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 
planule of Eudendrium indicate that these forms may be 
oriented in this way. Eudendrium planule are cone- 
shaped organisms with the wide end in front, and having 
the body covered by cilia by means of which they swim 
through the water. According to Mast, “if the ray direc- 
tion is but slightly changed after the planule are oriented 

Fra. 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 larve of Arenicola according to Lillie orient them- 
