REACTIONS TO LIGHT IN THE EARTHWORM. 33 



when extended, owing to the fact that when extended the sensi- 

 tive elements are spread out over a greater surface and become 

 more susceptible. 



3. In locomotion, as there are alternate extensions and con- 

 tractions, there is an alternation of the condition of lower and of 

 higher sensibility. This is important particularly in the sensitive 

 anterior end. 



4. As the worm begins each extension in a condition of 

 lower sensibility, it may project its anterior end toward the source 

 of light. This movement is checked as soon as the increased 

 sensibility of the extended anterior end appreciates the stimulus. 

 Movements away from the light do not meet such a check and 

 so are prolonged farther. Orientation is the result of a trial and 

 error method. 



5. In strong enough light, random movements toward the 

 light are suppressed altogether, and the worm appears to move 

 djrectly away from the light without noticeable trial movements. 

 This applies to worms which have been kept in the dark and are 

 in a perfectly fresh condition, as after a time they lose their dis- 

 crimination and begin to make random movements. 



6. Movements in the longitudinal direction are typical of the 

 normal burrow life of the animal, and the axial movements 

 initiated by the anterior and posterior ends are more definitely 

 controlled by the stimulation of light and by a weaker stimulus 

 than are the lateral movements. Lateral movements tend more 

 to be random and are directed only by stronger stimuli because 

 the organization of the worm is chiefly in adaptation to a burrow- 

 ing life and not to an open air life. 



7. The characteristic leaping motion of Perichcefa is a con- 

 spicuous example of random lateral movements, adapted to life 

 outside of the burrow. All gradations may be observed between 

 the ordinary reaction to a slight local stimulus by jerking back, 

 and also bending the body away, if the stimulus be stronger, up 

 to a complete series of leaping movements. 



8. Reactions to mechanical stimuli, as well as to other stimuli, 

 chemical, thermal and electric, show that the worm is like other 

 animals as highly organized as itself in responding to a local 

 stimulus of an injurious nature by contracting and bending away 



