286 LIGHT AND THE BEHAVIOR OF ORGANISMS 



different conditions of light are of vital importance to these 

 creatures. The reactions are in reality responses not to 

 light, but to what light represents. They appear to have 

 learned to use light as a guide in directing their course in 

 accord with the demands of their state of development 

 and general habits. 



Hadley (1908) made a very thorough study of the 

 changes in the photic reactions of lobster larvae. He 

 found that they are positive for about two days after 

 hatching, after which they become negative and remain 

 so until shortly before molting, when they again become 

 positive. Both the early second-stage, and the third- 

 stage larvae are negative but as in the first stage they 

 become positive before molting. The fifth and later stages 

 are persistently negative. 



It has long been known that changes in light cause 

 daily periodic movements in plants, the so-called sleep 

 movements of leaves and flowers, and that these move- 

 ments continue for some time if the plant is kept in con- 

 tinuous illumination. Pfeffer (1906, p. 108) says, ** The 

 periodic movements are at first pronounced, both in con- 

 stant light and in darkness, in the case of the leaves 

 of Acacia lophantha, Mimosa pudica, Impatiens noli-me- 

 tangere, and Sigesbeckia orientalis, and they continue 

 to be perceptible until after the lapse of four to eight 

 days." 



Similar after effects have been noted in certain animals. 

 Mitsukuri (1901) observed that the mollusk Littorina is 

 negative when under water during high tide and positive 

 when it is exposed to the air at low tide. Bohn (1905 and 

 1907) made similar observations on Littorina, Hedista 

 diversicolor, and Actinia equina, and claims for them 

 that these periodic changes in the sense of reactions to 

 light continue in harmony with the tide for some days in 

 specimens confined in aquaria where they are not directly 

 affected by the tides. I was unable to confirm the results 

 recorded by Bohn in observations on Littorina littorea at 



