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The common European limpet. Patella lulgala, clamps down tiglitly when tide is out but 

 moves about to browse on seaweed when water returns. ( England. D. P. Wilson ) 



it seems incredible for any other mollusk to occupy 

 space inside the doorway. Yet the boat shell Crephl- 

 iila often takes up residence there. Actually the boat 

 shell is not very selective about an attachment site, 

 for it will even cling to others of its own kind until 

 masses of as many as forty come to lie as a cluster 

 on the bottom. Crepidida holds to its oval, boat- 

 shaped shell by a horizontal shelf across the posterior 

 end inside — and because of this shelf, the shell is of- 

 ten called the "quarter-deck shell" or "slipper shell." 



Crepidida feeds on small plants and animals 

 trapped in a mucus film spread over the gills on each 

 side of its foot. About every four minutes, the mol- 

 lusk twists its head to one side or the other and gath- 

 ers the loaded mucus into its mouth. Small particles 

 are swallowed immediately, but large ones may be 

 stored in a pouch at the front of the mouth as emer- 

 gency rations. These are used when Crepidida must 

 clamp its shell down tight and hold on. 



Freshly matured boat shells are males. Later they 

 become bisexual, and still later completely female. 

 At breeding season the mother snail produces about 

 fifty or sixty membranous bags, each loaded with 

 around 250 eggs, and guards these for the month un- 

 til they hatch. The young swim freely for about two 

 weeks, and then settle on some surface where they 

 can become more or less permanently attached. 



The edible periwinkle Littorina littorea is collected 



by the ton along European shores and sold roasted on 

 the streets of London. About 1857 it was introduced 

 into Nova Scotia and gradually spread southward, 

 now well past Chesapeake Bay. The sexes are sep- 

 arate, the female being a little larger than the male, 

 about % of an inch in diameter and in height — a 

 squat, thick cone, usually olive-colored but often 

 banded with dark red or brown. Almost no one eats 

 winkles in America. Other species of Littorina 

 (Plate 56) are found on almost all of the world's 

 coasts, rasping algae from the rocks or cleaning the 

 film of vegetation from the surface of mud flats. 



The old-maid's curl or worm shell Vennicularia 

 starts out as a tightly coiled high spire, but the 

 snail inside changes its construction habits after a few 

 turns have been completed. It progresses with its 

 building of new shell in a very straggling manner, 

 producing a winding tube with lengthwise keel and 

 grooves, often totaling 6 to 9 inches in length. In 

 tropical waters these shells are found entwined 

 among sponges and corals, where they suggest the 

 products of tube-building worms. Occasionally they 

 become grouped in tangled masses or attach them- 

 selves to oyster shells and other mollusks. 



Conchs and whelks produce far more regular and 

 massive shells. These are aggressive, carnivorous 

 snails, whose large shells usually provide a "canal" 

 for the siphon through which water is discharged 



180] 



