382 Physiology of the Nervous System of the Crayfish. [Mar. 6, 



(b.) That on the presence of the supra-cesophageal ganglion depend 

 (1) the spontaneous activity of the animal as a whole, or what might 

 be called its volitional activity ; (2) the power to inhibit the aimless 

 and wasteful mechanical activity of the lower centres ; (3) the 

 power to maintain equilibrium ; and (4) the use of the abdomen in 

 swimming. 



(c.) That the sub-cesophageal ganglia are the centres for co- 

 ordinating (1) the locomotive* and (2) the feeding movements, and 

 (3) for the rhythmic swing described under II. (The stilted gait in 

 II and the vigour of the posterior maxillipedes in III, the limbs 

 connected with the other centres being then disabled for locomotion, 

 seem to show that the sub-cesophageal ganglion is the source of a 

 considerable amount of motor energy.) 



(d.) That there is much less solidarity, a much less perfect con- 

 sensus, among the nervous centres in the crayfish than in animals 

 higher in the scale. The brainless frog, e.g., is motionless except when 

 stimulated, and even then does nothing to suggest that its members 

 have a life on their own account ; whereas the limbs of a crayfish 

 deprived of its first two ganglia, are almost incessantly preening, and 

 when feeding movements are started, the chelate legs rob, and play 

 at cross purposes with, each other as well as four distinct individuals 

 could do. 



(e.) That some stimulus from other centres is more or less necessary 

 to the activity of any given centre. This conclusion is rendered, at all 

 events, probable (1) by a comparison of the activity of the antennae 

 and eye-stalks in I, II, and III ; (2) by the diminution in the spon- 

 taneous feeding movements in III ; and (3) by the simultaneous 

 increase in the preening movements — the excitations from the tail-fin 

 region having no longer a counterpoise. 



(/.) The " natural " discharge of a ganglionic centre (not ex- 

 hibiting li volition ") appears to be of a rhythmic kind ; the rhythmic 

 movements becoming converted into varied movements by temporary 

 augmentation or inhibition. f 



It remaius to mention one or two outlying points. There is much 

 in the action and inaction of the mandibles, to suggest very consider- 

 able independence between the centre for their movements and that for 

 the movements of the maxillipedes — which last is doubtless situated in 

 the sub-cesophageal ganglion. Thus the mandibles in several cases lost 

 the power to move while the maxillipedes continued unaffected, and 



* In further proof of this position it may be added that, when the commissures 

 are divided behind the second thoracic ganglia, the animal crawls with extreme 

 difficulty by alternate advances of the chelse alone ; and that when they are divided 

 behind the third it walks by alternate advances both of the chelae and the first pair 

 of legs : the other legs in each case being rucked together in confusion. 



f Is there such a rhythm at the bottom of " volitional " movements ? 



