STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 491 
glion, passes up the anterior face of the stomach to the gastric 
ganglion. This ganglion is spindle-shaped (fig. 2, gast. gang.), 
and from the opposite end of the spindle to that at which the 
azygos enters a stout nerve is given off, which bifurcates 
on the dorsal surface of the stomach (fig. 2). The gastric 
ganglion of Astacus is enclosed in a connective-tissue sheath, 
which can with care be removed. When this is done, the large 
unipolar cells can be seen protruding on either side, each in- 
vested in a nucleated sheath. One such cell with its sheath 
is represented in fig. 5. The central portion of the ganglion 
is occupied by a mass of neuropile made up of the arborescent 
branches springing from the processes of the ganglion cells, 
and the arborescent ends of fibres which enter the ganglion by 
the azygos nerve. As far as I have observed, only one fibre 
passes straight through this ganglion, namely, the fibre (NV) 
which springs from the bipolar ganglion-cell already described 
(fig. 2, N) in the lobster embryo, and which is also frequently 
stained in Astacus. All other fibres appear either to break up 
in the neuropile of the ganglion, or to be derived from the 
ganglion cells. 
Three pairs of lateral nerves spring from the gastric ganglion. 
These are represented in fig.2. The same figure shows several 
fibres starting from ganglion-cells and entering these nerves. 
In the adult Astacus the processes of the cells of the gastric 
ganglion are remarkable for the extraordinary richness of 
the arborescence of the branches which they give off to the 
neuropile. Figs. 6 and 7 represent two such cells with their 
processes. The cell of fig. 6 gives rise to one main fibre, which 
passes backwards to the bifurcation on the dorsal surface of 
the stomach. The figure, which is a camera drawing ren- 
dered as accurately as possible in all its details, illustrates 
not only the remarkable richness of the arborescence, but 
also the characteristic forms in which the finer branches 
terminate. 
Fig. 7 represents a cell from which two main fibres originate. 
Looking at the stomatogastric system as a whole, it is pro- 
bable that there are three main centres from which motor 
