56 THE TROPISMS 



tion or pleasure is probably associated with the phototactic 

 response. 



There are a few cases in which positively phototactic 

 species swim backward instead of forward toward the light. 

 An instance of this kind was found by Lyon in Palaemonetes. 

 Hadley found that larval lobsters swim with the head pointed 

 away from the light whether they are postive or negative 

 in their reaction. In the pycnogonid Anoplodactylus, 

 Cole observed that in crawling toward the light the anterior 

 end was in advance, but in swimming toward the light the 

 animals moved approximately backward. 



Fiddler crabs form an exception to most phototactic 

 animals in that in going toward the light the body is oriented 

 sidewise instead of with its longitudinal axis in the direction 

 of the rays. These animals are strongly positive and, like 

 Ranatra, their response becomes stronger with longer ex- 

 posure. They are timid animals and often a sudden move- 

 ment of the light will send them scuttling away in alarm, 

 but after following the light for some time they become 

 more oblivious to other stimuli and slavishy follow it with 

 less show of fear. Orienting movements are different from 

 those used in ordinary locomotion which is not the case with 

 most forms. Other animals may orient themselves by walk- 

 ing faster, so to speak, on one side than on the other, but 

 with lateral orientation special movements have to be per- 

 formed in order to change the direction of locomotion. 

 Here again it is difficult to apply the theory of forced 

 orientation in its usual form, and we are led to conclude that 

 the reaction to light is to a certain extent one of the pleasure- 

 pain type. 



According to Ra"dl phototaxis has an intimate relation to 

 vision. In a number of experiments Cole has found that 

 reactions to light are influenced by the extent to which the 



