NATURAL HISTORY OF ONCHIDIUM 469 
If a sharply defined narrow beam of sunlight, about 1.5 times 
the width of the animal, is reflected into a dark chamber con- 
taining an Onchidium, the snail orients and creeps away from 
the light. Slight deviations bring the animal within the zone of 
shadow, whereupon it retracts sharply, being thus confined to 
the light beam. After a few shading stimulations, however, this 
kind of reactivity is exhausted, a slight deviation puts the ani- 
mal's anterior end in the shade and the rest of the body follows. 
Similarly, if an animal be shielded from the light on one lateral 
half, and then illuminated from behind, it turns and creeps into 
the shade. 
The photic orientation of Onchidium is therefore a purely 
tropistic process, determined through photochemical transfor- 
mations localized in specific receptors. In the similar case of 
Holothuria it has been suggested (Crozier, '15) that the same 
photochemical system may afford the receptive basis for both 
the continuous action of light and the sudden decrease of light 
intensity; final proof for this suggestion cannot as yet be offered. 
According to the theory of responses obtainable under these 
conditions, the musculature upon either side of an unequally 
illuminated, bilaterally symmetrical animal undergoes differ- 
ential contraction, resulting in forced movements of orientation 
with reference to a single source of light (Garrey, '18). With 
such an organism as Onchidium it might then be conceived that 
when an individual placed upon its dorsum begins to carry out 
righting movements, the direction in which righting occurs 
should be strongly influenced by the relative illumination of the 
two sides of the body. We have made experiments of this kind 
with Onchidium. Tests of this point have been made previously, 
principally with echinoderms. There are reasons for regarding 
such organisms as the starfish as not well adapted for this pur- 
pose. The superiority of Onchidium consists in the fact that it 
is (minor morphological points aside) a pronouncedly bilateral 
animal. It is necessary to point out that tactile stimulation of 
the dorsal surface may complicate such tests. The righting of 
an Onchidium involves the lateral twisting ventralward of the 
anterior end of body and foot, so that the anterior portion of the 
