196 VEETEBKATE ANIMALS. 



which are known to anatomists as the " sympathetic " system. 

 Leaving the brain and spinal cord out of sight for a mo- 

 ment, we see that the lower or visceral tube of a Vertebrate 

 animal contains the digestive canal (b), the blood-vascular sys- 

 tem (c), and a system of nervous ganglia {n). Now, this is 



i— .^'i 



Fig. 93. — ^Diagrams representinfj transverse sections of one of the higher Invertehrata, A, 

 and one of the Vertebrata, B. a Wall of the body; h Alimentary canal; c Hsemal or 

 blood-vascular system ; n N'ervous system ; n' Cerebro-spinal axis, or brain and spinal 

 cord, enclosed in a separate tube ; ch Notochord, or chorda dorsalis. 



exactly what is contained within the visceral ca^'ity 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 low- 

 er wall of the tube which contains the cerebro-spinal axis, a 

 singular structure known as the " notochord " (Gr. notos, back ; 

 chord's, string) (Fig. 93, 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 condition, but, in most cases, it is replaced 

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

 Veiiiebrata derive their name. The general structure of the 

 vertebral column will be described shortly, and it is suflBcient 

 to state here that it consists of a series of more or less com- 

 pletely bony segments or " vertebrse," 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 noto- 

 chord, but this may remain persistent throughout life, and, in 

 many cases, the development of the spinal column is very im- 

 perfect. 



