56 
G. H. Parker 
other investigators, all may be classed under either the type of 
Branchipus or that of Astacus, Ganglia esseotially like those in 
Branchipus are to be found in Apus, Estheria^ Argulus^ and, judging 
from the figure given by Claus (76, PI. 26 Fig. 10), in Daphnia, 
Ganglia of the type in Astacus occur in Squilla aecording to the 
figure given by Bergee (78, PI. 16 Fig. 32) , in Mysis aecording 
to Grenacher (79, pag. 120), and, though somewhat differently 
described, in Nehalia aecording to Claus (88, pag. 65), the last two 
cases being confirmed also by my own observations. Ganglia of 
this type are likewise found in the decapods Palaemon and Homarus 
and in the isopod Anilocra, The optic ganglia in the amphipods 
are so crowded together that they are extremely unfavorable for 
study. Della Valle (93) , in his exhaustive account of the Gammarini, 
does not even give the number of ganglia present, nor have I been 
able to determine it. 
Meagre as these observations are, it will be seen at a glance 
that so far as present Information extends, the type represented by 
the optic ganglia in Branchipus is characteristically entomostracan, 
vvhile that represented in Astacus is peeuliar to the malacostraca. 
Notwithstanding the fact that these conclusions are based upon the 
examination of comparatively few Speeles, their validity seems to me 
largely assured by the fact that the species have been chosen from 
V7Ìdely separated groups. 
Although not directly connected vrith this question, it is of 
interest to observe that the optic ganglia in hexapods are almost 
identical in their structural features with those in Astacus, the simi- 
larity being so striking that one is tempted to hypothecise a direct 
derivation of the ganglia in insects from those in crustaceans, though, 
as will be seen from the next section, this resemblance may also be 
explained by assuming that the requirements of growth for these 
nervous structures have been similar in the two groups of organisms. 
7. Growth of Retina, Ganglia, and Optic Nerve. 
Several investigators, notably among them Claus, have suggested 
that the anatomical complications of the optic ganglia in the higher 
crustaceans were to be explained as a mechanical necessity of the 
method of growth in these structures; but no one, so far as I am 
aware , has attempted to formulate the exact Steps by which this 
