The visceral ganglion of Pecten, etc. 
15 
Kawitz figures an internal mass of cells, which is called the »in- 
nerer Zellkern« in his figure and he describes (p. 60) how some of the 
fibres of the connectives, muscle nerves and palliai nerves arise from 
these ganglion cells (»mittlere Zellkerne«). These cells however are 
evidently only the cortical cells situated at the base of the deep groove 
between the dorso-central lobes and the ventro-central lobe (see fig. 4) 
and a series of sections would soon have convinced Rawitz of this fact. 
The Lateral Lobes. The structure of the lateral lobes deserves 
separate description, since they give the complexity to the ganglion 
and do not occur in other lamellibranchs. These two bodies appear 
to be special developments in Pecten and one naturally turns therefore 
to the anatomy of the animai for any special organs whose presence 
might be correlated with these structures. The attention is imme- 
diately drawn to the eyes, for the presence of such numbers of excee- 
dingly complicated organs should presumably have some influence on the 
structure of the centrai nervous system. The eyes are innervated by 
nervo fibres from the palliai nerves, for I have been able to trace the 
fibres for some distance in the mantle, and the paUial nerves arise from 
the under surfaces of the lateral lobes. Hence the eyes are apparently 
in dose union with these lobes and it remains to see whether part of the 
fibres in the palliai nerves arise from them. It has already been shown 
that the palliai nerves receive some of their fibres from the cerebro-vis- 
ceral connective and the neuropil of the more centrai parts of the visceral 
ganglion, and it will be shown below that other fibres arise in the lateral 
lobes. Further confirmation of the association of these structures, the 
lateral lobes and the eyes, is however given by the foUowing interesting 
fact. The number of eyes on the left mantle lobe is greater than that 
on the right and this difference is greatest in Pecten jacobaeus where the 
eyes on the right lobe are also very small. It has been already mentioned 
that the lateral lobes are also asymmetrical and the greatest difference 
occurs in P. jacobaeus. We see therefore that the lateral lobe which is 
the larger is concerned in the innervation of the mantle lobe with the 
most eyes and the conclusion to be drawn is that the development of the 
lateral lobes has been determined by the development of visual organs 
on the mantle lobes. 
The lateral lobes have a cortical lay er of small ganglion cells (fig. 3 gx). 
The centrai part is composed of nervo fibrillae which however are not 
distributed so as to form a regulär nerve net as in the centrai neuropil 
of the ganglion but arranged in such a way that a section through the 
piane of the ganglion (fig. 4) reveals a number of rounded areas more 
