80 ON THE MOHniOLOGICAL RELATIONS 



that all the rings are united by a series of longitudinal abdo- 

 minal commissures. According to this view, the oesophageal 

 collar, with its superior, lateral, and inferior ganglions, is 

 homologous with each pair of segmental nerves, and the cor- 

 responding abdominal ganglionic centre; the cesophageal 

 collar being in a ])lane parallel to those in which the post- 

 cephalic ganglions and their pairs of nerves are situated, but 

 at right angles to the line of the series of abdominal gan- 

 glions. 



I first recognised what I believe to be the real morpho- 

 logical relations of the annulose nervous system during the 

 delivery of a course of lectures on Invertebrate Anatomy in 

 1849 ; but more fully and completely during courses on the 

 Anatomy of the Mollusca in 1850, and on the Anatomy of 

 the Crustacea in 1851. 



I now perceived that the fundamental difference between 

 the morphological relations of the annulose and vertebrate 

 nervous systems, consists in the position of the mouth. 



I saw that the entire axis or central portion of the nervous 

 system extends along the neural aspect of the body in both 

 types of organisation ; but that while, as is well known — 

 although its morphological importance does not appear to 

 have been perceived — the vertebrate mouth opens into the 

 hffimal, the annulose mouth passes through the neural aspect 

 of the body. 



In the annulose animal, therefore, the buccal entrance 

 interferes with the nervous axis — passing up between the two 

 lateral halves of one of its longitudinal commissural or inter- 

 ganglionic cords, so as morphologically to divide the con- 

 tinuous axis into a pre-sto7tial and a post-stomal portion. 



These relations are most satisfactorily seen in the Crus- 

 tacea, in which the so-called brain, or supra-oesophageal gan- 

 glion or nervous mass, is actually in front of the mouth, 

 and not above it. 



