OF THE NERVOUS SYSTEM. 79 



them does not appear, in the estimation of physiologists 

 generally, to present that obstacle to a satisfactory 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. Hilaire 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 intervenes in 

 the invertebrate between the aortic trunk and the neural 

 canal." 



I must here premise, that while I hold the 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 Carus. I conceived that 

 each segment of the annulose animal contains potentially an 

 annular nervous arrangement, 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 



