464 LESLIE B. AREY AND W. J. CROZIER 
fact that the snails never creep down to the actual water level, 
they do creep upward or downward with seeming indifference. 
Only a limited degree of negative geotropism may therefore be 
postulated for Onchidium. 
Vigorous vibratory mechanical stimulation serves to interrupt 
the creeping of an Onchidium, but neither the sharp tapping of 
a glass beaker containing some of the animals nor forcible blows 
struck upon natural rock bearing- them leads to any more pro- 
nounced form of response. Although the snail may altogether 
cerse creeping for some minutes when disturbed in this way, 
the tentacles are not retracted, nor is the mantle-fold depressed. 
2. Photic excitation 
The data presented in this section were all secured under 
laboratory conditions. In nature the heliotropism of Onchidium 
is inhibited (Crozier and Arey, '19c), although its reactivity to 
shading is quite pronounced. 
Onchidia gather in groups on the side of aciiuaria away from 
the light, and, once there, in the majority of cases stay on that 
side. Diffuse daylight and brilliant sunlight induce the same 
form of response, which obtains whether the animal is in air or 
under water. After reaching the side of the container farthest 
from the light, thej^ usually continue to creep upward to the 
water edge, where a brief halt commonly ensues, and then on 
up. To a source of unilateral horizontal light, an individual 
orients sharply, precisely, without 'trial' movements. Having 
oriented, the animal proceeds to move in a straight line away 
from the light source. 
In a dark room the surface of Onchidium was explored with a 
minute and fairly intense beam of light (cf. Patten, '15). The 
diameter of this beam was about 0.3 mm. The anterior end of 
the mantle was found the most sensitive part of the creature's 
surface. Even with this minute source of stimulation it was 
possible to make the animal move in any desired direction by 
appropriately placing the spot of light upon one side of the 
anterior end of the mantle. The photic excitability of the 
posterior part of the mantle was somewhat lower than that of 
