220 MANTON COPELAND 
next directed away, fails to receive the stimulating substance, 
but on the return swing again enters it, the osphradium is again 
stimulated and the animal moves into the stream of odorous 
material, swinging its siphon vigorously. The snail continues 
turning as long as the osphradium is most strongly stimulated 
by materials coming to it from one side of the siphon’s axial 
position. It may be said to be oriented to the odor stream 
when the foot is straightened out, and the effective stimulating 
substance is received by the siphon in its axial position, or when 
it is equalized on both sides of that position. The siphon 
continues conducting to the osphradium samples of water and 
odorous substance in changing concentrations from the region 
through which it moves; stimulations of the osphradium are 
followed by foot movements which take the snail toward the 
place where the strongest stimulating material entered the siphon, 
until slowly but surely the animal is directed to the source of the 
scent. The snail, therefore, provided with a water testing ap- 
paratus, which may be said to consist of a single receptor as- 
sociated with a long snout terminating in a shifting nostril, 
accomplishes what the dogfish does with its paired receptors and 
fixed nostrils. . 
When the siphon of Busycon is fastened to one side of. its 
axial position, so that it cannot be moved to the opposite side, 
circus movements occur after olfactory stimulations which are 
not due to disturbances resulting directly from tying up the 
siphon. The same form of locomotion may be observed when 
the organ is given its natural freedom of movement, but is 
allowed to receive odor only from one side. Under these cir- 
cumstances the snail continues exhibiting orienting movements, 
and thus behaves like a dogfish scenting food through a single 
nostril. 
In the case of the dogfish, it was found that olfactory stimula- 
tions called forth certain movements described as random ones. 
Comparable movements, as might be expected, were not ob- 
served in the responses of Busyecon. An animal moving as 
slowly as this one would be poorly adapted for finding food if, 
