i6 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 siib-classes of mammals are united, however, 

 by the corhmon 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 lu'ngs 

 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- 



