REACTIONS TO LIGHT 257 



obtain the following groups: a. Reaction to change of Inten- 

 sity; b. Reactions to constant Illumination; c. Reactions of 

 questionable cause. 



a. Reaction to change of intensity. — Response to change 

 of Intensity on the surface of the organism may result In 

 orientation or merely In a change In position or direction 

 of motion, {a) Orientation: Examples — Euglena, Chlam- 

 ydomonas, Trachelomonas, CVilorogonlum, ^swarm-spores, 

 Volvox, Stentor, Planaria, earthworms and fly larvae, {b) 

 Changes In direction of motion: Examples — Shock reac- 

 tions, or avoiding reactions which do not result In orienta- 

 tion, In all the forms mentioned above, {c) Changes In 

 position : Examples — Sudden contraction In the tublcolous 

 worms, Gonlonemus, a few actlnlans, various moUusks and 

 arthropods, and Amphloxus. {d) To these a fourth divi- 

 sion may be added consisting of reactions to shadows In 

 Clepslne, shrimps, prawns, mosquito larvae and Fundulus. 



b. Reactions to constant illumination. — Constant or 

 continuous Illumination affects the sense of the reaction of 

 organisms and their general activity, and it may possibly 

 produce orientation In some forms. Whenever a positive 

 organism becomes negative In light or vice versa, It Is In all 

 probability due to the action of light owing to Its continued 

 intensity, the absolute amount of light energy received, the 

 product of the Intensity and time of exposure. Reversal 

 In the sense of reaction Is not common to all organisms which 

 respond to light, but the general activity of all probably 

 depends upon the absolute amount of light energy received, 

 much as the activity depends upon the temperature or heat 

 energy received. The aggregation of Planaria In regions 

 of low light Intensity Is no doubt In part due to this effect 

 of light, since they come to rest even In a field uniformly 

 illuminated from above In such a way that there Is no per- 

 ceptible change of Intensity on any part of the organism. 

 The time of exposure is an important element In this 

 response. This Is contrary to Loeb's conclusions regarding 

 the cause of aggregation of Planaria. He claims the aggre- 



