408 



METAZOAN PHYLA 



The alimentary canal shows several distinguishing characteristics. 

 The mouth cavity is separated from the nasal chambers by a hard palate, 

 which is a shelf of bone covered by soft tissues (Fig. 219). This is 

 supplemented posteriorly by a fleshy soft palate. The passage from the 

 mouth into the pharynx is known as the fauces, on each side of which 

 lie the tonsils. The latter are masses of lymphoid tissue and their 

 significance is not definitely known. The opening from the pharynx 



into the windpipe is the glottis. It is 

 protected from the entrance of food during 

 swallowing by a fleshy and cartilaginous 

 cover called the epiglottis. At the junction 

 of the small and large intestines the latter 

 is prolonged into a blind sac called the 

 caecum, which is enlarged in herbivorous 

 mammals, where its purpose seems to be 

 to increase the capacity of the intestine. 

 -Po/p cavii-y The contracted tip of this caecum is known 



^Enamel 



penfine 



Cemen1-um 



Nerve 



Bloodvessel 



in certain mammals, including man, as the 

 vermiform, appendix. Excepting in the egg- 

 laying monotremes, there is no cloaca, the 

 anal opening being at the surface of the 

 body. 



The lungs are contained in coelomic 

 spaces called pleural cavities, which are 

 the lateral portions of the thoracic cavity, 

 the middle part of which is the pericardial 

 cavity. The thoracic cavity is separated 

 from the abdominal cavity by a thin, mus- 



FiG. 278. Canine tooth of a mam- c^i^^r diaphragm, which is convex anteri- 



mal, diagrammatic. " 



orly and concave posteriorly. The thoracic 

 cavity is expanded in breathing by raising the ribs and flattening the 

 diaphragm. Thus air is drawn into the lungs. By the relaxation of 

 the muscles of the ribs and diaphragm and the resulting contraction of 

 the thoracic cavity, air is forced out. At the upper end of the windpipe, 

 or trachea, lies a larynx, or voice box. 



Mammals are warm-blooded animals possessing a well-developed 

 heat-regulatory mechanism. The heart is four-chambered and the 

 systemic and pulmonary circulations are entirely separate (Fig. 279). 

 There is a single aortic arch which turns to the left. A hepatic-portal 

 system is present but there is no renal-portal system. 



The brain of mammals (Fig. 280) shows a high development of 

 the cerebrum and cerebellum, the latter always being convoluted but the 

 former showing convolutions only in the higher forms. The size of the 

 cerebellum of mammals is equal to that of birds, while the cerebrum far 



