OLFACTORY REACTIONS OF MARINE SNAILS 219 
ditions of the fish are disturbed by the complete occlusion of one nos- 
tril, the fish swims as though it were in water that was highly charged 
with odorous particles on the side of its body corresponding to the open 
nostril and devoid of these particles on the opposite side. The fish 
therefore turns toward the side of the open nostril, but since, under 
the artificial conditions of the experiments, this turn does not equalize 
the stimulus, the motion is continued and a circular form of locomotion 
results. Thus, in my opinion, the more or less circular movements 
induced in a dogfish with an occluded nostril by an odorous bait are to 
be explained upon the same basis as the circus movements of such 
invertebrates as crustaceans, insects, etc. 
Turning now to the snail, it is at once recognized that it has a 
receptive apparatus concerned with responses to odorous material 
which, in certain respects, is quite unlike that of the dogfish. 
Instead of possessing a pair of olfactory organs symmetrically 
placed on the right and left of the principal axis of the body, 
it has but one, the osphradium, which is approximately median. 
Extending forward over the head there is a long tube, the siphon, 
through which water is conducted not only to the gill but to the 
olfactory receptor at its proximal end. The more distal part 
of this siphon is not fixed in position but, when the snail is 
crawling, is almost continually in motion. For the most part 
it is moved back and forth from one side of the body to the 
other, but it may be directed more or less upward and downward 
and, in the case of the mud snail, even posteriorly. By this 
provision water and its contained odorous substances can be 
drawn to the osphradium from various directions. 
Let us assume that a whelk is crawling over the sea bottom in 
the vicinity of a dead fish. From what has been learned of its 
olfactory reactions under varied conditions, it is possible now to 
picture mentally the movements which must often oecur after 
the snail scents the food, and to interpret them with some degree 
of success. As the snail moves, the end of the siphon is shifted 
from one side to the other, although the foot is carrying the 
animal in a straight course. Perhaps as the siphon is directed 
laterally, it happens to enter a stream of odorous material 
spreading or drifting from the fish, a sample of which passes to 
the osphradium and stimulates it. The effect is seen in the foot 
which begins to turn in the direction of the odor. The siphon, 
