156 LIGHT AND THE BEHAVIOR OF ORGANISMS 



however is not true; movement from the source of Hght is 

 ordinarily relatively rare. There must therefore be a 

 greater tendency to travel when the oral end faces the 

 light than when it faces in any other direction. 



It may then be stated that Hydra tends to orient with 

 the anterior end directed either toward or away from the 

 source of light depending upon whether the specimens are 

 positive or negative, and that it tends to travel in the direc- 

 tion in which it orients. There are evidently two appar- 

 ently independent phenomena involved here, orientation 

 and locomotion. How can these phenomena be explained ? 

 Let us first consider orientation. 



Since Hydras tend to expose the anterior end to light 

 when they are positive and to shade it when they are nega- 

 tive, it is probable that the oral end in this organism, as in 

 Euglena and Stentor, is more sensitive to light than other 

 parts of the body. If this is true it may be that orientation 

 is, as Jennings suggests (1906, p. 213), '' due to the fact 

 that when it turns this end away, the change to relative 

 obscurity at the anterior end causes further movement, till 

 the light again falls on the anterior end." This explana- 

 tion fits the facts, as far as they are known, fairly well. It 

 is however difificult to see, since Hydra frequently retains a 

 position in which the anterior end is shaded for more than 

 two minutes, how " the change to relative obscurity at the 

 anterior end could cause further movement." There is 

 certainly no reaction in these animals comparable to the 

 avoiding reaction or shock movements in the lower forms, 

 for sudden changes of intensity even if extremely great 

 produce no immediate reactions. If it is assumed that the 

 organism tends to become oriented by random movements 

 and tends to remain oriented because of inhibition due to the 

 illumination of the anterior end, this dif^culty is obviated. 



As already pointed out, locomotion ordinarily occurs only 

 when Hydra is approximately oriented. Positive speci- 

 mens travel only when the oral end Is illuminated, not when 

 it is shaded. It Is therefore evident that the light itself 



