The Retina and Optic Ganglia in Decapods, especially in Astacus. 53 
itself. To carry out such a line of research to anything approaching 
completeness would, however, extend the present investigations far 
beyond what I originally set as their limits, and I content myself, 
therefore, with a few brief comparisons , in the belief that they 
will indicate the more iraportant conclusions to which such studies 
would lead. 
For raatters of comparison with the crayfìsh one of the most 
instructive crustaceans is Branchipus. The optic ganglia of this 
animai have already been carefully studied, especially by Claus (86), 
and there is little to add to his extended account of them. As can 
be seen in Figure 28 (PI. 1), the ganglionic mass in Branchipus 
consists of two portions; the distai of these is tongue-shaped (I), 
receives the retinal fibres, and gives off from its proximal face fìbres 
that connect it with the second or proximal portion; this (II) is 
considerably elongated and tapers proximally into the optic nerve 
(n.opt), These two masses are not only connected by bundles of nerve 
fibres, but they are actually confluent at their dorsal extremities [x). 
In spite of repeated attempts, I never succeeded in obtaining 
good preparations of the optic ganglia in Branchipus stained either 
with methylen blue or by the Golgi method, and I am, therefore, 
obliged to depend entirely upon ordinary preparations for the finer 
anatomy of these organs. 
The distai ganglion in Bra?ichipus has many resemblances to the 
first optic ganglion in the crayfìsh. Like the latter, it is dome- 
shaped with its convexity facing the retina. It is composed of two 
layers, a distai one of nuclei closely set and a proximal one of 
»Punktsubstanz«. In position the nuclear layer corresponds to the 
two nuclear layers and the fibrous layer in Astacus, for the single 
layer in Branchipus exiends from the distai surface of the ganglion 
to the »Punktsubstanz«. It is also penetrated by the retinal fìbres as 
the three layers in Astacus are. If these three layers in Astacus 
are equivalent to the single one in Branchipus^ the condition in the 
latter probably represents the more primitive one, an assumption 
agreeing well with what is known about the relations of these two 
crustaceans. The »Punktsubstanz« in Branchipus is divided into 
neurommatidia as in Astacus; the retinal fìbres pass into it, and 
from it emerge the fìbres that lead to the next ganglion as in 
Astacus. These facts seem to me to show that what I have called 
the distai ganglion in Branchipus is without doubt homologous with 
the fìrst optic ganglion in Astacus. 
