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of Edinburgh, Session 1881 - 82 . 
commissure is shown in every series of sections prepared. M. de 
Lacaze-Dutliiers * figures and describes three distinct commissures, 
of which the two upper are stout, while the lower one, which evi- 
dently corresponds to the lower one of the two that I have described, 
is slender, and gives off from the middle an unpaired nerve running 
backwards, the distribution of which was uncertain. Whether this 
double (or treble) commissure is an expression of the compound 
nature of the pedal ganglia, which are generally supposed to have 
originated from a primitive ventral ganglionated chain by the fusion 
of some of the ganglia and the suppression of others, is doubtful ; 
judging from analogous cases, it would appear probable that it is 
not so. M. Yung,f in a paper on the nervous system of the Decapod 
Crustacea, describes the abdominal ganglia as being joined by three 
commissures ; as there is one ganglion to each abdominal segment in 
the Macrura, it does not seem possible that they can have been 
originally represented by several ganglia. So much for Lymnseus. 
In Planorbis the arrangements are a little different, there are the 
same two commissures (fig. 2), hut the lower one is much stouter 
than it is in Lymnaeus, and therefore is much more easily seen, 
though it seems to have escaped notice. 
The great point of difference between the two types studied is, 
that in Planorbis there is a small unpaired median ganglion which, 
lying between the pedal ganglia and on the lower of the two com- 
missures, unites them. This is itself connected with one or both of 
the pedal ganglia by a commissure, which, though independent of that 
uniting the two pedal ganglia, yet, as is shown in the figure (fig. 6), 
lies upon it. This ganglion is shown in figure 2 in position, and in 
figure 6 more highly magnified, so as to bring out its constituent cells. 
It is very small in extent, not more than four or five cells in thickness, 
hut is almost impossible to miss in a carefully-cut series of sections ; 
the cells are of uniform size, and not large. From the under surface 
an unpaired nerve is given off, which is no doubt homologous with 
that described and figured by M. de Lacaze-Dutliiers in Lymnseus, 
hut in Lymnseus I have found no trace of this small unpaired lobe. 
The unpaired nerve, on the contrary, does not seem to he peculiar 
* Loc. cit ., pi. xvii. fig. 4. 
f Emile Yung, “ Systeme Nerveux chez les Crustacees Decapodes, ” Arch. 
Zool. Exper ., 1878, vol. vii. 
