VEETEBEATE ANIMALS. 



CHAPTER XXII. 



GENERAL CHARAC'TEES OF THE VEETEBRATA. 



The five sub-kingdoms which we have previously considered — viz. 

 the Protozou., Calenterata, Echinodermata, Anmdosa, and Mollusca — 

 were gi-ouped together b>- Lamarck into one great division which 

 he termed the Inverteh-ata. The remaining sub-kingdom, that of 

 the Vertehrata, 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 conve- 

 nient one. 



The sub-kingdom Vertebixcta includes the five great classes of the 

 Fishes (Pisces), Amphibians, Eeptiles, 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 " verte- 

 bral 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. 1.57, A) the body maj' 

 be regarded as a single tube, enclosing all the viscera ; and conse- 

 quently, when a distinct nervous system and alimentary canal are 

 present, these are in no way shut off from one another. The trans- 

 verse section, however, of any Vertebrate animal (fig. 157, B) shows 

 tiro tubes, one of which eontains the great nervous axis (•//), or brain 

 and spinal cord, whilst the other contains the alimentary canal, the 

 chief circulatory organs, and certain portions of the nervous system 

 (n) which are known to anatomists as the "sympathetic" system. 



