OLFACTORY REACTIONS OF MARINE SNAILS 191 
report them in detail. In the case of those snails which possessed 
their tentacles, but had their siphons shortened, the proboscis 
reactions were of greater frequency, as the tentacles are partic- 
ularly sensitive, and their stimulation was usually followed by a 
more thorough investigation of the surroundings with the siphons. 
That dilute food juices taken into the siphon or mantle cavity 
cause Alectrion to respond actively was demonstrated in another 
way. When a snail is placed-in an aquarium, the bottom of 
which is covered with sand, it usually soon buries itself completely 
except for the tip of the siphon which projects slightly above the 
surface and conducts water to the gill within the mantle chamber 
below. If fish extract and carmine are squirted over the end 
of the siphon, they disappear within, and the snail often reacts 
by coming to the surface and extending its proboscis. Different 
individuals were found to vary considerably in their reactions, 
certain ones always responding to the stimulus in a marked way, 
whereas others were more erratic in their behavior. In one 
instance, in over sixty trials involving many individuals, a snail 
came out of the sand immediately after sea water and carmine 
were squirted over the siphon tip, but in this case the proboscis 
was not extended as it was when the animal responded to fish 
juice. Snails deprived of their tentacles showed the same 
behavior as others; in fact, it is unlikely that the stimulating 
material reached any anterior portions of the body outside of the 
mantle chamber and siphon when the animal was buried in the 
sand. 
Conclusions. The experiments upon Alectrion all substanti- 
ate the conclusion already drawn, namely: that the reaction to 
food juices, consisting of an extension of the proboscis, is under 
the conditions of the tests the result of the stimulation of a 
receptor located either at the extreme base of the siphon or within 
the mantle cavity. The skin receptors of the foot, siphon and 
especially the tentacles are affected by these materials has been 
clearly demonstrated, but the part they play in the final reaction 
appears to be subordinate to some organ which, judging from 
the animal’s response, is much more sensitive to stimulation by 
dilute food substances. Asa result of skin sensitiveness, a change 
