xiii PHYLUM CHORDATA 663 



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

 permanent teeth. In ^bhe insectivorous Chiroptera (Bats) the 

 molars are provided with pointed cusps, while in the frugivorous 

 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 Hapalidse, 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 continu- 

 ous 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-buds. 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, also with taste-buds, 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 plate the epiglottis of which rudiments only are 

 found in certain 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 Babbit (p. 448). 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 complication 

 of this organ reaches its extreme limit in the ruminant Ungulata 

 and in the Cetacea. In a typical Ruminant (Fig.- 1210, E, Fig. 

 1211), such as a sheep or an ox, the stomach is divided into four 

 chambers the rumen or paunch, the reticulum, the psalterium, and 

 the abomasum, or rennet stomach. The first of these (Fig. 1211, 6) is 

 much larger than the rest ; its mucous membrane is beset with 

 numerous short villi. The reticulum (c), which is much smaller 

 than the rumen, has its mucous membrane raised up into a number 

 of anastomosing ridges, giving its wall the appearance of a honey- 

 comb with shallow cells. From the aperture by which the reticulum 

 communicates with the rumen to that with which it communicates 

 with the psalterium, runs a groove bounded by a pair of muscular 



