52 



THE TROPISMS 



tion and the nature of their locomotor organs. There are 

 comparatively few cases in which light orients an animal 

 by causing directly a greater contraction of the muscles 

 of the side most affected. The observations of Mast on the 

 planulse of Eudendrium indicate that these forms may be 

 oriented in this way. Eudendrium planulse are cone- 

 shaped organisms with the wide end in front, and having 

 the body covered by cilia by means of which they swin 

 through the water. According to Mast, "if the ray direc- 

 tion is but slightly changed after the planulse are oriented 



F IQ . 5. The water scorpion Ranatra, showing the different attitudes 

 assumed according as the light falls upon it from in front or from behind. 

 The arrows indicate the direction of the rays. 



they do not turn directly toward the source of light in its 

 new position, but merely swing the anterior end a little far- 

 ther toward it each time. In the meantime the body grad- 

 ually turns so as to become oriented again. If however the 

 direction of the rays is changed to such an extent that the 

 sides of the organism become fully exposed, they with very 

 few exceptions appear to turn toward the light at once. In 

 this process they swing the anterior end laterally until it 

 nearly if not quite faces the source of light." 

 The larvse of Arenicola according to Lillie orient them- 



