CoNGDON, Reactions to Light. 319 



toward the right or left. Animals in a healthy condition, but with head cut off, 

 showed a directive effect by an average bending of 57°. In case the initial direction 

 coincided with that of the light the deviation from a straight line dropped to 24° for 

 a normal animal and 35° for those without eyes. The conclusion is that phototaxis 

 is only in part due to the eyes. 



The amount of non-directive w andering which takes place was learned from the 

 angle obtained under vertical light. Animals with eyes wander on the average 

 27°. The bending of 78° by animals headed toward the light must be discounted by 

 this much to obtain the directive effect of light upon them. The course away from 

 the light with 24° of wandering has a turning of only 3°, due to the directive action 

 of light. The results of the comparison of the undirected with the directed course 

 of Planaria are in point with the criticism made by Torrey ('07) upon Jennings' 

 view of the manner in which animals may move forward after they have once become 

 oriented. The latter believes that a straight course may be regarded as due in 

 part at least to a lack of any stimulus of light or any other agent which would 

 tend to turn it. Torrey takes the position that the straight course may be due to 

 balanced rather than to non-stimulation. As a matter of fact, we have seen that 

 Planaria's course from the light is influenced only slightly by the light. 



Convoluta, to which Gamble and Keeble ('03, '03a) gave their attention, is a 

 sedentary planarian containing a large amount of chlorophyl. The animal gives 

 a positive response in strong illumination which is markedly greater or less depend- 

 ing upon whether the bottom of the aquarium is white or black. The conditions 

 of tonus found in this creature are peculiar, very likely because of the presence of 

 chlorophyl in its body. If kept in darkness for a while its muscles become contracted 

 and its movements sluggish. Very strong light produces a similar effect except 

 that the animal is now unusually susceptible to being broken to pieces if handled. 

 Convoluta lives within the tide lines and periodically moves to the surface of the sand. 

 The changes in tonus give Gamble and Keeble an explanation of this procedure. 



The study of planarian light reactions by Walter (07) is one of the most exten- 

 sive and many-sided contributions that have as yet appeared upon the light-reac- 

 tions of any group. 



A comparison is made between representatives of several genera in regard to 

 nine different varieties of response which he distinguishes in the animals. Diagrams 

 are given of typical paths followed by the various species if allowed to roam 

 in an aquarium until they come to rest. Walter says of these, "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." Two species of one genus show a nearer relationship 

 in behavior than do the different genera. 



Among the conditions of illumination which were applied to the worm are a 

 series of intensities of non-directive vertical light, including zero intensity, changes 

 in the strength of the entire field of non-directive light, two adjacent non-directive 

 fields of differing intensity, directive light of constant and of varying intensities. 

 Animals in the dark make many double turns which are termed " indefinite " as they 

 evidently are not of orienting value. In non-directive light they are replaced by 

 single turns. The stimulating effect of simultaneous change of intensity over the 

 entire animal varies with the rapidity of change. Decrease of illumination is more 

 of a stimulus than an increase. 



