GENEKAL CHAEACTERS OF THE VERTEBBATA. 219 



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 (6), the hfemal or blood-vascular system (c), and a 

 system of nervous ganglia (n). Now this is exactly what is con- 

 tained within the visceral cavity of any Invertebrate animal ; and 

 it follows from this that the " cerebro-spinal " nerve-system of the 

 Vertebrata is a structure for which no precise parallel exists among 

 Invertebrates. 



Another peculiarity which is present in all the Vertebrata is, that 

 at an early period of life there is developed in the lower wall of the 

 tube which contains the cerebro-spinal axis a singular structure 

 known as the " notochord," or "chorda dorsalis " (fig. 157, 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 noto- 

 chord remains permanently in this condition, but in most cases it is 

 replaced at maturity by the bony column or backbone, from which 

 the Vertebrata derive their name. The general structure of the 



Fig. 157. — Diagrams representing transverse sections of one of the Iiigher Invertebrata, 

 A — and one of the Vei-tebrata, B. a Wall of the body ; 6 Alimentary canal ; a 

 Hsemal or blood-vascnlar system ; n Nervous systejn ; n' Cerebro-spinal axis or 

 brain and spinal cord, enclosed in a. separate tube ; ch Notochord, or chorda 

 dorsalis. 



vertebral column will be described shortly, and it is sufiieient to 

 state here that it consists of a series of bony or cartilaginous seg- 

 ments or " vertebrae," arranged so as to form a longitudinal axis 

 upon which the spinal cord is supported. It is to be remembered, 

 however, that all Vertebrate animals do not possess a vertebral 

 column. They all possess a notochord, but this may remain per- 

 sistent throughout life, and in many cases the development of the 

 spinal colunm is very imperfect. 



The skeleton of all Vertebrate animals is internal, and the muscles 

 are attached to its several parts. The value of this character is in 

 no way affected by the fact that many Vertebrates, such as the 

 Tortoises, Crocodiles, and others, possess an external skeleton as 

 well. The limbs of Vertebrate animals are always articulated or 



