482 LESLIE B. AREY AND W. J. CROZIER 
photolysis in light of given intensity, and resynthesis in Ught of 
lower intensity — might be involved in stimulation by Ught and 
by sudden decrease of light intensity, respectively; here, both 
forms of irritabihty play roles of bionomic importance (Crozier, 
'15), whereas, according to this notion, but one phase of the 
matter, namely, sensitivity to shading, is permitted to exert an 
influence upon the normal behavior of Onchidium. The mechan- 
ism whereby the possibility of heliotropic response is normally 
prevented has already been discussed (Crozier and Ai-ey, '19 c). 
It appears to depend upon specific impulses originating in the 
oral lappets, at their points of contact with the substratum, for 
when these lappets are removed or are anaesthetized by MgS04, 
the Onchidium becomes photonegative, even if replaced upon the 
specific rock surface from which it was taken. 
The rhythm of the tides, ordinarily well defined, controls the 
emergence of Onchidium upon its feeding ground. Under cer- 
tain conditions the orderly succession of periods of low water is 
seriously interfered with. High winds and accompanying ocean 
currents, during times of storm, not infrequently cause such a 
'piling up' of water within the semi-enclosed sounds at Bermuda 
that the water may fail to fall appreciably for several tides; such 
a period is followed, also, by a certain irregularity in the tidal 
sequence. Only when the water level has become lowered to 
the proper degree, previously indicated, do the snails emerge. 
Conversely, extensive periods of low water, notably occurring 
at spring tides, may leave the rocks uncovered along the Onchid- 
ium zone for an uncommonly lengthy interval. The duration 
of the emergence of a given colony is practically fixed, however, 
and gives not the least indication of being normally terminated 
by the rising of the tide; rather, the duration of the feeding time 
is determmed in a quite different way, which we shall shortly 
consider. 
The facts thus far presented do not exhaust the curious intri- 
cacies of the behavior of our snails. It has been mentioned that 
Onchidium comes out from its nest only during the daytime, 
and never at night. Were it not for certain serious obstacles, 
all this might be understood in terms of an hypothesis advanced 
