CHAP. X MAMMALIA 467 



temperature of the blood, which remains 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 — thorax and abdomen — 

 by a transverse partition, the diaphragm ; the presence of two 

 ventricles as well as of two auricles in the heart, and of a 

 single systematic left aortic arch ; the higher differentation 

 of the brain, and also of the skeleton ; and the mode of 

 articulation of the lower jaw. The teeth, again, of the 

 rabbit, like those of the large majority of Mammals, are 

 differentiated into front-teeth for biting or seizing the food 

 and into cheek-teeth or grinders, and their succession is 

 limited to two 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, and 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 in which the young 

 are born ; there are a number of varieties, the habits and 

 general appearance of which have been modified by domesti- 

 cation (compare p. 227). 



H H 2 



