216 VERTEBRATE ANIMALS. 
nervous system (7) which are known to anatomists as the 
“sympathetic” system. Leaving the brain and spinal cord out 
of sight for a moment, we see that the lower or visceral tube of 
a Vertebrate animal contains the digestive canal (0), the haemal 
or blood-vascular system (c), and a system of nervous ganglia 
(7). Now this is exactly what is contained within the visceral 
cavity of any Invertebrate animal; and it follows from this that it 
is the “sympathetic” system of Vertebrate animals which is truly 
comparable with the nervous system of the Invertebrata. The 
brain and spinal cord, or “cerebro-spinal axis,” are to be looked 
upon as something not represented at all in the Invertebrata, 
Another peculiarity which is present in all the Vertebrata is, 
that at an early period of life there is developed in the lower 
Fig. 115.—Diagrams representing transverse sections of one of the higher Jnverte- 
brata, A—and one of the Vertebrata, B. a Wall of the body; 6 Alimentary 
canal; ¢ Hzmal or blood-vascular system; 2 Nervous system; 7’ Cerebro- 
spinal axis, or brain and spinal cord, enclosed in aseparate tube ; ci Notochord, 
or chorda dorsalis. 
wall of the tube which contains the cerebro-spinal axis, a singu- 
lar structure known as the “notochord,” or “ chorda dorsalis ” 
(fig. 115, B, ch), This is a semi-gelatinous rod, tapering at both 
ends, and extending along the floor of the cerebro-spinal tube. 
In some cases the notochord remains permanently in this con- 
dition, but in most cases it is replaced at maturity by the bony 
column or backbone, from which the Vertedrvata derive their 
name. ‘The general structure of the vertebral column will be 
described shortly, and it is sufficient to state here that it con- 
sists of a series of more or less completely bony segments or 
“vertebrae,” arranged so as to forma longitudinal axis upon 
which the spinal cord is supported. It is to be remembered, 
however, that all Vertebrate animals do not possess a vertebra 
