CHAP. XI MAMMALIA 483 



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 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. Moreover, 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 un- 

 dergo 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 high 

 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 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). 



I I 2 



ligher 



