538 ZOOLOGY SECT. 



diphyodont. All the teeth are rooted. There are never fewer 

 than two incisors on either side of the lower jaw. The canines 

 are not of large size. The crowns of the molars are beset with 

 pointed tentacles. 



In the Chiroptera the dentition is complete, and the teeth are 

 all rooted. There is a milk series which differs entirely from the 

 permanent teeth. Jn the insectivorous Chiroptera (Bats) the 

 molars are provided with pointed cusps, while in the frugi- 

 vorous forms (Flying Foxes) they are longitudinally grooved or 

 excavated. 



In the Primates the teeth are heterodont and diphyodont, and 

 always form roots. There are almost invariably two incisors 

 on each side in each jaw, and in all but the Hapalidaa three 

 molars. The dental formulae of the various families have been 

 given in the synopsis of the classification. The dentition of Man 

 differs from that of the rest of the order in the teeth forming 

 a continuous series not interrupted by a diastema, and in the 

 comparatively small size of the canines. 



The mouth in Mammals is bounded by fleshy lips. On the 

 floor of the mouth is situated the tongue, which is usually well 

 developed, but varies in size and shape in different orders. Its 

 surface is covered with papillae of different forms, in association 

 with certain of which are the special end-organs of the nerves 

 of taste the taste bulbs. The roof of the mouth is formed in 

 front by the hard palate, consisting of the horizontal palatine 

 plates of the maxillary and palatine bones covered with mucous 

 membrane. Behind the hard palate projects backwards the soft 

 muscular fold of the soft palate, which divides the cavity of the 

 pharynx into two chambers, an upper and a lower. In front of 

 the opening, leading from the lower division of the pharynx 

 into the larynx, is a cartilaginous lid the epiglottis of which 

 rudiments only are found in lower Vertebrates. 



The oesophagus is always a simple straight tube. The stomach 

 varies greatly in different orders, being sometimes simple, as in 

 the majority of Mammals, sometimes divided into chambers, as in 

 the Cetacea and the Ruminants. 



In the majority of Mammals the stomach is a simple sac, as in 

 the Rabbit (p. 431). But in certain groups it is complicated by 

 the development of internal folds, and may be divided by con- 

 strictions into a number of different chambers. The complica- 

 tion of this organ reaches its extreme limit in the ruminant 

 Ungulata, and in the Cetacea In a typical Ruminant (Fig. 

 1129, E, Fig. 1130), such as a Sheep or an Ox, the stomach is 

 divided into four chambers the rumen or paunch, the reticuhim, 

 the psalterium, and the abomasum or rennet stomach. The first 

 of these (6) is much larger than the rest ; its mucous membrane 

 is beset with numerous short villi. The reticulum (c), which 



