The visceral ganglion of Pecten, etc. 
17 
of the nerve fibrils running out in the palliai nerye (fig. 3 pall.n) approach 
each other at the point of entrance into the ganglion and for a short di- 
stance a tract of closely packed parallel nerve fibres results (fig. 3 x), 
which passes into the lateral lobe. Having reached the lateral lobe the 
fibres first separate a little and then branch repeatedly into delicate 
elementary fibrillae which form an intricate interlacing mass (fig. 3 s), 
probably a net, which is distinct from the surrounding neuropil and is 
roughly spherical in shape. 
Near the upper (ventral surface) of the lateral lobes, these fibrillae 
become gradually less marked off until finally they pass into the general 
neuropil. This is the finest example I am aware of in the moUusca of the 
origin of nerve fibres from the neuropil and it represents in ali proba- 
bility the origin of the sensory fibres which innervate the eyes, because 
they are the only nerve fibres in the palliai nerves which come from the 
lateral lobes. There is therefore a dendritic ending or a true nerve net 
interpolated between the sensory nerve fibres and the sensory ganglion 
cells and this is Rawitz' »spongiöse Substanz«. As there are more of 
these areas than palliai nerves, it is obvious that more than one are con- 
nected with each nerve. As for the primary origin of the elementary 
fibrillae which form the neuropil from which these sensory fibres arise, 
a certain part is played by the small ganglion cells of the cortex of the 
lateral lobes whose axones break up, a very short distance from the cells, 
into elementary fibrillae. The other part is played by nerve fibres from 
the centrai parts of the ganglion and by the fibres of the transverse tract 
already referred to which pass into the lateral lobes quite dose to the 
ventral surface. 
It is probable that ali the sensory nerve fibres break up in this way 
in the gangha so that direct or indirect connection with the sensory cells 
is brought about with the Interpolation of a nerve net or contact den- 
drites. It is only in the lateral lobes however that this is so distinct owing 
to the freedom from the various types of fibres which are found crossing 
confusingly in the more centrai parts of the ganglion. 
The responses of Pecten to stimuli under normal and 
experimental conditions. 
The observations referred to in this section of the paper, concern 
in the first place the normal responses made by the living animai to sti- 
muli, to which it might be accidentaUy subjected at any time, and se- 
condly, the results of some experiments made to throw a Httle light on the 
Mittheilungen a. d. Zool. Station zu Neapel. Bd. 20,1. 2 
