108 RORIE, ON LUMBRICUS TERRESTRIS. 
cells of that ganglion. This circumstance may account, in 
some measure, for the gradually tapering appearance of the 
whole chain. 
2. Arrangement of the nerve-fibres and nerve-cells in the 
first or sub-cesophageal ganglion. This ganglion, as I have 
already stated, differs from the ventral ganglia generally in 
being of a triangular rather than of a quadrilateral form. 
It also differs in the nerve-cells not being so uniform in 
their arrangement. The quadripolar cells appear to be in a 
greater measure mixed up with those giving off more branches. 
As in the other ganglia, so here also some fibres may be 
seen passing through the ganglion without being connected 
with its nerve-cells. A sketch of some of the cells and 
fibres of this ganglion is given at fig. 2. 
3. This microscopical anatomy of the supra-cesophageal 
ganglia (fig. 8). The minute anatomy of this portion of the 
nervous system differs to a considerable extent from that of 
the other ganglia. Each ganglion, or to speak more correctly, 
each hemisphere presents the appearance of being composed of 
two layers of nervous matter, differing from each other not 
only in colour, but also in microscopical character. The outer 
layer is of a reddish-grey colour, contains a considerable 
amount of pigmentary matters, and appears to be composed 
of spheroidal apolar cells, between which may be seen more 
or less granular matter. The inner layer, on the other hand, 
contains but little pigment, is transparent, and composed 
for the most part of multipolar cells. From these cells 
branches may be traced as follows :—From the outer cells 
fibres pass to form the ribbon-like collar surrounding the 
cesophagus, while, from the inner cells, fibres may be traced 
passing between the two hemispheres, thus forming a distinct 
commissure. Both classes of cells communicate freely 
with each other by means of other fibres; and, in addition, 
a few fibres are given off, which become lost in the granular 
matter of the outer layer. In fig. 3, a nerve is seen arising 
immediately from the cells of the transparent layer, and is, 
in all probability, the nerve referred to by Brandt. 
With regard to the manner in which the nerves terminate 
at their periphery little is known. On the walls of the di- 
gestive organs of many Insecta they may be seen dividing 
and subdividing until they become lost to view, when exa- 
mined with a power of 450 diameters. 
Fig. 4 represents the appearance presented by one of the 
ganglia of the mussel (Mytilus), from which a few fibres may 
be seen, terminating in cells situated apparently in connection 
with a few fibres of the adductor muscle; but whether or not 
