16 ANATOMY OF THE RABBIT. 
backward, of the jaw-musculature. The articulation of the lower 
jaw exhibits an elongated articular process fitting into a corres- 
ponding longitudinal fossa on the skull, the jaw being able to move 
forward and backward in addition to vertically and from side to 
side. The teeth are further arcuate in shape, and are provided 
with open roots, so that their growth is not limited, as it is in the 
majority of mammals. “The rodents are in many particulars 
primitive types. For example, they tend to retain the five-toed 
(pentadactyl), plantigrade foot, characteristic of primitive mam- 
malia and, indeed, of terrestrial vertebrates, and exhibit also un- 
elaborated cerebral hemispheres in the brain. In other respects, 
however, as in the rodent characters above-mentioned and in the 
elaboration of the intestine, especially the caecum, they exhibit 
the characters of highly specialized herbivores. 
Like all higher or placental mammalia (Placentalia), the 
rabbit is viviparous, the young being retained through a period of 
gestation in the maternal uterus, to the wall of which they are 
attached by a vascular connection, the placenta. In this feature 
the placental mammalia differ from the marsupial mammalia 
(Marsupialia) of Australia and South America, the latter being 
viviparous, but, with one exception, without placenta; also from 
the egg-laying mammalia (Monotremata) of Australia, the 
latter being oviparous, like the majority of the lower, reptilian 
forms. These three sub-classes of mammals are united, however, 
by the common features of the class Mammalia. They are 
warm-blooded animals, provided with a complete double cir- 
culation, and with a hairy investment for the surface of the 
body. In all, the young are nourished for a time after birth 
through the secretion of modified cutaneous, milk-producing, 
or mammary glands. 
Many of the more general features of the rabbit are such as are 
not recognized by group designation, but yet are shared with other 
terrestrial vertebrates, including mammals, reptiles, birds, and, in 
part, amphibians. This refers to the development of the lungs 
and associated respiratory tracts, both the true réspiratory 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- 
