VERTEBRATE ANIMALS. 



CHAPTER L. 



GENERAL CHARACTERS AND DIVISIONS OF THE 

 VERTEBRATA. 



THE five sub-kingdoms which \ve have previously considered 

 viz., the Protozoa, Ccelenterata, JEchinodermata, Annulosa, and 

 Mollusca were grouped together by the French naturalist La- 

 marck to form one great division, which he termed Invertebrata, 

 the remaining members of the animal kingdom constituting the 

 division Vertebrate. The division Vertebrata, though including 

 only a single sub-kingdom, is so compact and well marked a 

 division, and its distinctive characters are so numerous and so 

 important, that this mode of looking at the animal kingdom is, 

 at any rate, a very convenient one. 



The sub-kingdom Vertebrata may be shortly defined as com- 

 prising animals in which the body is composed of a number of 

 definite segments arranged along a longitudinal axis ; the nervous 

 system is in its main masses dorsal, and the neural and hcemal 

 regions of the body are always completely shut off from one 

 another by a partition ; the limbs are never more than four in 

 number, and are always turned away from the neural aspect of 

 the body ; mostly there is the bony axis known as the " spine" or 

 " vertebral column" and in all the structure known as the " noto- 

 chord" is present in the embryo, at any rate. These charac- 

 ters distinguish the Vertebrata, as a whole, from the Inverte- 

 brata ; but it is necessary to define these broad differences 

 more minutely, and to consider others which are of little less 

 importance. 



