INTRODUCTION. 



VERTEBRATA. 



As all the domesticated animals belong to the sub-kingdom 

 Yertebrata, it is of importance to gain a clear idea of the lead- 

 ing characteristics of a vertebrate animal ; 

 and, except in one or two of the lowest 

 orders, these are well marked. 



The animal kingdom is divided into the 

 sub-kingdoms Invertebrata and Verte- 

 BRATA. As the names would imply, the latter 

 is distinguished from the former by its 

 members possessing a vertebral column, 

 or backbone, which forms, as it were, 

 the axis of the bony framework, supports 

 the head, and is placed dorsally, or in the 

 region of the back, extending from one end . 

 of the body to the other. It is pierced 

 throughout the greater part of its extent 

 by a canal called the neural canal, which 

 is continuous with a cavity in the head 

 called the craniuw,. These cavities are 

 occupied by centres, from which 

 the large series of nerves termed the 

 cerebrospinal system. Underneath the 

 backbone there is a second series of nerve- 

 centres — those of the sympathetic system. 

 Thus in a vertebrate animal there are system.-^/<er Nicholson. 

 two systems of nerves, whose centres are separated by a partition 

 of bone. 



The remaining portion of the animal body may be regarded as 

 a second cavity, or canal, which contains, in addition to the 

 sympathetic system of nerves, the alimentary and the Jtcemal 

 systems. The former runs the whole len'gth of the body, being 

 a canal which gives passage to the food ; the latter consists of 

 a series of tubes, by which the blood passes through the body, 

 both systems being supplied with many accessory organs. 



In the higher invertebrate animal we find no backbone, no 

 neural canal, and no cerebro-spinal system of nerves ; but the 

 visceral canal exists, and its contents correspond mostly with the 

 .structures found in the vertebrate. (See Fig. 1.) The lowest 

 vertebrate animal known — the Lancelet, or Am2jhioxus, and the 



Fig. 1. 

 A, Diagramatic transverse 

 section through the body of a 

 radiate vertebrate. B, A similar sec- 

 tion through a highly-organised 

 invertebrate animal. 



a, Neural Canal ; b, Cerebro- 

 spinal nerve-centres ; c, Noto- 

 chord ; d, Sympathetic nerve- 

 centres ; e, Alimentary canal ; 

 /, Visceral canal wall ; g, Haemal 



