NATURAL HISTORY OF ONCHIDIUM 473 
ing orientation, yellow light is less so, and red light is relatively 
ineffective. In different series of experiments ray-filters of 
several kinds were employed; thus in the first series of trials we 
used the colored glasses previously described in the work on 
Chiton and on Chromodoris (cf. Ai-ey and Crozier, '19; Crozier 
and Arey, '19 b), with this result: 
Blue. When Onchidia, in the dark, are made to crawl directly 
toward the future source of light, and the blue light is then suddenly 
turned on, the animals stop, pivot sharply through 180°, without 
creeping, then move directly away from the light source. If started 
in such a way as to creep in a direction perpendicular to the future light 
source, the animals sometimes hesitate, lift the anterior end, and swing 
it sharply away from the light source, thus making a precise 90° turn. 
Green. AVhen creeping is so begun as to lead the animal into the 
light, a prompt turning through 180°, with subsequent locomotion 
away from the light, at once follows the admission of the green light. 
On the whole, the response is not quite so sharply carried out as with the 
blue light. To unilateral light the response is as in the case of blue. 
Yellow. To light impinging directly on the anterior end of the ani- 
mal, Onchidium responds bj^ creeping in a complete circle away from the 
light, locomotion continuing along a path parallel to that at first pursued. 
With unilateral light, the process of orientation is equally precise, but 
less rapidly effected than in the case of green. 
Red. In the majority of the trials, although Onchidium invariably 
creeps awa,y from the light, the orienting process is sometimes quite 
slow, the animal occasionally continuing to move onward for several 
millimeters after direct red light is turned on — then going off at a right 
angle or turning back on its path. No cases were observed in which the 
Onchidia pivoted immediately away from the hght, as with the blue; 
but on the contrary they maintained a steady creeping progress during 
the course of orientation. When orientation is completed, the new 
path does not quite coincide with the old, as in the case of blue or green 
light, but in addition to being in the reverse direction lies within 10° 
to 45° on one side or the other of the original course. 
These experiments were made in a dark chamber, the onl}^ 
source of light being a slit of appropriate size covered by the 
appropriate ray-filter. The result of these tests, showing the 
higher stimulating power of blue and green light, agrees with 
what was found in the case of shading: under the blue or the 
green ray-filter used in the phototropism tests, decrease of light 
intensity led to a normal reaction on the part of the Onchidium, 
while under the red or the green filter only a slight response, or 
