xii PHYLUM CHORDATA 491 



bate, the eggs until the young are hatched, but in the Aus- 

 tralian mound-makers (Mcgapodius) the eggs are buried in 

 heaps of decaying vegetable matter, the decomposition of 

 which generates the necessary heat. 



CLASS VI. MAMMALIA 



The class Mammalia, the highest of the Vertebrata, com- 

 prises the Monotremes and Marsupials, the hoofed and 

 clawed quadrupeds, the whales and porpoises and sea-cows, 

 the rodents, bats, and insectivores, the lemurs and apes, and 

 the human species. All mammals, though many are aquatic, 

 are air-breathers throughout life, lungs being, as in reptiles 

 and birds, the sole organs of respiration. The blood of 

 mammals has a high temperature, resembling in that re- 

 spect the blood of birds, and differing from that of reptiles 

 and amphibia. The scales of reptiles and the feathers of 

 birds are replaced in mammals by peculiar epidermal struc- 

 tures, the hairs, usually developed in such quantities as to 

 form a thick soft covering or fur. 



The rabbit (Lepus cuniculus} will serve as a convenient 

 example of the class. 1 



The rabbit (Fig. 297) is a four-footed or quadrupedal ani- 

 mal, having the whole surface of its body covered with soft 

 fur. The head bears below its anterior extremity the mouth 

 in the form of a transverse slit bounded by soft lips. The 

 upper lip is divided by a longitudinal cleft, running back- 

 wards to the nostrils and exposing the chisel-shaped incisor 

 teeth. Behind the incisor teeth the hairy integument pro- 

 jects on each side into the cavity of the mouth. At the end 

 of the snout, above the mouth, are the nostrils in the shape 



1 The following account will apply in all but very slight details to our 

 cotton-tail rabbit or to our American hare. 



