xiii PHYLUM CHORDATA 477 



always the teeth ia different parts of the jaw are clearly dis- 

 tinguishable by differences of shape into incisors, canines, and 

 grinding teeth, i.e., are heierodont ; in some instances the teeth are 

 all alike (homodont). A cloaca is absent as a rule except in the 

 Prototheria. 



A movable plate of cartilage — the epiglottis — represented only 

 by a rudiment in some Amphibia and Sauropsida — overhangs the 

 slit, commonly termed glottis, leading from the pharynx into the 

 cavity of the larynx. 



A partition of muscular fibres usually with a tendinous centre, 

 the diaphragm, 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 aperture, 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 : it therefore represents the lefb aortic arch of Reptiles. 

 The red blood -corpuscles are non-nucleated and usually circular. 



The two cerebral hemispheres, in all but the Monotremes and 

 Marsupials, are connected together by a band of tranverse 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 quadrigemina. On the ventral side of the hind- 

 brain is a transverse band of fibres — the pons Varolii — by which 

 the lateral portions of the cerebellum are connected together. 



The ureters, except in the Prototheria, open into the bladder. 



With the exception of the Monotremes, Mammals are all 

 viviparous. The foetus is nourished before birth from the blood- 

 system of the parent through a special development of the foetal 

 membranes and the lining membrane of the uterus, termed the 

 placenta. After birth the young Mammal is nourished for a longer 

 or shorter time by the milk, or secretion of the mammary glands of 

 the mother. 



Sub-class I.— Prototheria. 



Mammals in which the mammary glands are devoid of teats ; 

 the oviducts are distinct throughout, and there is a cloaca into 

 which the ureters and urinary bladder open separately. In the 



