162 THE ELEMENTARY NERVOUS SYSTEM 



Metridium marginatum is found commonly either be- 

 low 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 lucice 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 maintained by 

 Bohn for Actinia, stones covered with Sagartia lucite 

 were transferred to an aquarium and record was kept of 

 the subsequent condition of these actinians. The 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 con- 

 ditions were repeatedly observed and it, therefore, seems 

 quite certain that in Sagartia lucicc there is no persistence 

 of a tidal rhythm. In this respect these observations 

 agree with those of Gee (1913) on Cribrina, where no 

 trace of the persistence of tidal rhythm could be 

 discovered. 



Metridium marginatum is almost always under water 

 and is so responsive to light that it might well be sus- 





