196 INTBODTJCTION TO ZOOLOGY. 



though less striking characteristics. Vertebrate Animals pos- 

 sess i-ed blood, a muscular heart, distinct senses, a mouth fur- 

 nished with two jaws moving vertically, and hmbs which, 

 however modified in form, never exceed fom' 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;" "the mam- 

 malia, espeeialy 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 doli:)hin, 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. Eeptiles (^Tortoises, Lizards, Serpents, Frogs.) 



III. Birds. 



IV. MAMMALiAf {Man, Bats, Whales, Quadrupeds.) 



Two of these, Fishes and Reptiles, are, with few exceptions, 

 cold-blooded ; and the remaining two, Birds and MamraaUa, 

 are warm-blooded. 



* Professor Owen's Lectures on the Vei-tebrate Animals, p. 25. 



t Most of the animals belonging to this Class, being four-footed, it is not 

 unusual in systematic works of a popular character, to speak of them all 

 (including the bats and whales) as " Quadnipedx," instead of using the more 

 scientific term " Mammalia.'' 



