The visceral ganglion of Pecten, etc. 
35 
fibres from the cerebral as well as those from the visceral ganglion. It 
has not been shown that these fibres innervate the osphradium, and in 
any case the visceral ganglion plays a most important part. 
5. The visceral ganglion consists of a centrai neuropil and a cortical 
layer of ganglion cells, which varies in thickness. These ganglion cells 
are largest in the dorso-central lobes and are motor in function. The 
smallest cells occur on the ventral surface of the ventro-central and lateral 
lobes which are unpigmented. Neuroglia nuclei are present between the 
ganglion cells and in the centrai neuropil, and the ganglion cells are en- 
closed by a sheath of neuroglia fibres and celi processes. The ground 
substance of the ganglion sheath also sends extensions between the gang- 
lion cells. The latter are chiefly unipolar and the body of the celi is situ- 
ated towards the periphery of the ganglion whilst the axone is directed 
towards the neuropil. A large nucleus is present in the ganglion cells 
and there is also in the cytoplasm, an extensive neurofibril net, a mass 
of yellow pigment and Nissl's granules. 
6. Nerve fibres bave been traced into the muscle nerves, palliai nerves 
and connectives from the large ganglion cells of the dorso-central lobes, 
which are probably motor in function. Other fibres, presumably sen- 
sory, could not be traced directly to nerve cells in the ganglion, but 
branched or split up into delicate elementary fibrillae in the neuropil. 
7. The neuropil is made up of an intricate mass of neurofibrillae 
which may or may not form a true net. Numerous nerve fibres run through 
this mass, which is formed by the Splitting up of the axones of ganglion 
cells or the sensory fibres of peripheral nerves. Some run direct through 
the ganglion from one nerve to another. Others place the opposite halves 
of the ganglion in communication, and finally there are the axones of 
motor cells, which run through the ganglion and into peripheral nerves 
without the Interpolation of a nerve net. 
8. The neuropil of the lateral lobes contains several spherical regions 
which are formed of closely packed endings of sensory fibres from the 
palliai nerves. These sensory fibres split up into elementary fibrillae 
which may form a true net or be only in contact with the dendrites of 
ganglion cells. The development of the lateral lobes is considered to be 
due to the presence of numerous complicated visual organs, and the sen- 
sory fibres referred to above are probably in connection with the latter. 
9. Pecten is very sensitive to the presence of starfish. These animals 
are recognised by the olfactory sense and not by touch or sight. The 
osphradia and abdominal sense organs do not appear to play any visible 
part in the perception of starfish. 
3* 
