THE NERVOUS SYSTEM. 741 



forms of activity — one a mere local response to stimuli, the 

 other a more complicated and co-ordinated action. The 

 first is presided over by the external plexus, but for its com- 

 plete accomplishment the external plexus must be intact ; a 

 connection with the gullet is unnecessary, as the action is 

 quite as efficiently performed when the ambulacral nerves 

 are severed. Over the co-ordinated action of the spines and 

 tube-feet the internal nerve-plexus presides, but connection 

 with the gullet-ring is absolutely necessary. The gullet-ring 

 is thus of great importance, but the co-ordinating action is 

 not entirely limited to it. Each ambulacral nerve can co- 

 ordinate the action of the tube-feet of its own segment, 

 when quite detached from the ring and the other ambulacral 

 nerves. This nervous system is a considerable advance on 

 that of the jelly-fish, but the centralisation is still small. 



In the Arthropods, as in the Annelids, the question of 

 the value of the supra-oesophageal ganglia has been much 

 debated. In Insects, according to Krukenberg, they are 

 not of great importance as a co-ordinating centre, many 

 complex movements being performed without the head. 

 But this argument is hardly conclusive, for a decapitated 

 tortoise may continue to walk along for several yards. The 

 respiratory movements appear to be presided over by the 

 ganglia of the abdomen ; they are still performed by 

 separated segments, though their depth or frequency is 

 often disturbed by the separation from the brain. In spite, 

 however, of the independence of the ganglia of the ventral 

 chain, the brain here, as in higher animals, directs the move- 

 ments. In the crayfish the voluntary movements and the 

 maintenance of equilibrium depend on the supra-oesophageal 

 ganglia ; the infra-cesophageal contain the centres for the 

 co-ordination of the movements of eating, and are reflex 

 centres, like all the remaining ganglia. In the crab there 

 is both morphologically and physiologically a much greater 

 amount of concentration. 



Among the Mollusca we find that in the Lamellibranchs 

 the three sets of ganglia are of nearly equal importance. 

 There is no defined central nervous system, a fact which 

 we may correlate with the sedentary habit. The motor 

 nerves to the great retractor muscles pass out from the 

 adjacent ganglia ; that is, the cerebral ganglia innervate the 



