60 THE ELEMENTARY NERVOUS SYSTEM 



animals have been much more fully studied than the 

 lower ones, examples of this rather novel condition are 

 better known among these more differentiated types than 

 among the simpler forms. It is probable that a close ex- 

 amination of the lower invertebrates will disclose many 

 such instances. Thus in the sea-anemone Metridium the 

 circular muscle of the column is very much like that in the 

 vertebrate iris in that it is both under nervous control and 

 open to direct stimulation. This sea-anemone, like most 

 other members of its group, is a somewhat cylindrical 

 animal, one end of which is attached to some firm object 

 in the sea and the other provided with a ring of tentacles 

 that surrounds a centrally located mouth. The circular 

 muscle of the column is a sheet of muscle whose fibers in- 

 vest in a circular fashion the inner face of the cylindrical 

 outer wall of this animal (Parker and Titus, 1916). If 

 this muscle contracts locally it produces a ring-like con- 

 striction around the animal's body. In sea-anemones 

 that have been well fed recently, constrictions of this kind 

 form near the oral end of the animal and pass down the 

 cylindrical column of its body to the opposite end. They 

 may recur every four or five minutes and resemble in a 

 general way the peristaltic waves of the intestine. They 

 are probably concerned with the movement within of the 

 newly acquired food, for they occur almost invariably 

 after extensive feeding. It is probable, though this is not 

 definitely proved, that the peristalsis just described is 

 coordinated by the nerve-net contained within the wall of 

 the sea-anemone 's body. 



If an area on the outer surface of this wall is thor- 

 oughly anesthetized by allowing crystals of magnesium 

 sulphate to dissolve on it, it can be easily rendered so in- 

 sensitive to touch that the typical contraction of the ani- 



