792 PROF. G. C. BOURNE ON THE 



From what precedes, it follows' that the pleuro-pedal centres are 

 extremely complicated and are largely composed of definite tracts 

 of nerve-fibres, some commissural, some decussating, some passing 

 into the nerves issuing from this region. It is noticeable that there 

 are several connections of a complex kind between the pleural and 

 pedal centres as well as between the right and left pleural centres 

 and the right and left pedal cords, and that there are evidences of 

 numerous nervous relays throughout the region illustrated. No 

 doubt, on analysis, these apparently complicated nerve-tracts can 

 be reduced to five groups : (1) the pedal commissure, containing 

 both direct and decussating tracts of fibres ; (2) the pleuro-pedal 

 connectives ; (3) the cerebro-pedal and cerebro-pleural connec- 

 tives ; (4) the pleural commissure, peculiar to the Neritida^ and 

 Helicinidje ; (5) the subintestinal connectives, derived from 

 both right and left pleurals. Thus, the paths of the nerve-tracts 

 might be described as normal, and consistent with our knowledge 

 of the usual connections between the chief nerve-centres of 

 Molluscs ; but a study of sections will show that the arrangements 

 are not so simple as might be inferi-ed from a superficial exami- 

 nation of the ganglia and their commissures and connectives. 

 For example, it appears that the cerebro-pleural connectives 

 contain fibres derived from the pedal centres and that all the 

 principal nerves contain fibres derived from two or more areas in 

 the fused pleural and pedal ganglion-mass. I have been unable 

 to pursue the subject further at the present time, and it is diffi- 

 cult to make any further progress because of our ignorance of 

 the physiology of the molluscan nervous system. The main 

 nerve-trunks must contain both afi'erent and eflferent nerves, and 

 it seems evident that these pass to dilFerent areas of the cortical 

 layer of ganglion-cells, but as at present we have no means of 

 distinguishing between these two kinds of fibres further analysis 

 of the details of the nervous system is impossible. It may be of 

 use to future workers on this subject to remark that there are two 

 kinds of ganglion-cells in the cortical layer: larger cells with 

 clear nuclei staining faintly in hsematoxylin, and much more 

 numerous smaller cells with deeply staining nuclei. It should be 

 possible to trace the connection of the nerve-fibres with these 

 different kinds of cells, but such an investigation demands fresh 

 material, and could form no part of the present work on the 

 Helicinidse. 



As Bouvier found considerable difl:erences in the size and shape 

 of the cerebral ganglia in Helicina sagraiana and H. brasiliensis, 

 I have studied these centres with care in the hope that I might 

 discover characters of classificatory value, but I have been no 

 more successful here than I was in the case of the genital ducts. 

 The characters to Avhich Bouvier draws special attention are the 

 relative size of the cerebral ganglia (enormous in JI. brasiliensis) ; 

 the size and shape of the labial lobe ; the origin of the labio-probos- 

 cidean nerves, which all spring from the labial lobe in H. brasi- 

 liensis, but only one has this origin in //. sagraiana. As regards 



