BEHAVIOR OF LOWER INVERTEBRATES 393 



this causes the animal to swim upward unless the load is too 

 heavy. As the particles of iion become more evenly distributed 

 through the body the negative geotropism tends to disappear. 

 A strong magnet placed at one side of a dish containing Para- 

 mecia which have ingested iron causes the animals to swim 

 upwards until they reach a weaker part of the magnetic field. 



Henri and Henri (15) and Henri and Larguier des Bancels 

 (16) (17) in several papers have given an account of their ex- 

 periments upon the reactions of Cyclops to ultra-violet rays. 

 These rays stimulate the animals to rapid activity, but if the 

 duration of the stimulus is too short no reaction is pioduced. 

 When the stimulus is of longer duration the Cyclops respond 

 unifoimly after a certain interval of time, which remains re- 

 markably constant for many successive experiments. The 

 latent period is decreased as the intensity or. the stimulus is 

 increased. Starting with a stimulus which does not last long 

 enough to evoke a reaction, it is found that if a number of stim- 

 uli are applied in rapid succession, the effect is the same as if 

 the stimulus were applied continuously throughout a period 

 equal in length to the sum of the partial stimulations. If the 

 stimuli are separated by longer intervals, there is a partial sum- 

 mation of the effects, which diminishes with the increase of the 

 interval until it is no longer apparent. The authors discuss the 

 relationship of these results to certain phenomena of memory 

 in man. 



The sea-urchin Arbacia punctulata was found by Holmes (r8) 

 to react negatively to light of a great range of intensity, although 

 it is occasionally positive in w r eak light. After it has ceased to 

 respond it may be made to resume its phototactic movements by 

 mechanical disturbance. Movements are effected by the combined 

 action of the spines and tube feet, although the animal may 

 crawl away from the light when either of these sets of organs 

 has been removed. Arbacia responds to shadows by erecting 

 the spines, while local stimulation by light causes the spines to 

 bend towards the stimulated region. Strong light thrown upon 

 the tube feet causes them to be withdrawn. Phototaxis is not 

 dependent upon stimulation of both sides of the body. A sea- 

 urchin which is stimulated only by a small spot of light will 

 crawl away from the course of stimulus in a fairly direct line. 

 Cutting the oral nerve ring, while not interfering with the 



