480 LESLIE B. AREY AND W. J. CROZIER 
puzzle of 'homing' behavior. Although we have to offer some 
suggestions toward such an interpretation, they are presented 
with due reserve as to their probable finality. No more striking 
instance is known to us of apparent incompatibility between the 
results of controlled experimentation, repeatedly verified, and 
the most obvious activities of the same animal's natural life. 
First, as to heliotropism. In the laboratory Onchidium 
behaves as a typical negatively heliotropic animal. In nature 
its behavior is in every essential at variance with this finding 
(Arey and Crozier, '18; Crozier and Arey, '19 c). Not only does 
this mollusk creep out of its dark nest into the glaring sun, when 
the tide is falling, to feed, but it does so only during daylight 
hours, and never at night, no matter how bright the moon. 
During the period of its emergence an Onchidium's movements 
seem not in any degree influenced by the sun. An individual 
quietly creeping in brilliant sunlight may be shaded by a board, 
and after re-expansion following its response to the shading, it 
creeps on as before. If now sunlight be thrown suddenly on 
this animal from a new direction, with a mirror, it may momen- 
tarily 'hesitate' somewhat, perhaps turn very slightly to one side, 
but soon it continues as before. 
It is not merely the presence of a normal kind of shore sub- 
stratum which determines this suppression of heliotropism, 
because animals brought from laboratory stock (secured several 
days before) and placed upon the same rock are oriented pre- 
cisely by the sun. The presence of the specific rock surface 
normal to the Onchidia emanating from a given nest, namely, of 
that surface within a certain small radius of their nest entrance, 
is the deciding influence. An Onchidium moving toward its 
nest entrance, but heading directly into the sunset, may be 
picked up and then replaced upon the rock without interfering 
with its course. But if a glass plate be slipped under the ani- 
mal before replacing it, it is now at the mercy of its heliotropism. 
If an Onchidium from one section of the shore be quickly trans- 
ferred to a strange zone, again the animal is oriented by the 
light. Obviously, there is no question here of the mere rapid 
exliaustion of the heliotropic mechanism or other kind of 'light 
adaptation.' 
