COMPLEX RESPONSES 163 



pected to be a species that would exhibit a pronounced 

 daily or nychthemeral rhythm. On August 9th, at 10.30 

 in the morning, a large pool in full sunlight was plotted 

 and twenty large specimens of Metridium were accu- 

 rately 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 

 reidentified. All were fully expanded. A number of 

 other observations 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; com- 

 pletely retracted in the day. Observations on sea-anem- 

 ones in dimly lighted situations, such as under bridges 

 and so forth, showed that these animals 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 (1907) 

 has observed in Eloactis, and Pieron (1908 c) in Sagartia 

 troglodytes, and what has been claimed by Bohn (1906 

 fc, 1907 b) to occur in Actinia equina, though the nych- 

 themeral rhythm in this species has been questioned by 

 Pieron (1908 c, 1908 e}. That in Metridium it is depen- 

 dent upon light, as maintained in general by Bohn (1908 

 a, 1910 a), and not upon oxygen, as was claimed for other 

 species by Pieron (1908 c, 1908 e), has already been 

 shown. 



Nothing apparently has ever been observed about the 

 activities of Metridium that would lead to the supposi- 

 tion that its nychthemeral rhythm is ever reversed or is 

 ever exchanged for a tidal rhythm as has been claimed 

 for some species by Bohn (1908 6, 1909); 



