VERTEBRATE ANIMALS. 



CHAPTER LIII. 



General Characters and Divisions of the 

 Vertebrata. 



The five sub-kingdoms which we have previously considered — 

 viz., the Protozoa, Cxlmteraia, Annuloida, Annulosa, and Mol- 

 /uscO'-^'weTe grouped together by the French naturalist Lamarck 

 to form one great division, which he termed Invertebraia, the 

 remaining members of the animal kingdom constituting the 

 division Vertebrata. The division Vertebrata, though includ- 

 ing 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 hamal 

 regions of the body are always completely shut off from one an- 

 other 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- 

 braia; but it is necessary to define these broad differences 

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

 importance. 



One of the most obvious, as it is one of the most funda- 

 mental, of the distinctive characters of Vertebrates, is to be 

 found in the shutting off of the main masses of the nervous 



