160 THE ELEMENTARY NERVOUS SYSTEM 



on this conclusion several vessels were filled with tidal 

 water and after it had come to rest stones carrying 

 Metridium were introduced into it. Although this quiet 

 tidal water retained its characteristically lower tempera- 

 ture and its higher oxygen content, the sea-anemones re- 

 mained closed in it, thus confirming the conclusion al- 

 ready expressed that motion is the element in tidal water 

 that induces expansion. 



The effect of water currents and other forms of agi- 

 tation were not only observed under natural conditions 

 but were tested likewise in the laboratory. If a Metrid- 

 ium is put in a darkened vessel through which seawater 

 is running, it quickly assumes a condition of maximum 

 expansion both as to its oral disc and its column. If, now, 

 the current is shut off, in about a quarter of an hour the 

 oral disc will be found covered, but the column will re- 

 main more or less elongated. The same was found true 

 of groups of Metridium on stones. Five, in one group, 

 were made to expand fully in running seawater in the 

 dark. The current was then cut off and in eighteen min- 

 utes the oral discs of all five specimens were covered and 

 some of the animals a little shortened. An hour and a 

 half after the current had been stopped all were still 

 closed except one which had partly expanded its oral disc. 

 Still an hour later all were retracted, whereupon the cur- 

 rent was reestablished and in seven minutes all were ex- 

 panding, a, process completed by all five in about thirteen 

 minutes. These responses were found to occur as well at 

 8 degrees centigrade as at the more usual temperature of 

 21 degrees centigrade. 



The agitation of the seawater, in a purely mechanical 

 way and without reference to oxygen and the like, ap- 

 pears, therefore, to be a means of inducing the expansion 



