OF THE NERVOUS SYSTEM. 79 



them does not appear, in tlie estimation of physiologists 

 generally, to present that obstacle to a satisfactoiy com- 

 parison which its essentially fundamental character would lead 

 us to expect. The difficulty has, however, been clearly stated 

 by Professor Owen, who, in discussing the relations of the endo- 

 and exo-skeletons in his Lectures on Fishes, page 21, ed. 1846, 

 says — " Geoffroy St. liilairc thought it needed but to reverse 

 the position of the crustacean — to turn what had been 

 wrongly deemed the belly upwards — in order to demonstrate 

 the unity of organisation between the articulate and ver- 

 tebrate animal. But the position of the brain is thereby 

 reversed, and the alimentary canal still inter\'enes in 

 the invertebrate between the aortic trunk and the neural 

 canal." 



I must here premise, that while I hold tlie general mor- 

 phological relations of the annulose and vertebrate ner- 

 vous systems to be identical, I do not consider these two 

 types of organisation to be mutually reducible. On the con- 

 trary, they are fundamentally distinct, presenting differences 

 which demand careful consideration. It is, nevertheless, 

 incumbent on the morphologist to ascertain in what re- 

 spects they correspond, so as to determine their distinctive 

 limits. 



My earlier conception of the morphology of the annulose 

 nervous system was based on that of Cams. I conceived that 

 each segment of the annulose animal contains potentially an 

 annular nervous an-angement, set in a plane at right angles 

 to the axis of the segment, or longitudinal axis of the animal ; 

 that the only complete nervous ring is that one through 

 which the oesophagus passes ; that the ganglions on this ring 

 are arranged in the various forms of superior, lateral, and in- 

 ferior oesophageal masses ; that the nervous rings in the post- 

 cephalic segments are all incomplete above, and have their 

 ganglions united into a single or double mass below ; and 



