xii PHYLUM CHORDATA 517 



distinct socket or alveolus in the substance of the bone of 

 the jaw ; and nearly always the teeth in different parts of 

 the jaw are clearly distinguishable by differences of shape 

 into incisors, canines, and grinding teeth, i.e., are heterodont : 

 in some instances the teeth are all alike (homodonf). A 

 cloaca is absent except in the Prototheria. 



A movable plate of cartilage the epiglottis overhangs 

 the glottis or passage leading from the pharynx into the 

 cavity of the larynx. 



A partition of muscular fibres, the diaphragm, usually with 

 a tendinous centre, divides the cavity of the body into two 

 parts an anterior, the thorax, containing the heart and 

 lungs, and a posterior, the abdomen, containing the greater 

 part of the alimentary canal with its associated glands, the 

 liver and pancreas, and the renal and reproductive organs. 



The lungs are freely suspended within the cavity of the 

 thorax. The heart is completely divided into two halves, a 

 right and a left, between which there is no aperture of 

 communication. Each half consists of an auricle and a 

 ventricle, opening into one another by a wide opening, 

 guarded by a valve composed of three membranous cusps 

 on the right side, two on the left. The right ventricle gives 

 off the pulmonary artery ; the left gives off the single aortic 

 arch, which passes over to the left side, turning round the 

 left bronchus in order to run backwards as the dorsal aorta. 

 The red blood-corpuscles are non-nucleated and usually 

 circular. 



The two cerebral hemispheres, in all but the Prototheria 

 and Marsupials, are connected together by a band of 

 transverse fibres, the corpus callosum, not represented in 

 the lower Vertebrates. The dorsal part of the mid-brain is 

 divided into four optic lobes, the corpora quadrigimina. On 

 the ventral side of the hind-brain is a transverse band of 



