734 COMPARATIVE PHYSIOLOGY. 



view is that the regular contractions are due to the activity 

 of the muscles themselves. On this hypothesis, building-up 

 processes go on in the muscles until extremely unstable 

 substances are produced ; these explode and break down 

 into simpler compounds, the process being accompanied 

 by an evolution of energy manifested by the contraction 

 of the muscle. The process, repeated at regular intervals, 

 causes the regular contractions. But this view seems to 

 minimise unduly the function of nerve cells. 



In Beroii, representing the Ctenophora, we can only 

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

 pole, has to do with the movements. In contra-distinction 

 to the conditions found in the Medusa;, 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. 



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 physio- 

 logical importance. 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 

 LuinbricHs, 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. 

 In the starfish, the nervous system consists of a rin^ round 

 the mouth, from which nerves pass out to the rays! giving 

 off branches to the tube feet. The whole surface' of the 

 body is sensitive to stimuli. The ring round the mouth 

 co-ordinates the action of the different rays ; if it is severed 

 the rays lose their power of acting in concert. 



In Echinus also 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 



