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18 A'tnerican Seashelh
the foot of this snail are flapped up and down much in the manner of a bat
in flight.
However, the pelagic habitat and the ability to swim are the exception
among the snails. The intertidal zone which is intermittently flooded and
drained by the moving tides is well stocked with many kinds of creeping
snails. Adany Nassarius Mud Snails live exclusively on the warm, flat mud-
FiGURE 3. a, The Nassa Mud Snail, Nassarius, crawling under the sand with its
siphon extended into the water above; b, cutaway view of a prosobranch snail
showing the direction of water currents (arrows) down the siphon, over the gills
and out from the right side of the body. (After Ankel 1936.)
bars of quiet bays. Among the carnivorous snails, we find that their ecologic
stations are determined by the location of the worms or bivalves upon which
they feed. One or two species of Terebra and Polinices Moon Shell are found
burrowing in the sand of beach slopes where they are able to find their favorite
clams, but the majority of these snail genera are found from low-tide mark
to a depth of several fathoms. Since most marine gastropods are nocturnal in
habit and shun bright sunlight, many species spend their time hidden in crev-
ices under rocks. This affords protection to themselves and their eggs from
predators, bright sun and violent wave action.
A great number of species live in deep water, and frequently their verti-
cal distribution is limited to relatively narrow ranges. From some 500 dredg-
ing samples taken off southeastern Florida by the late J. B. Henderson's yacht
"Eolis," Bayer's Dwarf Olive (Olivella bayeri) was found in depths ranging
from 25 to 115 fathoms. On the other hand, the Greenland Moon Shell has
been found from twelve feet to over two miles in depth.
In their experimental search for new living places, a few gastropods have
evolved strange associations with other marine animals. Tlie dwarf Cypho-