324 MANUAL OF ZOOLOGY. 



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

 brate animals, without exception, the body (fig. 115, A) may 

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

 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, 



Fig. 115. A, Tranverse section of the body of one of the higher Invertebrata : 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 ; ' Cerebro-spinal system 

 of nerves ; ch Notochord. 



however, of a Vertebrate animal exhibits two tubes (fig. 115, 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 ner- 

 vous system, known as the " ganglionic" or "sympathetic" 

 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 con- 

 tained in the visceral cavity of any of the higher Invertebrate 

 animals ; and it follows from this, as pointed out by Von Baer, 

 that it is the sympathetic nervous system of Vertebrates which 

 is truly comparable to, and homologous with, the nervous sys- 

 tem of Invertebrates. The cerebro-spinal nervous centres of 

 the Vertebrata are to be regarded as something superadded, 

 and not represented at all amongst the Invertebrata. 



The tube containing the cerebro-spinal centres is formed as 

 follows : At an early period in the development of the em- 

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

 which development is going on the "germinal area" be- 

 comes elevated into two parallel ridges, one on each side of 

 the middle line, enclosing between them a long groove, which 

 is known as the " primitive groove" (fig. 1 16, A, B). The ridges 

 which bound the primitive groove are known as the " laminae 



