CLASS I.: MAMMALS (MAMMALIA). 



Waem-blooded animals, as a rule covered with hair, breathing 

 through lungs, and producing living young (with the exception of cloacal 

 animals), which for a time at least they nourish with milk. The limbs, 

 as a rule, are in the form of legs. 



1. Body Covering. — The body is, as a rule, provided with a coating 

 of hair. The air spaces enclosed between the hairs prevent excessive 

 diffusion of the body-heat into the external medium, air being a bad 

 conductor of heat. Animals, being very sensitive to an excessive loss of 

 heat, are in need of such a covering. Experiments have shown that 

 rabbits did not recover after their bodies had been cooled down to 

 18° C. (about 42° F.) and brought into a chamber of the same tempera- 

 ture. Hibernating animals, however — e.g., the bat — can endure a much 

 greater degree of cold. 



2. Skeleton (compare pp. 2, 9). — The most important bones may 

 be seen in the accompanying illustration, to which may be added the 

 following remarks in regard to particular parts of the skeleton :* 



(a) Skull. — The bones of the skull are not, for the most part, com- 

 pletely united, but are placed edge to edge, with projections of one 

 bone fitting into notches of the other ; in this way sutures are formed. 

 The upper jaws (maxillary bones), however, are firmly united with the 

 bones of the skull, while the lower jaw (mandible) is connected with the 

 skull (temporal boneB) by means of an articulation. 



(b) Vertebral Column and Thorax. — The vertebral column may be 

 subdivided into the following regions : Cervical, dorsal, lumbar, sacral, 

 and coccygeal. Only the dorsal vertebras carry ribs. 



(c) The Shoulder Girdle, or Scapular Arch, consists of the shoulder- 

 Made (scapida], which in animals with specially active fore-limbs (e.g., bats) 

 is further supported by a collar-bone, or clavicle. 



(d) Pelvis. — The bones of this skeletal part unite with each other on 

 the ventral side, forming a closed girdle. 



* The statements which follow are limited to an explanation of the special features 

 mentioned in the description of mammals, and of the deviations of structure exhibited by other 

 classes of the vertebrata, 



