212 INTRODUCTION TO ZOOLOGY, 



though less striking characteristics. Vertebrate Animals 

 possess red blood, a muscular heart, distinct senses, a mouth 

 furnished with two jaws moving vertically, and limbs which, 

 however modified in form, never. exceed four in number. 



The skeleton of Vertebrate Animals presents considerable 

 variety, not only in its form, but in the material of which it is 

 composed. Bone consists of animal matter, chiefly gelatinous, 

 hardened by a general diffusion of earthy particles. The 

 proportion of the animal and of the earthy parts, or, in 

 other words, the proportion of the organic and inorganic 

 matter, varies in different classes. " Fishes have the least, 

 birds the largest proportion of earthy matter; 77 " the mam- 

 malia, especially the active predatory species, have more 

 earth, or harder bones, than reptiles." In each class there 

 are differences in the density of bone among its several 

 members. For example, in the freshwater fishes the bones 

 are lighter, and retain more animal matter, than in those 

 which swim in the denser sea; and in the dolphin, a warm- 

 blooded marine animal, they differ little in this respect from 

 those of the sea-fish.* 



The Vertebrate Animals are distributed into four classes, 

 namely: 



I. FISHES, 

 II. KEPTILES (Tortoises, Lizards, Serpents, and Frogs), 



III. BIRDS. 



IV. MAMMALIA. (Man, Bats, Whales, and Quadrupeds), 



Two of these, Fishes and Reptiles, are, with few excep- 

 tions, cold-blooded; and the remaining two, Birds and Mam- 

 malia, are warm-blooded. 



* Professor Owen's Lectures on the Vertebrate Animals, p. 25. 



