NATURAL HISTORY OF ONCHIDIUM 465 
the anterior end of the mantle. The stimulation of the posterior 
region of the mantle led, however, to a type of response not seen 
when the anterior portion was activated. Wlien the light-spot 
was applied to a point anterolateral with respect to the mid- 
posterior point, the anterior end of the Onchidium was swung 
sharply toward the stimulated side; when the body was then, 
immediately afterward, straightened, the stimulated spot was 
swung out of the region of activation. The body of Onchidium 
is not readily twisted sidew^ays, and it seems that when such 
twisting does occur it is always initiated by the anterior end, and 
is of such a character as to contract the animal on the side of 
the stimulated spot. This spot must, however, be located on 
the posterior half of the body. In the case of photic irritation, 
the resulting maneuver is very efficient in withdrawing from 
activation a given stimulated posterior part, and seems pur- 
posive; but it appears to be the only form of reaction possible 
under the circumstances. At the anterior end of the mantle, 
however, the reaction elicited by a spot-light is a more purely 
directive one, leading to less pronounced differential body con- 
traction, but to more vigorous locomotor movements. 
The whole mantle dorsum is photosensitive. When one point 
was activated by the spot-light, a puckered depression quickly 
appeared and extended as a furrow transversely across the 
animal's back. 
Stimulation of the anterior end of the foot or of the head 
region (beneath the mantle) was, so far as could be detected, 
without directive effect. 
The tentacles, also, seemed themselves to be non-reactive to 
illumination, nor when stimulated either by the spot-light or by 
concentrated sunlight did they induce directive locomotion. 
The tentacles are not important for orientation by light. 
Onchidia from which the tentacles have been amputated still 
orient away from the light as sharply and as promptly as with 
the tentacles intact. 
These experiments suggest that the dorsal mantle surface 
contains the only photoreceptors important for phototropism. 
This is confirmed by the following experiment: 
