PHYSIOLOGY OF NERVOUS SYSTEM. 735 



out through the shell to the outer nerve plexus. If any 

 spot on the outside of the shell be lightly stimulated, all the 

 spines, pedicellariffi, and tube-feet in the neighbourhood 

 bend towards the spot ; if it be more strongly irritated, the 

 spines and tube feet of the other segments come into play, 

 and by their co-ordinated activity move the animal in a 

 straight hne away from the point of injury. The spines and 

 tube feet thus exhibit two different forms of activity — one a 

 mere local response to stimuli, the other a more compli- 

 cated and co-ordinated action. The first is presided over 

 by the external plexus, but for its complete accomplishment 

 the internal plexus must be intact ; a connection with the 

 gullet ring 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 jellyfish, but the centralisation is still small. 



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

 the value of the supra-cesophageal 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 gangha of the ventral 

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

 ments. In the Crayfish, while voluntary movements and the 

 maintenance of equilibrium depend on the supra-oesophageal 

 ganglia, the infra-oesophageal contain the centres for the 

 co-ordination of the movements of eating, being reflex 

 centres, as are all the remaining ganglia. In the Crab there 



