212 G. H. PARKER 



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 

 wjll remain 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 minutes 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 current was reestablished and in seven minutes all were 

 expanding, a process completed by all five in about thirteen 

 minutes. These responses were found to occur as well at 8°C. 

 as at the more usual temperature of 21°C. 



The agitation of the seawater, in a purely mechanical way 

 and without reference to oxygen and the like, appears, there- 

 fore, to be a means of inducing the expansion of Metridium, 

 especially of its oral disc. This form of reaction has already 

 been observed in Actinia by Pieron ('06 b, '08 d), who, however, 

 points out that certain forms of mechanical agitation also induce 

 retraction (Bohn, '07 a). 



Since the expansion of the oral disc is dependent chiefly upon 

 the relaxation of the sphincter muscle, it seems probable, as 

 already pointed out (Parker, '16), that the mechanical stimu- 

 lus of the moving water in one way or another has a verj'- spe- 

 cific effect on this muscle. The condition of relaxation thus 

 induced is apparently exactly like that seen in such sponges 

 as Stylotella, where the oscular sphincter remains relaxed in 

 running water but contracts when the current ceases (Parker, 

 '10). 



The foregoing account shows quite clearly that the expansion 

 and retraction of such a sea-anemone as Metridium is dependent 

 upon a variety of factors. Light and high temperature, espe- 

 cially when suddenly applied, produce retraction; food an^d water 

 currents, expansion; the oxygen supply, in Metridium at least, 



