8 
W. J. Dakin 
tissue which surrounds the ganglion cells and can be distinguished from 
the ganglion sheath by reason of the absence of the ground substance. 
It is no doubt these extensions of ground substance of the sheath which 
caused Kawitz to state that processes from the ganglion sheath pene- 
trated the ganglion only in Pecten. This connective tissue of the 
sheath does not penetrate the neuropil. 
The ganglion cells vary considerably in size, the largest occurring 
in the dorso-central lobes, whilst the smallest which are rather uniform 
in size, are to be found in the cortex of the lateral lobes and the ventro- 
central lobe. 
The great majority of the ganglion cells are unipolar, but multiand 
bipolar cells also occur (fig. 13). I have not made a detailed study of 
the shape of the cells, which requires careful methylene blue staining, and 
cannot therefore give the actual proportion of the different types of cells 
beyond the statement above. The diameter of the large unipolar cells 
(fig. 10, 11 and 12) averages about 27 ^u, and the diameter of the spheri- 
cal nuclei is as much as 13 fx. The smaller cells are only about 10 ^ 
in diameter. Kawitz gives a large number of figures of ganglion cells 
from various lamellibranchs and attempts a Classification of them which 
appears to be carried to an unnecessary detail. Practically all his figures 
are drawn from macerations which is a very satisfactory way of deter- 
mining the shape of the cells if true cell processes are distinguished from 
adhering glia fibres. Rawitz has however apparently passed over the 
neuroglia altogether, although it is often very distinctly seen in mace- 
ration preparations (fig. 12). Hence it is very probable that some of his 
multipolar cells are really unipolar cells, and the fine processes figured 
are neuroglia fibres. For the same reason the direct connection of ganglion 
cells by these processes (Rawitz figs. 63 and 68) is also open to doubt. 
Freidenfelt says there are only contact connections of the ganglion 
cells in Änodon and Bochenek endeavoured to find connections but 
found it impossible, though he adds no definite proof against their pre- 
sence. I have found no direct connections, but this would not be ex- 
pected from the sections, and it is quite possible that such connections 
were broken in macerations. 
The large unipolar cells are roughly pear shaped; the body often 
being slightly angular owing to the pressure of neighbouring cells. The 
apical end from which the axone arises is directed towards the central 
neuropil. The axone is at first broad and can be traced easily, but on 
entering the neuropil it branches and may break up into very delicate 
fibrillae which take part in the formation of the neuropil. 
