364 THE ANIMAL KINGDOM. 



period without cutting the gum (Seal), or aborted altogether (e. g. Rat and 

 some other Rodents). The teeth are composed of dentine capped above 

 the gum by enamel, partially or completely, and in the root or fang coated 

 with cement. The exposed portion, or crown, varies much in character, 

 and when folded the interval between the folds is often filled with cement. 

 Enamel is entirely absent in Edentata and the Dugong among Sirenia. 

 Dentine with vascular channels (vaso dentine) makes up the whole tooth in 

 most Edentata ; and is found in the Manatee (Sirenia} and Tapir (Ungulata). 

 The pulp sometimes grows throughout life, e. g. incisors of Rodents, teeth 

 of Edentata ; or up to a certain age, e.g. molars of Horse and some Rodents; 

 and in Cetacea it either atrophies or is calcified. 



The mouth has soft fleshy lips, except in Cetacea and Prototheria. 

 The tongue is well developed, is rich in glands, in tactile papillae, and pos- 

 sesses special gustatory bulbs lodged at the sides of certain papillae or folds. 

 There is a soft palate or velum pendulum palati. Three pairs of glands 

 parotid, submaxillary, and sub-lingual open into the oral cavity. They 

 are wanting in Cetacea, and much reduced in Seals. The labial glands are 

 small in size. The stomach is always sharply marked off from the oeso- 

 phagus, and is generally of complicated structure in vegetable feeders. The 

 small intestine is of considerable length, and is as a rule separated from 

 the large intestine by an ileocaecal valve, rarely wanting as in some Carni- 

 vora. The caecum is sometimes absent, e. g. some Carnivora, and is of 

 great size in most herbivorous Mammals. It is small, and double only in 

 the two-toed Sloth among Edentata. The large intestine (colon) is of 

 great relative length : the rectum is very short, and the anus is posterior, 

 i. e. dorsal to the urogenital aperture. The whole tract is remarkably rich 

 in glands, and the small intestine possesses innumerable minute villi. The 

 liver is often complex. The ligamentum teres, or remnant of the um- 

 bilical vein, divides it into a right and left lobe. The latter is sometimes 

 subdivided into a central and lateral lobe. The right lobe is usually sub- 

 divided in a similar manner, and often has a caudate lobe cut off from the 

 lateral lobe and fitting on the right kidney and a Spigelian lobe projecting 

 behind the entrance of the portal vein. A gall-bladder is rarely absent 

 (Horse, Cetacea, some Rodents) and is always a diverticulum of the hepatic 

 duct. The pancreas is usually compact, but is a scattered gland in some 

 Rodents, e. g. Rabbit, Rat, and its duct as a rule unites with the hepatic 

 duct to form a ductus communis choledochus. 



The red-blood corpuscles are biconcave discs, circular in outline, ellip- 

 tical in the Camel and Llama. Their size varies much, and in one and the 

 same group are largest in the larger species. They are largest in the 

 Elephant (^TTT")' some Cetacea and Edentata, smallest in the Tragulidae 

 among Ungulata ( T ^|^ T "). The heart has two auricles and two ventricles. 

 The right auricle has no sinus venosus, and the auricular septum is marked 



