ZOOLOGICAL POSITION 9 



malia (Order Marsupialia of the Infraclass Aletatheria) of Australia 

 and South America, the latter being viviparous but, with one 

 exception, lacking a placenta. The members of both these groups 

 (which are associated in the subclass Theria) differ from those of 

 the order Monotremata (subclass Prototheria) of Australia, which 

 are not viviparous but oviparous, or egg-laying, like the majority 

 of reptiles and other lower animals. These three divisions of 

 mammals, however, are united by the common features of the 

 class Mammalia. For example, they all are warm-blooded, are 

 provided with a complete double circulation and a hairy investment 

 for the surface of the body, and nourish the young for a time after 

 birth by the secretion of cutaneous glands modified to produce 

 milk, the mammary glands. 



Many of the more general features of the rabbit are not charac- 

 teristic of any one group but are shared with other terrestrial 

 vertebrates, including mammals, reptiles, birds, and, in part, 

 amphibians. Such features are the development of the lungs and 

 associated respiratory tracts, both the true respiratory tracts and 

 the accessory respiratory passages traversing the skull. Further, 

 the loss of the branchial- or fish-type of respiration and the new 

 disposition of the branchial structures; the development of a tri- 

 segmented type of limb with a full complement of muscles, and 

 originally a pentadactyl, plantigrade foot, for support of the body 

 and for locomotion; the regional differentiation of the vertebral 

 column, especially the mobility of the neck, the free occipital 

 articulation, and the definition of the sacrum, the latter associated 

 with the elaboration of the pelvic girdle, are all features of general 

 significance in the terrestrial vertebrates. 



The rabbit is like all Vertebrata or Craniata in the possession 

 of an axial skeleton formed by the segmented vertebral column and 

 of an organized head region with differentiated brain, special sense 

 organs, and enclosing primary skull. Also, the vertebrates exhibit 

 a basic transverse segmentation (metamerism) of a considerable 

 part of the body. 



Finally, the rabbit agrees with other members of the phylum 

 Chordata in the ground plan underlying the most general features 

 of its organs and the position, arrangement, and plan of develop- 

 ment of its organ-systems. Particularly it possesses a dorsal, 



