THE NERVOUS SYSTEM. 777 



intervals, causes the regular contractions. This view seems 

 to minimise unduly the function of nerve cells, but yet it is 

 to be noticed that the destruction of the nerve-centres in 

 Aurelia does not permanently arrest the movements. 



In Beroe, representing the Ctenophora, it has been 

 observed that the sense organ, which is placed at the aboral 

 pole, has to do with the movements. In contradistinction 

 to the conditions found in the Medusae, we find that special 

 parts of the central nervous system preside over special 

 areas of the organism. This is a distinct advance in the 

 direction of division of labour, and recalls the state of 

 affairs in higher forms, where clusters of brain cells form 

 what are called centres, which preside over particular organs. 

 It is of interest to note that in the Ctenophora the move- 

 ment is due to cilia, as contrasted with the muscular 

 movement of other Coelentera. 



Little is known of the nerve physiology of the members 

 of the very heterogeneous group of " Worms." It is said 

 that a decapitated earthworm can regenerate the anterior 

 end with its cerebral ganglia. This would seem to indicate 

 that there is little centralisation of the nervous system, and 

 that the ganglia are all of nearly equal physiological import- 

 ance. It seems more likely, however, that in, at any rate, 

 most Annelids, the so-called "brain" does perform to some 

 extent the function of a central nervous system, although 

 the centralisation is only partial. In Lumbricus sensory 

 and motor nerve-fibres are differentiated. 



The nerve physiology of the Echinoderms has been very 

 fully worked out, except in the case of the Holothurians. 



As a type we may take the sea-urchin. Here the ring 

 round the mouth has a co-ordinating function ; only when it 

 is intact do the segments of the body act in unison. The 

 ambulacral nerves branch freely to form the inner nerve- 

 plexus ; from this nerves pass out through the shell to the 

 outer nerve-plexus. If any spot on the outside of the shell 

 be lightly stimulated, all the spines, pedicellariae, 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 line away from the point of 

 injury. The spines and tube-feet thus exhibit two different 



