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 nervoussystem. 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- 
