THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 53 
phageal ganglia of the cray-fish have been shown to be constituted 
of six prosomatic ganglia. 
In Fig. 25 I give figures of the central nervous system (with the 
exception of the abdominal or metasomatic ganglia) of Branchipus, 
Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammo- 
ecetes. In all the figures the supra-cesophageal ganglia are lined 
horizontally, and their nerves shown, viz. optic (lateral eyes (II) and 
median eyes (II’)), olfactory (1) (first antennze, camerostome, nose) ; 
then come the prosomatic ganglia (dotted), with their nerves (A) 
supplying the mouth parts, and the second antenna or cheliceree ; 
then the mesosomatic (lined horizontally), with their nerves (B) 
supplying respiratory appendages. These figures show that the con- 
centrated brain mass around the cesophagus of an arthropod which 
has arrived at the stage of Astacus, is represented by the supra- 
cesophageal ganglia and the fused prosomatic ganglia. 
The next stage in the evolution of the brain is seen in the 
gradual inelusion of the mesosomatic ganglia, one after the other, 
into the infra-cesophageal mass of the already fused prosomatic 
ganglia. With this fusion is associated the loss of locomotion in 
these mesosomatic appendages, and their entire subservience to the 
function of respiration. Dana urges that cephalization is a conse- 
quence of functional alteration in the appendages, from organs of 
locomotion to those of mastication and respiration. Whether this be 
true or not, it is certainly a fact that in Limulus,.the ganglion 
supplying the first mesosomatic appendage has fused with the 
prosomatic, infra-cesophageal mass. It is also a fact that the proso- 
matic appendages are the organs of mastication, their basal parts 
being arranged round the mouth so as to act as foot-jaws, while the 
mesosomatic appendages, though still free to move, have been 
reduced to such an extent as to consist mainly of their basal parts, 
which are all respiratory in function, except in the case of the first 
pair, where they carry the terminal ducts of the genital organs. In 
the next stage, that, of the scorpion, in which the mesosomatic 
appendages have lost all power of free locomotion, and have become 
internal branchie, another mesosomatic ganglion has fused with the 
brain mass, while in Androctonus two of the branchial mesosomatic 
ganglia have fused; and finally, in Thelyphonus and Phrynus, all 
the mesosomatic ganglia have coalesced with the fused prosomatic 
ganglia, while the metasomatic ganglia have themselves fused 
