578 
Proceedings of the Roycd Society 
to these two species ; it is figured by Spengel * in Gastropteron and 
Aplysia, and by Yon Ihering f in these two types as well as in 
Bulla and Acera. A double commissure between the pedal ganglia 
is very general, and in a few cases, as in Lymnseus, according to 
Lacaze-Duthiers, there is a treble commissure; a glance at Yon 
Iherings plates will show this. Whether this ganglion just de- 
scribed is limited to Planorbis, or whether it is general, I do not at 
present know, but hope to study the nervous systems of a number 
of Mollusca, with a view of determining its nature and distri- 
bution. 
In the same two sections (figs. 1 and 2) may be seen a mass, 
unpaired in Planorbis and paired in Lymnseus, lying above the 
cerebral ganglia, on the commissure uniting them. It has been 
hitherto regarded merely as a part of the cerebral ganglion, and if 
it is really so, its structure is very peculiar; from studying it in 
sections and by teasing, it appears to me to be rather of a glandular 
nature, judging from its structure and relations. 
It will be best to commence with a description of this structure 
in Lymnseus, and I may say that the description below refers to 
Lymnceus stagnalis, while the species of Planorbis described is 
Planorbis corneus. Fig. 5 represents the cerebral ganglion of 
Lymnseus, and that portion of it lettered a is now under 
discussion; it will be seen that the commissure &, uniting the 
ganglion to its fellow, has in reality nothing to do with this lobe, 
but takes its origin from the larger part of the ganglion, and merely 
passes under the lobe a. In the fresh ganglion this part is 
readily distinguishable from the rest from its being of a whitish 
colour, containing none of the red pigment which is distributed 
universally in the cells and their processes of the rest of the 
cerebral ganglion and in the other ganglia. Lacaze-Duthiers says 
of it : X “ II se fait distinguer sur l’animal vivant par son opacity et 
sa teinte blanchatre, tr&s saillant surtout dans le Lymnceus stagnalis , 
il Test un peu moins dans les deux autres especes. II est forme de 
corpuscles relativement fort petits et qui ressemblent a ceux que 
* “ Die Geruchsorgane und das Nerven system der Mollusken,” Zeitschr. fur 
Wissensch. Zool., 1881, plate xvii. figs. 7, 8. 
t Ncrvensystem der Mollusken , Leips., 1877, plates iii. and iv. 
J Loc. cit., p. 443. 
