[continued from page 160] 



from side to side as it shifts its weight from the one 

 being moved. 



The abundant httle salt-marsh snails (Melainpiis), 

 which breathe by means of a lung, hitch themselves 

 along by clinging alternately with the forward half 

 and then the hinder portion of the foot. As they do so 

 they suggest a cautious child taking short, sliding 

 steps on glare ice. By contrast, the curious little tube 

 shell Caecum glides along on a ciliated foot, holding 

 steady its blunt-ended limy cone with its many en- 

 circling rings. 



Gastropods show tremendous variation in the de- 

 gree of development of the shell. The horse conch 

 Pleiiroploca gigantea of Atlantic shores from North 

 Carolina to Brazil, and an Australian marine snail 

 Megalotractus awitaims, each with a 24-inch shell, 

 share the distinction of being the world's largest uni- 

 valves. On land the giant is Achatina achatina of 

 African jungles, especially in Liberia, with a shell 8 

 inches long and 4 in diameter. All of these animals 

 can pull well back into their shelters and remain hid- 

 den for days or even months if conditions are un- 

 favorable. 



In other gastropods, the shell offers so little space 

 that it can serve only as a sort of badge, proving 

 snailhood. Among sea butterflies (pteropods) it 

 scarcely hinders progress as the animal swims freely 

 near the ocean's surface far from land, waving a pair 

 of winglike expansions from the sides of the foot. Or 

 the shell may be lost altogether at a tender age, as in 

 another sea butterfly (Clione). found in great 

 schools, swimming in cold waters between the Arctic 

 and New York, Scandinavia, the northern parts of 

 the British Isles, northern California, and Japan. 

 Clione serves as a major food source for several 

 kinds of whalebone whales. The sea slugs (nudi- 

 branchs) and many land slugs also lose their embry- 

 onic shells and manage thereafter with almost none. 



Some small snails lack a radula. Most of these are 

 parasites on and in echinoderms, or they browse on 

 the slime accumulating at the end of a clam's ex- 

 posed siphon. Otherwise a radula seems important 

 in rasping off minute particles of food, whether this 

 is vegetable matter or flesh. Oyster drills and whelks 

 use the radula to cut neat circular holes through the 

 shells of clams and oysters, and then thrust the organ 

 into the victim's shelter to remove the meat — killing 

 the shellfish in the process. In tropical seas, many 

 whelks and conchs with this habit are collected for 

 human food and as bait. 



Auger snails (Terebra) and cone snails {Conns 

 marmoreiis; Plate 59) have a barb at the end of the 

 radula. connected by a duct to a poison gland near 

 the gullet. These carnivorous snails use the radula as 

 a weapon, "stinging" victims to subdue them. Some 

 of the larger cone snails, whose shells are much 

 prized by collectors, have been known to sting a hu- 



man hand and inject a fatal dose of poison. For this 

 reason they are quite honestly feared in the South 

 Pacific. 



Quite apart from any coiling of the body in rela- 

 tion to the shell, all gastropods go through a strange 

 process found in no other group of mollusks. During 

 their embryonic development, the body mass atop 

 the head and foot undergoes a torsion through 180 

 degrees until the anus, mantle cavity, and any 

 respiratory organs (ctenidia) come to lie at the back 

 of the head instead of the rear of the body. 



Among the majority of marine snails, from aba- 

 lones and limpets to whelks, this arrangement per- 

 sists. They are "prosobranchs," with the respiratory 

 organs at the front. Other snails and all slugs undo 

 the twist in one way or another, placing the respira- 

 tory organ at the rear again or replacing it with some- 

 thing else. These are the "opisthobranchs," such as 

 sea hares (with a very reduced shell) and nudi- 

 branchs (with no shell at all), or the "pulmonates" 

 in which the mantle cavity has become functional as 

 a lung — these last animals usually are found in fresh 



The sea butterfly Clione is a shell-Iess, swimming 

 mollusk found in great schools in surface waters, 

 particularly of the colder oceans, where it becomes 

 an important food of whalebone whales. (Delaware 

 Bav. William H. Amos) 



177 



