The Reactions of Planarians to Light 105 



toward than away from the source of the light, but very soon wig- 

 wagging motions set in, and after every exercise of these move- 

 ments, which were apparently in the nature of explorations, a 

 change in the direction of the course was effected. As might be 

 expected, such abrupt changes in direction were more difficult of 

 execution when the worm was on the surface-film. 



Characteristic movements of individuals of the genus Planaria 

 are shown in Figs. 13 and 14. From these two typical records it 

 would be difficult to select any diagnostic points which would dis- 

 tinguish the behavior of P. gonocephala from that of P. maculata. 

 There is no doubt, however, that taken together the behavior of 

 representatives of these two species presents a distinct (generic) dif- 

 ference from that of the representatives of the other genera studied. 

 The most striking feature of the Planaria records (Figs. 13 and 14) 

 is the high degree of response exhibited by members of this genus 

 to the directive action of light. Although many attempts were 

 made by the individual worms to penetrate the half of the aqua- 

 rium nearer the light, yet they seemed as unable to keep to that 

 direction as they would have been had a solid barrier been inter- 

 posed between them and the light. This characteristic respon- 

 siveness to directive light helps to explain why (as shown in Fig. 8, 

 B, p. 84) P. gonocephala was unable to come] to rest in the area of 

 lessened illumination as it would naturally have been expected to do. 



From the cases cited in this section, at least, it may be affirmed 

 that the generic differences are so pronounced that one could 

 take a miscellaneous, unidentified assortment of such records 

 and correctly assign the great majority of them to the proper 

 genera. 



Duration of Movement. When worms of different genera are 

 subjected to the same light intensity there is considerable variation 

 in the time required to bring them to a standstill. Bdelloura is 

 usually the first to stop, followed in order by Dendroccelum, Pla- 

 naria and Phagocata. Of the two species of Planaria, P. gono- 

 cephala, although averaging somewhat smaller in size, usually 

 keeps in motion for a longer time than P. maculata. The individ- 

 ual records of the duration of movement given in Figs. 10-14 

 may be taken as a typical set of records. They were as follows: 



