MORPHOLOGY OF THE HELICINID^. 793 



the relative size of the cerebi^l ganglia I find that the proportion 

 of each cerebral ganglion to the pleuro-pedal mass is expre>ssed by 

 the following figures : — in Alcadia palliata, | ; Alcadia hollandi^ 

 + 1 ; Eutrochatella pidchella, | ; Lucidella aureola, | ; Palceo- 

 helicina idee, nearly ^ ; Orohophana ponsonbi/i, 4 ; Aphanoconia 

 gouldiana, +| ; A phanocoiiia rogersii, f. The relative length of 

 the cex^ebro-pedal and cerebro-plenral connectives varies greatly : 

 they are longest in the two species of Alcadia and in Palceo- 

 helicina idee ; of moderate length in Orobopka^ia, Ajjhanoconia, 

 and Eutrochatella ; extremely shoi't in Lucidella aureola. I have 

 counted four labio-prolioscidean nerves on each side with more 

 or less certainty in all the species examined with the excep- 

 tion of Apjhanoconia rogersii, in ^^'hich there appear to be five. 

 Of these, counting fi'om above downwards, the fii-st and third 

 are invariably stout nerves wbich brancli soon after their origin ; 

 the second and fourth are slender and only divide into blanches 

 at their extremities. The labial lobe in all the species at 

 my disposal has the form of a rounded boss projecting inwards 

 from the antero-inferior edge of the ganglion : the shape and 

 relative size of this lobe differ somewhat in the various species, 

 but the differences are too slight to express in words. Fig. 53 

 (PI. XXXIX.), representing the left cerebral ganglion of Palceo- 

 helicina idee, and fig. 54, representing the same ganglion in Alcadia 

 palliata, show the extremes of difference in shape observed by me, 

 and, on the whole, the cerebral ganglia of the Polynesian species 

 T-esemble those of Palceohelicina, those of the West-Indian species 

 those of Alcadia. The cei-ebral ganglia of the Helicinidje, wedged 

 in as they are between the antei-ior end of the pharyngeal bulb 

 and the walls of the head, are nearly flat, the labial lobe projecting 

 inwards beneath the pharyngeal bulb. Because of their flatness 

 they are very I'eadily stained and mounted as tiunspaient objects, 

 and figs. 53 &l 54 give some idea of the complexity of the nerve- 

 tracts and centres within the ganglion. One may distinguish an 

 ocular centre, which is large relatively to the size of the ocular 

 nerve, a tentacular centre, small relatively to the size of the 

 tentacular nerve, and a relatively smaller commissural centre. 

 Each of the labio-proboscidean nerves has a more or less well- 

 defined centre of its own, but the buccal and labial commissures 

 have no distinct centres at their origin. There is, further, a 

 median lobe which possibly serves as a relay for various nerve- 

 tracts running into ami around it. It is evident that the nerve- 

 fibres of the cerebro-pleural connective make direct and intimate 

 connection with the ocular and tentacular lobes, and that a 

 stout band of fibres curves i-ound from the I'oot of the cerebro- 

 pleural connective to the base of the labial lobe, receiving on its 

 way an accession of fibres from the cerebro-pedal connective. 

 This nerve-tract makes connections with the centres of origin of 

 the buccal and labial commissures and the labio-proboscidean 

 nerves, and a well-defined curved band of fibres sweeps round 

 from the origin of the first labio-pi'oboscidean nerve to enter the 



