56 
G. II. Parker 
other investigators, all may be classed ander either the type of 
Branchip^is or that of Astacus, Ganglia essentially like those in 
Bratichipus are to be found in Apus^ Estheria^ Argulus^ and, judging 
from the figiire given by Claus (76, PI. 26 Fig. 10), in Daphnia, 
Ganglia of tbe type in Astacus occur in Squilla according to the 
figure given by Berger (78, PL 16 Fig. 32) , in Mysis according 
to Grenacher (79, pag. 120), and, thoiigh somewhat differently 
described, in Nehalia according to Claus (88, pag. 65), the last two 
cases being confirmed also by my own nbservations. 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. Bella 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, 
while that represented in Astacus is peculiar to the malacostraca. 
Notwithstanding the fact that these conclusions are based upon the 
examination of comparatively few species, their validity seems to me 
largely assured by the fact that the species have been chosen from 
widely separated groups. 
Although not directly connected with this question, it is of 
interest to observe that the optic ganglia in hexapods äre 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 
