xii PHYLUM CHORDATA 487 



unless the temperature is kept up to about 40 C. : this is 

 usually done by the heat of the body of the parent Birds, 

 one or both of which sit upon, or incubate^ the eggs until 

 the young are hatched ; but in the Australian Mound-makers 

 (Megapodius) 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 respect 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 epi- 

 dermal structures, 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. 



The Rabbit is a four-footed or quadrupedal animal, 

 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 backwards 

 to the nostrils and exposing the chisel-shaped incisor teeth. 

 Behind the incisor teeth the hairy integument projects on 

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



