468 LESLIE B. AREY AND W. J. CROZIER 
made with the aid of a glass plate upon which there was a small 
opaque dot of india-ink, thus enabling a small shadow to be 
cast upon the back of an Onchidium. In a resting animal, if 
the shadow spot so produced was made to move slightly upon 
the back of the mantle, a response was usually provoked, in 
the form of a shght retraction of the tentacles with or without 
the depression of the mantle. 
Under water the tentacles of Onchidium are never extended so 
fully as when the animal is in air. The response to shading is 
therefore not so conspicuous when the animal is under water, 
since the tentacular retraction is not so obvious; otherwise, the 
behavior of Onchidium when shaded is identical under water 
and in air. 
Onchidium gives no response whatever to increase in hght 
intensity, as such. 
Onchidium is but one of a number of animals in which it has 
been demonstrated that precise negative orientation by light 
may occur simultaneously with the presence of definite and 
conspicuous negative responses to decrease of light intensity, 
reactions initiated b}" increase of light intensity as such being 
absent (Euglena, Bancroft, '13; the leech Dina, Gee, '13; holo- 
thurians, Crozier, '14, '15; Chiton, Crozier and Arey, '18). 
Onchidium resembles Chiton and the holothurians especially in 
the fact that the regional distributions of the two kinds of irri- 
tability are seemingly identical. Phenomena of this kind nullify 
the supposition that the orienting stimulus can originate, for 
these animals, in the changing intensity of light; because the 
sense of their only known form of response to changes of light 
intensity is incompatible with the manner in which photic orien- 
tation occurs. The significance of this fact, especially as demon- 
strated in bilaterally sj^mmetrical animals, seems to have been 
unduly ignored. 
Additional evidence was secured from Onchidia repeatedly 
shaded at various rates until they ceased to respond to shading 
at all. The orientation of these individuals in a field of light 
was in no particular different from that of snails with undimmed 
reactivity to shading. 
