MAMMALIA 63 



breathing air by means of lungs and quite devoid of gills and gill- 

 slits. For it is a familiar commonplace that the frog starts life 

 as a "pollywog" or tadpole, which to all intents and purposes is 

 a fish, and would be classed as such if it developed no further. 

 In due course, however, the tadpole grows lungs, while at the 

 same time its gills shrivel up and its gill-slits close, changes which 

 are accompanied by growth of limbs and loss of tail, the ultimate 

 product of these revolutionary proceedings being a frog. 



All Vertebrates possess a blood system, consisting of a set of 

 tubes in which blood is circulated, usually by the pumping action 

 of a heart placed near the under or ventral side of the animal. 

 In the higher groups the heart is much more complicated in 

 structure than in the lower ones. 



The chief part of the central nervous system in a Vertebrate is 

 a thick-walled tube, the front part of which is larger than the rest 

 and is termed the brain, while the rest is the spinal cord. This 

 tube has a dorsal position. The sensitive parts of the characteristic 

 eyes are developed as outgrowths of the brain. 



A brief survey of the Vertebrate groups, with their subdivisions, 

 may now be appropriately entered into, beginning with the Mam- 

 mals and ending with the Protochordates. 



MAMMALS 



Mammals are justly regarded as being the highest group of 

 Backboned animals, and a number of characters mark them off 

 pretty sharply from the remaining groups. 

 These characters, as well as others of 

 less importance, have been for the most 

 part dealt with in the sketch already 

 given of Man, the highest member of 

 the group. 



Probably the two most striking fea- 

 tures which characterize a Mammal are pig. 36._sect!on of skin, showing Hair, 

 the possession of (i) hair and (2) milk- ^^^^::^^f^^^^^, 



P^lands, both of which belongr to the skin, a, sebaceous glands,- e, muscle attached to 

 o ' c hair follicle. 



(i) Hair (fig. 36).^ — In all cases we 

 find that the epidermis gives rise to a more or less pronounced 

 hairy covering, each hair growing from a deep narrow pit in the 

 skin, called a hair-follicle. Opening into this are two little glands 



