492 LESLIE B. AREY AND W. J. CROZIER 
section of the shore in order to witness the complete unmasking 
of its heUotropic impulses. 
Since strychnine has in some instances been shown to produce 
negative phototropism, even in animals naturally indifferent to 
Ught (Moore, '12), it should be clearly understood that the 
strychnine effect in Onchidium cannot be regarded as of this 
sort.^ 
Certain animals are known to become photonegative upon 
immersion in sea-water. Isopods of the genus Ligia, which in 
certain places occupy territory also frequented by Onchidium, 
have been said by Abbott ('18) to reverse their phototropism, 
perhaps under control of humidity, in the sense that at low tide 
they come out from hiding places above flood-tide level and 
wander over the exposed intertidal zone. That this behavior 
really involves phototropism of any kind, and is not rather a 
case similar in certain features to that of O. floridanum, remains 
to be proved. It would be of interest to test this matter, for 
Abbott states that "in the laboratory they (Ligia) give a nega- 
tive reaction to sunlight." Moreover, an understanding of 
the situation in Onchidium may be important for the elucidation 
of other curious cases in which an animal's heliotropism seems 
fundamentally at variance with its mode of life (e.g.. Para- 
vortex, described by Ball, '16, p. 464). 
According to Mitsukuri ('01), the specific habitat of Littorina 
is determined by changes in its phototropism, from negative in 
air and under water to positive when splashed by waves. Here, 
again, the evidence that phototropism is really primarily involved 
is somewhat defective. The heliotropism of Onchidium is in no 
respect altered by complete immersion in sea-water. 
^ Whether the action of str3'chnine in producing negative heliotropism with 
an animal naturally photopositive or even indifferent to light (Moore, '12, '13) 
can be always referred to chemical modifications within the primary receptors, 
rather than to some more strictly central nervous (synaptic) effect, remains 
unanswered. Even with the human eye, where visual acuity (retinal resolving 
power) is notably augmented by strychnine, one cannot at present be sure that 
the removal of certain central inhibitions is not at bottom responsible. As an 
example of the inhibition of one sensory impulse by another, we might cite the 
heightened tactile responsiveness of certain de-eyed fishes (Crozier, '18). 
