1908.] ASPIDOBBAXCH GASTROPOD MOLLUSCS. 857 



of the left kidney. The great elongation of the ganglionated 

 postei^ior tract of the visceral commissure is readily explicable 

 when it is borne in mind that, as I have already described for 

 Seplaria and shall describe further on for Xerita, the embryonic 

 ■condition of the coelom has been retained in this family and its 

 ■cavity stretches across the body from left to right, the two 

 primitive kidneys being separated by a considerable space, and 

 opening at widely separate points into the coelom. In short, 

 this region of the body is broader from right to left than is usual 

 in Gastropoda, and the visceral ganglionic swelling is elongated 

 accordingly. 



I must admit that it may be urged against this view that if the 

 " organe creux " described by Lenssen in X. Jfuviatilis, and 

 found by me in a corresponding position in the various species 

 •examined, is really a vestige of the right ctenidium, tlie swollen 

 right end of the ganglionic enlargement bears the same topo- 

 graphical relation to it that the subintestinal ganglion bears to 

 the right ctenidium in dibranchiate Rhipidoglossa. I confess 

 that I was at first inclined to agree with Boutan and Haller in 

 identifying what I now regard as a specialized part of the visceral 

 ganglion as the subintestinal, and was inclined to argue that the 

 outgrowth must represent the right ctenidium because of its 

 relation to the ganglion, and that the ganglion must be the 

 subintestinal because of its i-elation to the relic of the ctenidium. 

 This was so flagrant an example of the circidus in definiendo that 

 I was led to reconsider the question, and I am satisfied that the 

 origin of the genital nerve and the relationship to the oviduco- 

 coelomic funnel and to the right side of the coelomic cavity 

 •outweighs all other evidence. 



If, then, Boutan and Haller were wrong, de Lacaze-Duthiers 

 and Bouvier were right in their identification of the subintestinal 

 ganglion. To test this point, I have made a careful study of 

 sections through the pleuro-pedal centres. These goto show that 

 the enlargement from which the subintestinal nerve proceeds is 

 really a nerve-centre, and is therefore the representative of the 

 •subintestinal ganglion. To make this point clear, the question 

 must be asked, what do we mean when we speak of a ganglion ? 

 It is a swelling caused by the presence of the nerve ganglion- 

 cells investing a central core of nerve-fibrils. It is something 

 more than this, it is a nervous relay, in which some of the fibres 

 enter into nerve ganglion-cells, and in which the dendrites of 

 the ganglion-cells are intermingled and in contact with one 

 •another. As far as I am aware, nobody has yet attempted to 

 work out the courses of the nerve-fibres and to trace them to 

 their connection with groups of nerve-cells in the Mollusca. I 

 am unable to do more than touch the fringe of a subject that 

 offers a large field for future research, as my prejiarations were 

 not made with this purpose and are inadequate to disentangle the 

 complex of nerve-fibies and cells. I have, however, ascertained 

 the following facts : — Each nerve-centre consists of a core of nei've- 



