OBSERVATIONS ON UNICELLULAR FORMS 87 



which the Hght intensity from it is much lower than that 

 from the stationary glower to a position in which it is much 

 higher. Under such conditions one can clearly see these 

 organisms, especially the free-swimming forms, gradually 

 change their direction of motion through an angle of nearly 

 90°. (Just how this change is brought about will be 

 demonstrated later.) By regulating the relative intensity 

 of the light from the two sources, it is thus possible to 

 cause Euglenae to move toward any point between the two 

 sources of light without changing the direction of the rays. 

 It is evident then that the direction of the rays does not 

 absolutely control the direction of motion. These results are 

 in harmony with those which I obtained in experiments on 

 Volvox (1907, p. 134). Identical results were also obtained 

 in light from two sources with Stentor coeruleus, Trachelo- 

 monas, Chlamydomonas, Oedogonium swarm-spores, Eu- 

 dorina, Pandorina, Planulae of Eudendrium, Limulus 

 polyphemus larvae, Musca larvae, Allolobophora foetida, 

 medusae of Bougainvillea superciliaris, trochophores of 

 Hydroides dianthus, Arenicola larvae, zoeae, several forms, 

 and Leptoplana tremellaris. Judging from these results 

 it is highly probable that all individuals without image- 

 forming eyes orient in the same way under like conditions. 

 All of these forms can be induced to change their direc- 

 tion of motion by varying the relative light intensity on 

 opposite sides of the body, or by changing the intensity 

 on the same side, without changing the direction of the rays. 

 It may therefore be concluded that difference in the inten- 

 sity of light on opposite sides, or a change of intensity on 

 the same side of the body of all these creatures, may deter- 

 mine orientation independently of the direction of the 

 rays. The orientation of organisms without image-form- 

 ing eyes can therefore not be explained by the application 

 of Sachs' ray direction theory, nor are the orienting reac- 

 tions in harmony with the statements of Loeb expressed 

 in the following quotations: (1905, p. 2), " It is explicitly 

 stated in this and the following papers that if there are 



