BEHAVIOR IN LOWER METAZOA 



243 



by Pearl (1903), may be summed up under the same formula set forth 

 in the preceding paragraph. Varied movements which subject the ani- 

 mal to many different conditions, are seen even in the unstimulated 

 specimen. As the flatworm glides along by means of its cilia, the head 

 is held upward (Fig. 135) and moved frequently from side to side, while 

 its margins wave up and 

 down, and are extended 

 and contracted. The flat- 

 worm thus seems to "feel 

 its way" with its head. 



Sometimes these feeling FIG. 135.- Side view of moving Planaria. After 



movements become much Pearl. A, body; B, mucus; C, cilia; D, substratum. 



accentuated, the animal almost or quite stopping, then raising the 

 whole anterior part of the body and waving it about in the water. 

 These movements of course serve to test the environment on each side; 

 in other words, they subject the sensitive anterior end to varied con- 

 ditions. 



The testing movements are specially marked under certain condi- 

 tions. When the active planarian is about to come to rest, it stops and 

 moves the anterior end from side to side, touching any object that may 

 be found in the neighborhood. After thus thoroughly testing the sur- 

 roundings, the muscles relax and the animal comes to rest. When later 

 the animal resumes its active progression, this begins again with the 

 testing movements of the head. 



The same testing movements are seen under various sorts of stimu- 

 lation. On coming to a solid body, the flatworm moves the head 

 about over its surface. If it turns out to be some- 

 thing fit for food, the animal now feeds upon it, other- 

 wise it moves away again. If while a number of 

 specimens of Planaria are moving in a certain direc- 

 tion, the direction of the light is changed so as to fall 

 upon their anterior ends, they usually turn the head 

 from side to side two or three times, then follow up 

 one of these movements by turning the body till it is 

 finally directed away from the light. These testing 

 movements are also seen when the animal begins to 

 FIG 136 Re- d r y> an ^ when the water is heated ; the worm gives the 

 action of Planaria to impression that it is seeking about for other conditions. 

 Other features of the reactions to drying and to 

 temperature changes are of interest from our present standpoint. If 

 the planarian is laid on a glass plate, as soon as the tendency to dry 

 becomes evident the worm curls up closely and thrusts the head under 



