it>6 ELEMENTARY BIOLOGY. [CHAP. 



the peritoneum, its anterior face by a membrane of similar 

 character, the pericardial membrane, which lines the peri- 

 cardium and is reflected on to the heart, in the same way as 

 the peritoneum lines the peritoneal cavity and is reflected on 

 to the intestine. 



A layer of the muscular fibres which enter into the walls 

 of the abdomen is continued inwards at the anterior boundary 

 of the pleuroperitoneal cavity and is attached to the sides of 

 the oesophagus and to those of the pericardium, thus consti- 

 tuting a rudimentary diaphragm ; which, it will be observed, 

 is situated in front of the lungs, and not behind them, as in 

 the higher animals. 



Thus, in the trunk, on the ventral side of the vertebral 

 column, the body presents two cavities, one large posterior 

 pleuroperitoneal cavity, and one small, anterior to the fore- 

 going, the pericardial cavity, and neither of these communi- 

 cates directly with the exterior, though in the female there 

 is an indirect communication by the oviducts. 



'On the ventral side of the head, the very wide mouth 

 opens into a spacious buccal cavity, the roof of which is hard 

 and firm, while the floor is soft and flexible, except so far as 

 the middle of it is occupied by a broad, flat, for the most 

 part gristly plate, the body of the hyoid bone. Within the 

 lips the upper jaw is beset with numerous sharp small teeth, 

 and two clusters of similar teeth are to be seen in the fore 

 part of the roof of the mouth; the latter, being attached to 

 the bones termed the vomers, are the vamerine teeth, while 

 the former, attached to the premaxillce and maxillce, are 

 maxillary teeth. The lower jaw or mandible bears no teeth. 



At the sides of the clusters of vomerine teeth are the 

 apertures termed posterior nares, by which the nasal chambers 

 communicate with the mouth. At the sides of the back part 

 of the throat two wide passages, the Eustachian recesses, lead 



