54 
G. E. Parker 
The homologies of the proximal ganglion in BrancJiipus are, 
perhaps, not so easily determined. This ganglion may be said to 
he composed of three layers: on its ventral face is a layer of nerve 
fibres (Fig. 28 /Z»r.^) derived in part at least from the first ganglion, 
next to this dorsally is a layer of »Punktsubstanz« (II), and finally 
on its dorsal side is a layer of ganglionic cells [d.gn). In regarding 
this ganglion from a comparative Standpoint, the question arises: does 
it correspond to some one of the remaining optic ganglia in Astacus 
or to several of them united? An answer to this question could. of 
course, be found if the exact extent of the neurons were known in 
BrancJiipus. Unfortunately , I have been unable to determine this, 
but there is still other evidence, beside that derived from Golgi or 
methylen blue preparations, that may lead to a satisfactory conclusioD. 
In Branchipus there are, as I have just mentioned, two masses of 
)) Punktsubstanz«, that in the first ganglion and that in the one in 
question. These masses, judging from the conditions in Astacus^ 
must represent the termination of one system of neurons and the 
beginning of another, and it is, therefore, probable that in passing 
from the retina to the brain in Branchipus three sets of neurons 
would be traversed. An inspection of figure 28 (PI. 1) will show 
rather clearly the extent these three neurons must have. The neurons 
of the first order would be represented by the retinular cells and 
their processes, the retinal fibres, which together reach from the 
retina to the «Punktsubstanz« of the first ganglion. The neurons of the 
second order would reach from the »Punktsubstanz« of the first 
ganglion to that of the second one. As the most ventral of these 
terminate at the proximal end of the « Punktsubstanz « of the second 
ganglion, one is justified, I believe, in assuming only another set of 
neurons to reach to the brain. These neurons would be represented 
by the fibres of the optic nerve and would constitute a third order. 
In Branchipus^ therefore, only three neurons are necessarily con- 
cerned in connecting retina with brain. These observations lead 
me, therefore, to conclude that the whole of what 1 have called the 
proximal ganglion in Branchipus is homologous with the second optic 
ganglion in Astacus and that the tbird and fourth optic gauglia in 
the crayfish are not represented in Branchipus. 
Although the first and second optic ganglia in Branchipus 
show many points of resemblance to the homologous structures in 
Astacus^ there are, nevertheless, some important differences. It will 
be remembered that in Astacus a decussation occurs among the 
