OLFACTORY REACTIONS OF MARINE SNAILS 179 
provided with sharply pointed tips. The slender tapering tenta- 
cles, about five millimeters long, are borne on stout processes 
projecting in an antero-lateral direction three millimeters from 
the head, and the eyes are situated at the base of the tentacles 
on the outer side. The siphon, which runs forward above the 
head, represents a prolongation of the mantle. The ventral 
wall is split its entire length, but when the edges are brought 
together a tube is formed, open at the tip, through which water 
passes to the gill within the mantle chamber. The organ pro- 
trudes from ten to fourteen millimeters beyond the anterior 
border of the shell, and can be swung about in various directions. 
Alectrion occurs commonly on tide flats, where it is a thorough 
scavenger, feeding on a variety of food substances, but often 
to be seen collected in great numbers on some dead animal such 
as a fish or a crustacean. Dimon (’05) writes of its food as 
follows: 
Among the various food materials upon which Nassa was seen to 
feed were hen’s egg shells, dead hermit crabs and Squilla, live Nereis, 
ulva, the alga that grows upon the shell of Nassa itself, the thick black 
mud of the inner harbor, and the alga that gathers on the glass of 
aquaria. . . . . Nassa did not usually attack a live uninjured 
clam, though I have seen the snails collected at the edge of the mantle 
of a clam that was apparently alive, devouring it and preventing the 
shell from shutting by pushing themselves into the opening between the 
valves. 
Belding (10) has shown that Alectrion occasionally feeds 
upon the living scallop, gaining access to the soft parts of the 
mollusk by entering between the open valves. 
/ 
1. Reactions to olfactory stumuli 
That an olfactory sense plays an important part in the dis- 
covery of food by Alectrion seemed probable. Dimon (’05) 
reports finding a Squilla about six inches in Jength 
on the mud, with ninety-eight mud-snails crowding about it. It 
was then taken away from these and put into a pool about eighteen 
inches in diameter, which was full of quiet mud-snails. These snails 
immediately became active, but without moving definitely toward the 
Squilla. When one happened to reach it, it stopped and began to 
