244 CLASSIFICATION OP THE ANIMAL KINGDOM. 



variety and shape, is either simple, as belonging to the greater number of 

 mammals, complex, made up of several communicating compartments of a 

 similar organization, as in the kangaroo, porcupine, etc., or compound, as 

 that possessed by the ruminants, or animals which chew the cud, consisting 

 of four distinct cavities of a different organic construction. Some animals, 

 as the camel and dromedary, that have to subsist many days without a 

 fresh supply of water when journeying over the desert, in addition to these 

 cavities, have deep bags or reservoirs where water may be retained without 

 mixing with the contents of the stomach. The rest of the alimentary 

 canal in most quadrupeds is divided into small and large intestines, the 

 division of which is marked by ccecal and vermiform appendages, or by 

 an internal valve, as in the sloth and armadillo tribes. 



The remaining viscera, the chylopoyetic and the lymphatic system, as 

 well as the circulatory and respiratory organs, correspond in their general 

 development with that found in man, breathing being effected by the alter- 

 nate motions of the diaphragm and thoracic cavity, and the heart, which 

 offers the same arrangement throughout the entire class, consists of two 

 auricles and two ventricles, the arterialized blood being collected by the 

 pulmonary veins into the left auricle, and being expelled from its corres- 

 ponding ventricle through the aorta into the general structure, and thence 

 again collected by the venous system through the vense cavse into the 

 right or pulmonic auricle, and its corresponding ventricle, to be again passed 

 on as before. 



The skin of all mammals consists of three parts, — the cutis or true skin, 

 the epidermis or cuticle, and a thin layer of pigment between the two, and 

 the hair and other epidermic appendages with which it is clothed, although 

 apparently of very diversified structure, always consist of the same material 

 in a more or less modified state. The soft wool of the sheep, the sharp 

 quills of the porcupine, the tough armour of the armadillo, and the horn 

 on the snout of the rhinoceros are all made of the same material, and 

 are of the same construction as that of the human hair. "Widely dif- 

 ferent, however," says Professor Jones, "are the so-called horns of the deer 

 tribe, which in reality consist of bone, and, being deciduous, have to be 

 reproduced from year to year by a most peculiar and interesting process." 



In their generative apparatus mammals in their lowest orders approximate 

 to that of birds. The marsupials are ovo-viviparous, but in the highest 

 orders these organs present a perfect type of structure; and lastly, as regards 

 their nervous system, we find the fullest development of all its parts, 

 necessarily commensurate with their high state of organization. The brain 

 assumes its maximum development; the cerebrum and cerebellum, with 

 additional lateral lobes, are greatly increased; and the various senses of 

 smell, touch, sight, and hearing, according to the never-failing rule, have 



