneath the skin in the female which secrete milk for 

 nourishing the young ; the subdivision of the body-cavity 

 into two portions tlwrax and abdomen by a transverse 

 partition, the diaphragm ; the presence of two ventricles 

 as weU as of two auricles in the heart, and of a single 

 systemic aortic arch (that of the left side) ; the higher 

 differentiation of the brain, and also of the skeleton ; 

 and the mode of articulation of the lower jaw. More- 

 over, in the large majority of Mammals, the teeth are 

 differentiated into front-teeth for biting or seizing the 

 food and cheek-teeth or grinders, and their succession 

 is limited to two functional sets ; an external ear or 

 pinna is present ; there is no cloaca, the anus and 

 urinogenital apertures opening separately on the 

 exterior, while the ureters open directly into the 

 bladder ; the ova are minute ; the young undergo their 

 early development in the oviduct, where they are 

 nourished by diffusion from the blood-system of the 

 parent by means of an organ known as. the placenta, 

 and after birth they are suckled by the mother. 



Bearing in mind these essential characters of the 

 higher Mammalia as compared with the Vertebrates 

 previously studied, we can now proceed to examine the 

 structure of the rabbit in greater detail. 



External Characters. The rabbit (Lepus cuniculus] 

 is a very abundant and widely distributed animal which 

 in the wild state makes burrows in the earth, and the 

 practically hairless young are born in these or in special 

 nests. There are a number of varieties, the habits and 

 general appearance of which have been modified by 

 domestication (compare p. 227). 



