214 G. H. PARKER 



In this respect my observations agi'ee with those of Gee ('13, p. 

 310) on Cribrina, where no trace of the persistence of tidal rhythm 

 could be discovered. 



Metridium marginatmn is almost always under water and is 

 so responsive to light that it might w^ell be suspected to be a 

 species that would exhibit a pronounced daily or nychthemeral 

 rhythm. On August 9 at 10.30 in the morning a large pool in 

 full sunlight was plotted and twenty large specimens of Metri- 

 dium were accurately located. All were fully retracted. At 10 

 o'clock on the evening of the same day, the sky being overcast 

 with clouds and the night dark, the pool was again visited and 

 by means of a hand light the twenty sea-anemones were reiden- 

 tified. All were fully expanded. A number of other observa- 

 tions of this kind and many casual records were made of the 

 condition of pool animals in daytime and at night, and always 

 with the same results; the sea-anemones were fully expanded at 

 night and partly or completely retracted in the day. Observa- 

 tions on animals located under bridges and in other dark situ- 

 ations showed that they were more or less continuously expanded, 

 but aside from such exceptions it was clear that Metridium in 

 its natural surroundings exhibited a well marked nychthemeral 

 rhythm. 



This form of rhythm agrees with what Hargitt ('07) has 

 observed in Eloactis, and Pieron ('08 c) in Sagartia troglodytes, 

 and what has been claimed by Bohn ('06 b, '07 b) to occur in 

 Actinia equina, though the nychthemeral rhythm in this species 

 has been questioned by Pieron ('08 c, '08 e). That in Metridium 

 it is dependent upon light, as maintained in general by Bohn 

 ('08 a, '10 a), and not upon oxygen, as was claimed for other 

 species by Pieron ('08 c, '08 e), has already been shown in an 

 earlier part of this paper. 



I have never observed anything about the activities of Me- 

 tridium that would lead me to suppose that its nychthemeral 

 rhythm is ever reversed or is ever exchanged for a tidal rhythm 

 as has been claimed for some species by Bohn ('08 b, '09 b). 



A persistence of the nychthemeral rhythm in Metridium after 

 its removal from the influence of day and night is apparently 



