100 THE ELEMENTARY NERVOUS SYSTEM 



mals. On applying suck a rod to the pedal edge of the 

 column of a sea-anemone the oral disc as a whole is de- 

 pressed, an operation that involves the simultaneous ac- 

 tion, as far as can be seen, of all the longitudinal muscles 

 in the numerous complete and incomplete mesenteries of 

 the animal. This means that scores and perhaps hun- 

 dreds of these muscles act in unison. Is the nerve-net 

 capable of only this gross form of activity or does it rep- 

 resent a system of finer gradations by which under an 

 appropriate stimulus a part of this musculature can be 

 excited to specific action while the rest remains essen- 

 tially quiescent? 



An answer to this question can be seen in the follow- 

 ing experiment. If a Metridium is allowed to remain for 

 some time in running seawater in a situation relatively 

 dark, its muscular tonus will be reduced to a minimum and 

 it will assume the condition of fullest normal expansion. 

 If, under such circumstances, it is generally and briefly 

 illuminated, it will quickly shorten its length quite notice- 

 ably though it will by no means go into what would be 

 described as a state of contraction. This shortening of 

 the animal as a whole is due to the simultaneous moderate 

 contraction of its longitudinal mesenteric muscles. The 

 fact that the shortening is symmetrical and uniform shows 

 that a complete ring of these muscles have contracted in 

 unison. If, instead of subjecting the fully expanded sea- 

 anemone to a general illumination, light is thrown on only 

 one of its sides, it responds usually by turning its oral disc 

 toward the light, precisely as some flowers come to face 

 the light. In the sea-anemone this is due to the contraction 

 of those longitudinal mesenteric muscles that lie on the 

 side illuminated, in consequence of which that side 

 shortens and the oral disc is tilted in that direction. Thus 



