16 INTRODUCTION. | 
vous or “neural” system. The side of the body on which the 
“hemal” or blood-vascular system is placed is called the 
“hgemal aspect ;” whilst the side of the body on which the 
main masses of the nervous system are situated is called the 
“neural aspect.” When there is any skeleton, this is eaternal 
(forming an ‘“exo-skeleton”), and it is really nothing more than 
a hardening of the skin. The limbs, when present, are turned 
towards the neural aspect of the body. 
In the Vertebrata, on the other hand, the body, if transversely 
divided, exhibits ¢wo tubes. In one (fig. 1, B) is placed the main 
mass of the nervous system (the brain and spinal cord). In the 
Fig. 1.—Diagrams representing transverse sections of one of the higher Invertebrata, 
A—and one of the Vertebrata, B. a Wall of the body; 4 Alimentary canal; 
¢ Hzmal or blood-vascular system ; z Nervous system; 2’ Cerebro-spinal axis, 
or brain and spinal cord of the Vertebrata, enclosed in a separate tube ; ch Noto- 
chord or chorda dorsalis. (Slightly altered from Huxley.) 
other tube are the alimentary canal, the hemal or blood-vascu- 
lar system, and certain other portions of the nervous system 
which are known as the “sympathetic ” system of nerves, and 
which correspond to, or are homologous with, the entire ner- 
vous system of the Invertebrata. Further, in the Vertebrata 
there is always an zwz¢ermal skeleton (or endo-skeleton), the cen- 
tral stem of which is usually constituted by a true backbone or 
“vertebral column.” When this is not present, there is always 
a structure which will be afterwards described as the ‘ noto- 
chord” or ‘“chorda dorsalis.” Lastly, the limbs of the Verde- 
érata, when present, are never more than four in number, and 
they are always turned away from the neural aspect of the 
body—away, that is, from the side on which the main masses of 
the nervous system are placed. 
Subjoined is a short tabular view of the main existing divi- 
sions of the Animal Kingdom, the characters and smaller divi- 
sions of which will be considered hereafter at length :— 
