ANATOMY OF THE NERVE CENTRES. 443 
between the brain in the Orthoptera and in the Decapoda, con- 
tents himself with generalisations, and does not descend to 
particulars; he says: ‘The cerebroid ganglion, or brain, is con- 
stituted on the same plan in the Macroura and the Brachyura, 
and this is analogous to that described by various authors in the 
Insecta.’ 
It has long been known that the ganglia, from which the 
nerves of the antennules spring in the Crustacea, corres- 
pond with those of the antennz in insects, and there is 
not the slightest difficulty in recognising the identity of the 
optic ganglia in the two classes; but, beyond these points, our 
FG. 53.—The brain of the Crayfish: 7. Seen from above ; 2. Seen from the side. 
a, olfactory ganglion ; a’, nerve to antennule ; a, ganglion of the crus; a’, nerve 
to great antenna ; ¢. a, ¢. a, esophageal connectives ; #, 7’, mesocerebron ; ¢h, 
thalamon ; #7, m/, metacerebron ; 0, optic peduncle. 
knowledge of the actual structure of the brain in insects has 
been too incomplete to permit any accurate comparison. 
I have taken the supra-cesophageal ganglia of the Crayfish 
(Astacus) asa fair type of the preoral centres of the generalised 
Arthropod. In the dorsal view (Fig. 53, 7) three pairs of 
ganglia are recognisable in the central brain mass; three pairs 
of sensory ganglia also exist, connected with the eyes, anten- 
nules and antennz. So that there are six pairs of ganglia. 
Three pairs which belong to the brain proper, the central 
ganglia, and three to the sensory organs, sensory ganglia. 
The anterior and posterior pairs of central ganglia each con- 
sist of a single reticular nucleus, but the lateral pair are more 
complex ; each contains two reticular nuclei, separated by a 
