THK RABBIT. 133 



the discoidal form of the placenta* They constitute an 

 order of the animal kingdom, that of Roclentia. 



Similarly we have the order Carnivora, typified by the 

 cat and dog ; the Insectivora, by hedgehog and mole ; 

 Ungulata, by ox and horse ; Chiroptera, the bats ; Cetacea, 

 whales and dolphins ; Primates, by man and monkeys ; 

 Sirenia, by the manatee ; Edentata, by the armadillo ; 

 Marsupialia, by the kangaroo ; and Monotremata, by the 

 ornithorhynchus. Widely as the species in these orders 

 differ among themselves, they agree on certain points 

 which mark them off from all other animals. Therefore 

 these orders are united as a Class, to which the name 

 Mammalia is given. It is as a type of the Mammalia that 

 we have been studying the rabbit in this book. 



10. Mammalia and other Vertebrata. The rabbit 

 agrees with all mammals, and differs from all other verte- 

 brata (i.e., birds, reptiles, amphibia, and fishes), in having 



(a) Hair; 



(6) Mammary glands ; 



(c) A diaphragm ; 



(d) Only one aortic arch, and that on the left side of 



the body ; 



(e) Its young born alive. (But the Monotremata lay 



eggs, and certain fish and reptiles bear living 

 young) ; 



Epiphyses to its vertebral centra ; 

 Seven cervical vertebrae (but some Edentata have 

 more) ; 



!h) The cerebral hemispheres covering the mid- brain ; 

 i) The optic lobes transversely divided (hence corpora 



quadrigemina) ; 



(/) A corpus callosum, uniting the two cerebral hemi- 

 spheres ; 



() A spirally coiled cochlea to the internal ear ; 

 (I) Small, non-nucleated red blood-corpuscles. 



(In respect to/ and k also, the Monotremata are scarcely 

 mammalian.) 



* This is explained in chapter xx. 



