NATURAL HISTORY OF ONCHlDItJM 481 
It is not that the Onchidium, 'seeking its best interests/ 
comes out into the Hght to feed and in so doing ignores the 
dictates of its negative hehotropism. Definite evidence, on the 
contrary, is available to the effect that under normal circum- 
stances the central nervous mechanism of heliotropic movements 
is inhibited, perhaps ordinarily by impulses originating in the 
oral lappets which compete successfully for the control of the 
body musculature. This inhibition can be abolished ('reversed') 
by means of strychnine (Crozier and Arey, '19 c). Reversal of 
inhibition within the ganglia of mollusks is known in Chiton, 
in Chromodoris, and in Cephalopods (cf. Crozier, '20). 
A possible explanation for the existence of negative hehotro- 
pism under any circumstances may be found in an imperfection 
of the photoreceptive system. The response of Onchidium to 
sudden shading is clearly of conceivable survival value; it leads 
to the retraction of the tentacles, the cessation of locomotion, 
and the depression of the mantle-fold to the substratum; the 
mantle is thus enabled to assist as a hold-fast, for the suction 
power of the foot is but poorly developed, and in any event the 
algal-covered surface affords a difficult field for contact attach- 
ment by the small foot of an animal so hght in weight that it 
does not flatten out the algae. Our suggestion as to the nature 
of the heliotropic irritability, depending on the continuous 
chemical activity of incident hght, involves the assumption that 
the stimulation so produced is in this case bound up with sub- 
stances forming a necessary part of the receptive mechanism for 
the response to shading. The obviously efficient manner in 
which heliotropic impulses are normaUy blocked, in such fashion 
that they play no detectable part in controlling the creature's 
movements, may account for the fact that this deficiency in the 
photosensitizing mechanism has failed to suffer selective elimina- 
tion. A balanced system of photocatalizable reactions was 
postulated to account for the phenomena of photic irritabihty 
in Holothuria (Crozier, '14). The general features of such a 
system have been (in Mya) admirably treated in a quantitative 
manner by Hecht ('19). For Holothuria it was suggested that 
both phases of activity within the photosensitive system — 
