ACTINIAN BEHAVIOR 213 



seems to have very little if any direct influence on retraction 

 and expansion. 



These operations in sea-anemones have been regarded by some 

 investigators, notably Bohn and Pieron, as occurring in rhythmic 

 fashion, and two types of rhythm have been distinguished; a 

 tidal rhythm and a daily or nychthemeral rhythm. According 

 to Bohn ('06 b, '09 b, '10 b) Actinia equina retracts when it is 

 exposed to air by the falling tide and expands when it is again 

 covered by water. This rhythm may be retained for from 3 

 to 8 days in an aquarium though the animals under such con- 

 ditions are always under water. Pieron ('08 c) on the contrary 

 questions the presence of a pronounced tidal rhythm in Actinia 

 equina. 



Metridium marginatum is found commonly either below low- 

 water or in pools that do not empty on the falling of the tide. 

 When exposed to the air it usually retracts though this is not 

 invariable. This species, partly from the situations in which it 

 is found and partly from its irregularity of response, is not a 

 very favorable one in which to seek evidence of tidal rhythm. 



In this respect Sagartia luciae is very much more promising. 

 This species attaches itself to stones, shells, and other fixed 

 objects that are commonly exposed to air by the falling tide. 

 When thus exposed this species is very regularly retracted, and 

 when covered with water it is expanded though not invariably 

 so. To ascertain whether this rhythm would persist, as main- 

 tained by Bohn for Actinia, I transferred at various times to 

 an aquarium stones covered with Sagartia luciae and kept records 

 of their subsequent conditions. My results were quite uniform 

 and may be well illustrated by a single example. On July 7 at 

 11.00 in the morning a stone that had been exposed by the 

 tide for some hours and that had upon it twelve contracted 

 Sagartia was transferred to an aquarium. At half past eleven 

 all the sea-anemones had expanded and they remained so for 

 the next thirty hours, after which they began to close irregularly. 

 Similar conditions were repeatedly observed and I am quite sure 

 that in Sagartia luciae there is no persistence of a tidal rhythm. 



