470 LESLIE B, AREY AND W. J. CROZIER 
foot is enabled to attach itself. If illuminated from one side 
only, Onchidium almost invariably contracts the musculature of 
the opposite side when righting is begun. 
We may consider at this point the nature of certain evidence 
sometimes adduced in adverse criticism of the idea that photo- 
chemical transformations are responsible for the activation pro- 
ducing heliotropic movements. The theory of heliotropic move- 
ments centers upon the fact of orientation in a field of light ; if 
the light for any reason produces unequal effects in the symmet- 
rical receptive areas, the rates of photochemical transformations 
will not be the same in these areas ''and the rate at which the 
symmetrical muscles of both sides of the body work will no 
longer be equal; as a consequence the direction in which the ani- 
mal moves will change." It has been deduced from such state- 
ments that the rate of locomotion of an animal, once oriented 
(or already oriented), should be proportional to the acting light 
intensity (Dolley, '17). 
This idea is not necessarily correct, and it may be pointed out 
that there are forms for which it can at most have but a limited 
applicability. 
The locomotor progress of Onchidium is determined bj^ the 
succession of transverse neuromuscular waves upon the sole of 
the snail's foot. These waves traverse the length of the foot 
at a rate of about 16 cm. per minute, succeeding one another at 
intervals of approximately five seconds (at 27°C.). The speed 
of these waves and their frequency are very largely independent 
of the proximal stimulus, once a certain threshold of activity has 
been exceeded. But in orientation, the bending of the body is 
due to the differential contraction of symmetrically located 
parietal muscles, quite distinct from those producing the pedal 
waves. The rapidity of orientation thus depends upon the 
differential action of the symmetrical halves of the neuromus- 
cular mechanism which controls the lateral bending of the body 
— upon the degree to which one side is contracted, the other 
side reciprocally relaxed. It should therefore not prove surpris- 
ing to find the speed of photic orientation (within limits imposed 
by the snail's anatomy) proportional to the intensity of the 
