VERTEBRATE ANIMALS. 



CHAPTER XXIII. 



GENEEAL CHAEACTEES OF THE YEETEBEATA. 



THE five sub-kingdoms which we have previously consid- 

 ered, namely, the Protozoa, Codenterata, Annuloida, Annu- 

 losa, and Mottusca, were grouped together by Lamarck into 

 one great division, which he termed the Tnvertebrata. The 

 remaining sub-kingdom, that of the Vertebrata, is so well 

 marked and compact a division, and its distinctive characters 

 are so numerous and so important, that this mode of viewing 

 the animal kingdom is, at any rate, a very convenient one. 



The sub-kingdom Vertebrata includes the five great classes 

 of the Fishes (Pisces), Amphibians, Reptiles, Birds (Aves), 

 and Mammals ; and the name of the sub-kingdom is derived 

 from the very general, though not universal, presence of the 

 bony axis known as the "vertebral column" or backbone. 

 One of the most fundamental of the distinctive characters of 

 Vertebrate animals is to be found in the fact that the main 

 masses of the nervous system (that is to say, the brain and 

 spinal cord) are completely shut off from the general cavity 

 of the body. In all Invertebrate animals (Fig. 93, A), the 

 body may be regarded as a single tube, enclosing all the vis- 

 cera ; and, consequently, when a distinct nervous system and 

 alimentary canal are present, these are in no way shut off 

 from one another. The transverse section, however, of any 

 Vertebrate animal (Fig. 93, B) shows two tubes, one of which 

 contains the great nervous axis (') or brain and spinal cord, 

 while the other contains the alimentary canal, the chief circu- 

 latory organs, and certain portions of the nervous system (ri), 



