164 THE ELEMENTAEY NERVOUS SYSTEM 



A persistence of the nychthemeral rhythm in Metrid- 

 ium after its removal from the influence of day and night 

 is apparently as little in evidence as the tidal rhythm in 

 Sagartia lucice. 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. These observations agree with those of Gee 

 (1913), who was unable to find any evidence for the per- 

 sistence 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 Bolrn and 

 by Pieron, there can be not the least question, but that 

 these rhythms may persist even for a few days in the 

 absence of the external stimulus, as maintained espe- 

 cially by Bohn, is certainly not true for Metridium mar- 

 ginatum or Sagartia lucice. The fact that a persistence 

 of rhythm in aquarium specimens has not been seen by 

 Appelloff (Retterer, 1907), Gee (1913), and others throws 

 grave 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 is reinvestigated. 



Bohn and Pieron (1906) and especially Pieron 

 (1906 b, 1908 a, 1910) have claimed that in Actinia 

 equina the tidal rhythm is carried out a little in 

 advance of the actual tidal changes, thus giving evi- 

 dence of what may be called an anticipatory reaction. 

 This reaction, according to Pieron (1910), may be lost 

 when the animals are placed in an aquarium and may be 

 regained after a week or so when they are again sub- 

 jected to the tides. Metridium has been watched very 



