LEPUS CUNICULUS 267 



the body wall termed the mammae or teats, and these structures with 

 their associated glands are much better developed in the female 

 than in the male. Well marked external ears or pinnae are also present, 

 and typically they are capable of being moved by a special set of 

 muscles. In the adult of higher mammals there is no cloaca, so that 

 the anus opens quite separately from the urogenital aperture in both 

 sexes. Both openings lie on a characteristic area known as the 

 perineum on the posterior ventral abdominal wall in the region of 

 the hinder end of the pelvic girdle, and with this certain perinea! 

 glands are associated. A notochord is- of course present in the 

 embryo, but with the complete ossification of the vertebrae in the 

 adult it disappears save for a small remnant in the middle of the 

 centra of certain species. The vertebral column consists of a mode- 

 rate number of vertebrae which are differentiated into five regions, 

 the cervical, the thoracic, the lumbar, the sacral and the caudal. The 

 first cervical vertebra, termed the atlas, is modified for articulation 

 with the skull, while the second, the axis or epistropheus, forms a very 

 striking joint with the atlas by means of which the rotation of the 

 skull can be brought about. The skull itself is a far more solid and 

 compact stiucture than we have previously encountered and has the 

 orbit sunk deeply in it, so that this cavity becomes largely or com- 

 pletely surrounded by bones on the internal side. The suspensory 

 apparatus of the lower jaw is completely incorporated with the 

 cranium, so that the jaw actually articulates with that structure, a 

 type of suspension that is termed autostylic. In the frog the internal 

 nares open into the front end of the buccal cavity, but in the mammal, 

 owing to the formation of a sort of false roof to the mouth, the 

 palate, partly consisting of bone, the hard palate, and partly of 

 mucous membrane, the soft palate, the internal nares come to open 

 far back at the begining of the pharynx. No columella auris is 

 present, but its place is taken by a very characteristic chain of 

 tiny bones, the auditory ossicles. During the life of the animal two 

 distinct sets of teeth make their appearance in the jaws ; the first 

 occurring in the young animal being termed the milk teeth, and these 

 are later replaced by the so-called permanent teeth of the adult. 

 1 his condition is termed diphyodont, to distinguish it from that in 

 the dogfish, where there are many successions of teeth. Not only 

 this, but we also find that the teeth are heterodont,that is to say, may 

 be differentiated into different types to subserve special functions, 

 and in a typical mammal we can recognise incisors, canines, premolars 

 and molars. A series of thin curved movable bony rods, the 

 ribs, are developed and they articulate on the dorsal side with the 

 vertebrae and on the ventral side with tjie sternum. These afford 

 protection to the heart and lungs, and as the hind limbs are more 



