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 favor- 

 ite 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- 



