COMPLEX EESPONSES 165 



closely for signs of this preparatory activity, but no con- 

 clusive evidence of it has ever been observed. It is 

 astounding how quickly Metridium will begin to expand 

 on the entrance of the tide into a pool in which this sea- 

 anemone is located. With this species expansion often 

 begins within a few minutes after the arrival of the 

 first new tidal water. As already pointed out, this ex- 

 pansion appears to depend upon the movement of the 

 water and not upon its temperature or its oxygen con- 

 tent. Since the first water that enters the pool, often 

 by indirect and not easily visible channels, may cause all 

 the water in the pool to move somewhat, a stimulus im- 

 perceptible to the observer may be given to every actin- 

 ian there and thus induce expansion in what seems to be 

 an anticipatory manner, whereas in reality it is a re- 

 sponse to a direct stimulus. It is in some such way as 

 this that Pieron has probably been deceived, for Bohn 

 (1908 a) is very likely correct in questioning the exist- 

 ence of reactions really anticipatory. Such rhythms as 

 have thus far been studied in sea-anemones seem, there- 

 fore, to depend upon immediate rhythmic stimuli ex- 

 ternal in origin, as the changes of the tides or the change 

 from day to night and the reverse, and not upon rhythmic 

 organic operations of a more internal nature. 



In jellyfishes rhythmic movements are seen in the 

 pulsing of the bell. In this instance, as already pointed 

 out, the normal pulse depends very commonly on certain 

 sense organs. These are often grouped together and 

 constitute the so-called marginal bodies. There are com- 

 monly eight or more of these bodies on each bell. Their 

 complete removal, as already mentioned, is followed by 

 at least a temporary cessation of pulses. It is also well 

 known that the presence of a single marginal body is all 



