54 THE TROPISMS 



ventral muscles near the stimulated point. It is not im- 

 probable that the negative phototaxis of an eyeless Planaria, 

 like its turning away from strong stimuli in general, is due 

 to the local effect of light in the region on which it impinges. 



In the earthworm or leech turning away from strong light 

 is accomplished in all probability by the contraction of the 

 longitudinal muscles on the opposite side of the body, the 

 result being in the nature of a crossed reflex which is so com- 

 mon in animals with an axial central nervous system. In the 

 crustacean Eubranchipus which usually swims on its back 

 there is a marked positive phototaxis if the animals are sub- 

 jected to a rather small source of light. In ordinary day- 

 light before a window they pay little heed to the light, but 

 if taken into a dark room and exposed to light from an 

 electric bulb they will swim toward it and follow it about 

 in any direction. If they are oblique to the rays they 

 bend the tail suddenly toward the more illuminated side 

 one or more times until they become oriented. 



In most Crustacea as in most insects orientation is affected 

 through the unequal action of the appendages on the two 

 sides of the body. In a form which is positively phototactic 

 light entering one eye sets up impulses which passing into 

 the brain and nerve cord, cause, directly or indirectly, move- 

 ments predominantly of flexion of the legs of the same side 

 and of extension of the appendages of the opposite side of 

 the body. If this is a sort of mechanical reflex process we 

 should expect that, in a positively phototactic form, if one 

 eye were destroyed or blackened over, the animal would 

 move continually toward the normal side. This experi- 

 ment of blackening over one eye was tried by the writer on 

 the large sand flea, Talorchestia, and it was found that the 

 specimens no longer went straight toward the light, but 

 performed circus movements toward the side of the un- 



