580 Proceedings of the Poyal Society 
cells of the “ white lobe ” are tinted of a uniform pale lilac, there 
being but a very slight distinction between the nucleus and the cell 
protoplasm. This “ white lobe ” also presents the appearance of a 
firm solid cellular mass, while the other part of the cerebral ganglion 
consists of an outer sheath of nerve cells of various sizes, and an 
inner core of nerve fibres. When hardened and stained by osmic 
acid, there is still a noticeable difference ; the ordinary nerve cells 
within the ganglia are coloured brown, the nucleus being left 
unstained, or coloured more or less faintly ; while the cells making 
up the white lobe are coloured on the whole much more faintly 
than the ganglion, but the nuclei are rendered conspicuous by being 
stained more deeply than the surrounding protoplasm. 
The homologous structure in Planorbis (see figs. 1, 2) is a large 
single mass lying over the commissure uniting the two cerebral 
ganglia ; it is figured by Lacaze-Duthiers,* but is not specially 
described by him. Though this mass in Planorbis is single, it has 
a deep depression in the middle, which gives it a kidney-shaped 
outline in transverse section. In structure it is altogether similar 
to the corresponding part in Lymnaeus, that is to say the cells which 
make up the mass are quite alike in both animals, but their arrange- 
ment is rather different in Planorbis. As is shown in figure 3, the 
whole mass is distinctly lobulated, the lobules being separated from 
each other by trabeculae of connective tissue ; it is not unlike the 
salivary gland of the same animals, but in a number of sections 
both structures were visible in the field of the microscope at once, 
and then there was no possibility of confounding them ; the cells 
of which the salivary gland is composed are a great deal larger, and 
the nucleus is far more distinct — there are of course numerous other 
minute points of difference ; but what I wish to dwell on in the 
present paper are the points of resemblance rather than difference, 
to show that this “ white lobe ” must be considered as a gland. 
The facts then about the anatomy of the “ white lobe ” in 
Planorbis appear to point to one conclusion — that this part of the 
cerebral ganglion must not be considered as really forming a part of 
the cerebral ganglion, but as being distinct and of a glandular 
nature ; and if this is granted in the case of Planorbis, it must be 
the same with Lymnseus, as their position and general resemblance 
leaves no room for any other view than that they are homologous 
* Loc. cit., pi. iii. fig. 10. 
