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 antennae 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 



CM. 



FIG. 53. The brain of the Crayfish : /. 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 ; c. a\ c. <r, oesophageal connectives ; nt, /;/', mesocerebron ; th, 

 thalamon ; it, mt, metacerebron ; o, 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) as a fair type of the preoral centres of the generalised 

 Arthropod. In the dorsal view (Fig. 53, /) 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 antennae. 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 



