VERTEBRATE ANIMALS. 
CHAPTER XXIII. 
GENERAL CHARACTERS OF THE VERTEBRATA. 
THE five sub-kingdoms which we have previously considered— 
viz., the Protozoa, Celenterata, Annuloida, Annulosa, and Mol- 
/usca—were grouped together by Lamarck into one great division 
which he termed the /zvertebraia. 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 (Pzsces), 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. 115, A) the body may be regarded 
as a single tube, enclosing all the viscera; 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. 115, B) shows 
two tubes, one of which contains the great nervous axis (z’) or 
brain and spinal cord, whilst the other contains the alimentary 
canal, the chief circulatory organs, and certain portions of the 
