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- 

 ccetes. 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 (I) (first antenna?, camerostome, nose); 

 then come the prosomatic ganglia (dotted), with their nerves (A) 

 supplying the mouth parts, and the second antenna? or chelicera? ; 

 then the mesosomatic (lined horizontally), with their nerves (B) 

 supplying respiratory appendages. These figures show that the con- 

 centrated brain mass around the oesophagus of an arthropod which 

 has arrived at the stage of Astacus, is represented by the supra- 

 ossophageal ganglia and the fused prosomatic ganglia. 



The next stage in the evolution of the brain is seen in the 

 gradual in lusion 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 branchiae, 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 



