ACTINIAN BEHAVIOR 215 



as little in evidence as the tidal rhythm in Sagartia luciae. 

 Specimens of Metridium in a retracted condition were removed 

 from a quiet pool at noon and placed in running water in the 

 dark. In less than an hour all were fully expanded and remained 

 so for over 36 hours, Tljese observations agree with those of 

 Gee ('13, p. 310), who was unable to find any evidence for the 

 persistence of the nychthemeral rhythm in Cribrina. 



That sea-anemones may exhibit in the sequence of their states 

 of expansion and retraction a tidal rhythm or a nychthemeral 

 rhythm, as pointed out by Bohn and by Pieron, there can be 

 not the least question, bui; that these rhj^thms may persist even 

 for a few days in the absence of the external stimulus, as main- 

 tained especially by Bohn, is certainly not true for Metridium 

 marginatum nor Sagartia luciae. The fact that a persistence 

 of rhythm in aquarium specimen has not been seen by Appellof 

 (Retterer, '07), Gee ('13), and others throws great doubt on the 

 occurrence of this phenomenon at all, but a decisive answer to 

 this question can not be given till the species for which these 

 peculiarities have been claimed are reinvestigated. 



Bohn and Pieron ('06), and especially Pieron ('06 b, '08 a, 

 '10) have claimed that in Actinia equina the tidal rhythm is 

 carried out a little in advance of the actual tidal changes, thus 

 giving evidence of what may be called an anticipatory reaction. 

 This reaction, according to Pieron ('10), may be lost when the 

 animals are placed in an aquarium and may be regained after a 

 week or so when they are again subjected to the tides. I have 

 watched Metridium very closely for signs of this preparatory 

 activity, but I have never seen any conclusive evidence of it. 

 It is astounding how quickly Metridium will begin to expand on 

 the entrance of the tide into a pool in which this sea-anemone is 

 located. With this species expansion often begins within a few 

 minutes after the arrival of the first new tidal water. As already 

 pointed out I believe this expansion to depend upon the move- 

 ment of the water and not upon its temperature or its oxygen 

 content. Since the first water that enters the pool, often by 

 indirect and not easily visible channels, may cause all the water 

 in the pool to move somewhat, a stimulus imperceptible to the 



