268 Proceedings of Eoyal Society of Edinburgh. [sess. 
is also to be seen in the same fig. {os.g.). In Aphjsia the ganglion 
cells are concentrated into a definite osphradial ganglion and into a 
definite branchial ganglion some distance along the branchial nerve. 
Here, however, the cells are scattered almost indiscriminately along 
the nerve. This condition throws some light upon the question as 
to the homology of the various ganglia, occurring at this point. The 
position of the osphradium on the efferent vessel differs from that 
in NotarchuSj where it is more in front, and resembles that of 
Aplysia, where it is decidedly under the vessel. In connection 
with the innervation of the osphradium it may be stated that the 
visceral ganglion (the right), from which the osphradial nerve 
comes, was found to be lying over on the left, and consequently the 
pleuro-visceral connectives were crossed, — that is, the arrangement 
was chiastoneure. 
Nothing calling for special attention is to be noted in the gills. 
The position of the anus in all these forms is important, as 
throwing light upon the difficult problem of the torsion of the 
Molluscan body. In a general account of the pallial organs of the 
Opisthobrancliiata I have endeavoured to show that there is in co- 
ordination with the loss of the shell a tendency of the different 
organs to return to their original position at the posterior end of 
the body, and that, as a rule, the anus is the leading organ in this 
backward movement. In Aplysia the anus practically reaches the 
posterior median position, by the aid of an exhalent sipho, in the 
middle of which it opens. In the form under consideration, how- 
ever, this sipho (fig. 3, s) has been moved forward towards the head 
region, and is to a large extent unavailable for this purpose. We 
find, accordingly, that here the anal opening has shifted further to 
the posterior median position, to such a degree that it has all but 
broken through the edge of the mantle and passed over it (fig. 
3, a.n.). This condition explains how the arrangement in Notar chus 
may have been phylogenetically brought about. With the gradual 
growth of the parapodia behind, the sipho has been shifted farther 
and further forward, followed by the anus ; while the part of the 
mantle behind the sipho has gradually disappeared, so that finally, 
as in Notarchus, only a part in front is left, and that for a special 
function connected with conjugation, as explained elsewhere. 
The position of the hypobranchial gland is peculiar. It occurs. 
