XII PHYLUM CHORDATA 333 



limb, thigh, leg, and foot in the posterior ; and each hand 

 and each foot contains five slender digits, each provided 

 at its extremity with a curved and pointed horny claw. 

 Slight rhythmical movements of dilatation and contraction 

 of the anterior portion of the trunk are the movements of 

 respiration, by means of which air is alternately drawn into 

 and expelled from the lungs through the nostrils. 



In the rabbit the place of the scales of the lizard is taken 

 by the coating of hairs constituting the fur. The limbs 

 present the same main divisions as in the lizard, though the 

 proportions of the parts are very different, and the hind foot 

 has only four toes. Between the head and trunk the neck 

 region is more sharply marked off than in the lizard. Aris- 

 ing from the posterior part of the head, behind the eyes, 

 are a pair of very prominent auditory pinnce or auricles, at 

 the base of each of which is the corresponding ear-opening. 

 Movements of respiration resembling those of the lizard, 

 but much more marked, are to be detected in the living 

 animal. 



When the skeletons of these three animals are examined 

 and compared, it will be found that they are constructed on 

 the same general plan with differences in details. In the 

 dogfish it is mainly composed of cartilage ; in the others, 

 mainly of bone. In all there is a rod-like axis, the spinal 

 or vertebral column supporting the trunk and tail, but not 

 continued into the head, where its place is taken by the 

 skull. The spinal column consists of a row of similar 

 segments, the vertebra, which articulate with one another. 

 Each vertebra consists of a neutral solid portion, the cen- 

 trum or body ; an arch of bone or cartilage, the neural 

 arch, situated on the dorsal side of the centrum, and cer- 

 tain processes. The series of centra form together a strong 

 axial support for the entire body and tail ; the series of 



