EVOLUTION OF NERVOUS SYSTEM 87 



discharged to the exterior through the single aperture already 

 mentioned which serves thus both as mouth and anus. This 

 aperture in almost all sea-anemones is surrounded by one or 

 more circles of tentacles. 



When an expanded sea-anemone is stimulated by being 

 touched or otherwise excited to action, it commonly responds 

 by a quick general contraction, whereby the seawater contained 

 in its digestive cavity is discharged through its mouth and the 

 whole volume of the animal is greatly reduced. This general 

 contraction may be called forth from almost any point on the 

 surface of the animal, showing that the muscles that lie within 

 the walls of the creature and are responsible for the contrac- 

 tion, are collectively accessible to nervous impulses from al- 

 most every point on the surface. This accessibility is- insured 

 through the presence of a nerve-net which spreads throughout 

 the living substance of the animal and which brings its surface 

 into connection with almost its whole musculature. This 

 nerve-net nowhere shows a special concentration but extends 

 rather uniformly throughout the body and thus affords an easy 

 path over which impulses may spread from the surface of the 

 animal to its musculature. A nervous system of this type is 

 commonly called a diffuse nervous system as contrasted with a 

 centralized one and is characterized by the absence of a cen- 

 tral organ, or adjuster, through which all impulses must pass 

 on their way from the receptors to the effectors. 



The characteristics by which a diffuse nervous system may 

 be distinguished from a centralized one are well shown in a 

 number of activities exhibited by sea-anemones. When small 

 fragments of meat or other bits of food are placed on the 

 tentacles of a sea-anemone, these organs wind around the bits 

 of food and, by bending in the appropriate direction, deliver 

 them to the mouth. If, now, a distending tentacle on a quiet 

 and expanded sea-anemone is suddenly seized at its base by 

 forceps, cut off and held in position so that its original rela- 



