444 MANUAL OF ZOOLOGY. 



One of the most obvious, as it is one of the most funda- 

 mental, of the distinctive characters of Vertebrates, is to be 

 found in the shutting off of the main masses of the nervous 

 system from the general cavity of the body. In all Inverte- 

 brate animals, without exception, the body (fig. 238, A) may 

 be regarded as a single tube, enclosing all the viscera; and 



Fig. 238. A, Transverse section of the body of one of the higher luvertebrata : a 

 Body-wall ; b Alimentary canal ; c Haemal system ; n Nervous system. B, Trans- 

 verse section of the body of a Vertebrate animal : a Body-wall ; b Alimentary 

 canal ; c Haemal system ; n Sympathetic system of nerves ; n' Cerebro-spinal system 

 of nerves ; ch Notochord. 



consequently, in this case, the nervous system is contained 

 within the general cavity of the body, and is not in any way 

 shut off from the alimentary canal. The transverse section, 

 however, of a Vertebrate animal exhibits two tubes (fig. 238, B), 

 one of which contains the great masses of the nervous system 

 that is, the cerebro-spinal axis, or brain and spinal cord 

 whilst the other contains the alimentary canal and the 

 chief circulatory organs, together with certain portions of 

 the nervous system known as the " ganglionic " or " sym- 

 pathetic" system. Leaving the cerebro-spinal centres out 

 of sight for a moment, we see that the larger or visceral 

 tube of a Vertebrate animal contains the digestive canal, the 

 haemal system, and a gangliated nervous system. Now this 

 is exactly what is contained in the visceral cavity of any of 

 the higher Invertebrate animals; and the opinion has been 

 generally entertained that it is the sympathetic nervous sys- 

 tem of Vertebrates which is truly comparable to, and homolo- 

 gous with, the nervous system of Invertebrates. On the other 

 hand, there are Invertebrates with a sympathetic system of 

 nerves, and the development of the nerve-chain of the Annu- 

 losa resembles that of the cerebro-spinal axis of the Vertebrata. 

 The tube containing the cerebro-spinal centres is formed as 

 follows : At an early period in the development of the embryo 

 of any Vertebrate animal, the portion of the ovum in which 



