The visceral ganglion of Pecten, etc. 
3 
tissue. A glance at fig. 1 will show that we bave a single ganglion to 
deal with, representing the two fused visceral ganglia of other lamelli- 
branchs. This compound ganglion is however not merely a fused pair 
but a higher development. It is divided up into several lobes and is 
moreover asymmetrical. Rawitz pointed out this asymmetry but figu- 
red the ganglion on the contrary as perfectly symmetrical. The muscle 
surface of the ganglion is the dorsal and the superficial surface the 
ventral. The lobes are most distinctly seen on the ventral surface which 
is unbroken by the entrance of nerves (with the exception of one on 
each side). 
Visible on the ventral surface is, first, a centrai unpigmented hemi- 
spherical lobe, which rises above the general surface of the ganglion and 
occupies the place of greatest thickness. This is the ventro-central lobe 
(fig. 1 V.C.I.). On the right and left sides of this, the surface falls to two 
lower slightly pigmented areas, the ventro-lateral grooves (fig. 1 lat. gr\ 
from which arise the single pair of nerves leaving the ventral surface 
(fig. 1 os.h.n). Most laterally are two crescent shapedlobes (fig. 1 r.l.l. and U), 
which are swoUen so that they rise above the level of the intervening 
grooves. These lateral lobes are unpigmented. The area occupied by 
the left lateral lobe and groove is greater than that occupied by the right. 
It is in this respect that the ganglion is asymmetrical (fig. 1). 
Anteriorly and situated between the two cerebro-visceral connectives 
(fig. 1 c.v.con) and the lateral lobes, are two very conspicuous deeply 
pigmented bodies, which appear almost spherical (fig. 1 d.c.l). An exa- 
mination of the dorsal surface of the ganglion (fig. 2) clearly shows that 
these are the anterior ends of two elongated lobes which lie side by side 
in a median position on the dorsal surface (fig. 2 d.c.l). The anterior 
ends of these dorso-central lobes wrap round the centrai mass of the 
ganglion and are visible on the ventral surface. These lobes are divided 
by a deep groove and are uniformly pigmented deep orange brown. 
Posterior to the dorso-central lobes can be seen the under surface 
of the ventro-central lobe (fig. 2 v.c.l). This part of it is pigmented and 
on it, and situated in the median line, is a very small pigmented body 
(fig. 2 h). The two lateral lobes already seen on the ventral surface, 
extend dorsally right up to the dorso-central lobes, and from this part 
of the gangUon arise most of the nerves. The dorsal surfaces of the la- 
teral lobes are unpigmented. 
Those parts of the ganglion which may be seen between the lateral 
and the dorso-central lobes (fig. 1 a, fig. 2 a) bave not been considered 
as separate lobes. 
1* 
