CHAPTER II 



ESSENTIAL CHARACTERS OF VERTEBRATE ANIMALS. 

 STRUCTURE AND CLASSIFICATION OF MAMMALS 



Vertebrates and Invertebrates. — It has already been 

 pointed out (p. 8) that Aristotle in his natural history drew 

 what is perhaps the most important boundary line in zoology, 

 i.e. the one dividing Backboned or Vertebrate Animals from 

 Backboneless or Invertebrate ones. 



The description which has just been given of the structure 

 and functions of the human body will have given a rough idea of 

 the leading characters of the Vertebrates, and it is now necessary 

 to indicate those features which characterize the group as a whole. 

 The following classes are here included: — 



I. Mammals, or ordinary warm-blooded quadrupeds. — Exs.\ Lion, 

 Horse, Rabbit, Bats, Monkeys, Human Beings. 

 II. Birds. — Exs.-. Fowl, Parrot, Ostrich. 



III. Reptiles. — Exs.-. Crocodiles, Lizards, Turtles, Snakes. 



IV. Amphibians. — Exs.: Frogs, Toads, Newts, Salamanders. 

 V. Fishes. — Exs.: Herring, Salmon, Shark, Lamprey. 



VI. Protochordates, including various low forms, none of which 

 are familiar to most persons. 



Essential Characters of Vertebrates. — The body is nearly 

 always bilaterally symmetrical, though cases are known where this 

 is only true for the very young animal while the adult is more or 

 less asymmetrical. A good case in point is that of Flat-fishes, 

 such as turbot, sole, and plaice, in which the young fry closely 

 resemble those of other fish, but as development advances one 

 eye is shifted round to the other side of the head. In a turbot 

 (fig. 34), for example, the dark and light sides are not dorsal and 

 ventral, as might at first sight be supposed, but left and right, both 

 the eyes being situated on the former side, which is kept turned 

 upwards during life. 



