VERMES, FLY LARVAE, AND ECHINODERIIS 207 



ever also numerous head movements which bear no definite 

 relation to the location of the stimulus. 



Cole, studying the reactions of Bipalium kewense to 

 sources of light of different size, refers to the process of 

 orientation as follows (1907, p. 365): "Like most plana- 

 rians, it creeps with an even, gliding motion, the head being 

 slightly raised and waved to right and left apparently in 

 searching movements, as the worm crawls forward. . . . 

 Bipalium kewense is exceedingly sensitive to light, of even 

 a very low intensity, falling upon It from the side, and 

 responds Immediately by turning away from the light." 



Walter (1907) made an extensive study of the light reac- 

 tions of the following species: Planaria maculata; Planaria 

 gonocephala; Phagocata gracilis; Dendrocoelum lacteum; 

 and Bdelloura Candida. He found that all of these species 

 orient more or less accurately under certain conditions and 

 concluded that orientation '' Is primarily due to asymmetri- 

 cal response resulting from asymmetrical stimulation." He 

 also found that these animals frequently respond by raising 

 the anterior end and throwing It from side to side, and 

 noticed that such movements are caused either by sudden 

 increase or by sudden decrease of intensity. These move- 

 ments, however, he thinks are functional in orientation only 

 " by assisting an organism to secure asymmetrical stimula- 

 tion " (1. c, p. 153). 



I made some observations on the orienting reactions of 

 Leptoplana tremellarls and several other polyclads, all of 

 which were positive in their light reactions, and found that 

 all of these forms orient directly. There Is no evidence of 

 preliminary trial movements in the process. The lateral 

 head movements are always slight and frequently appar- 

 ently absent. When exposed to light from two sources all 

 of these forms crawl toward a point between the lights. 

 The location of the point toward which they move, however, 

 depends upon the relative Intensity of light from the two 

 sources. It is always nearer the source from which the 

 more intense light comes. This indicates that orientation 



