National Geographic : 1955 Mar
like the corrugations of steel roof ing or a cardboard carton. Con sider also the ridges on a scallop or whelk, another beautiful ex ample of engineering design. They add little weight but greatly in crease strength. Some mollusks grow spines along the lip for defense. When the ani mal grows new spirals, it obviously cannot leave the spines sticking into the cavity which will house its tender body. But the creature, which has parts of its mantle specially equipped to secrete new shell, can also dissolve shell. It removes spines as soon as they get in the way, perhaps using the lime for new deposits else where. In addition the animal secretes a porcelaneous or pearly layer inside. Some mollusks economize on weight by thinning partitions with in the shell, as in the giant margi nella (page 430). Others, like Florida's bleeding-tooth shell, dis solve away almost all the inside. Diving on the Florida reefs, I have taken young cowries with the growing lips almost paper-thin. Rarely have I brought one home undamaged. But the fully grown cowrie has thickened and rolled the lip for strength, until it re sembles the edge of a pewter plate (page 430). If you turn over rocks on the 434 National Geographic Photographer Robert F. Sisson outer reefs, you will often find Clench's Helmet Shell Calls to Mind cowries alive-beautiful fleshy ob a Conquistador's Armored Hat jects with bushy or feathery pro Several varieties of the helmets are valued for cameo cutting. jections. Pick them up, and the Clench's Helmet Shell weighs about 7 pounds. If filled with mantle draws back to uncover the the live animal, it would scale about 20. The meat is often used in chowder. This large shell is found off the Florida Keys. mottled shell, as perfect as a piece of fine porcelain. this column, the animal pulls back into the Just as the mantle hides the shell, so the house when danger threatens. To give the shell hides those spiraling inner chambers that muscle a better grip, the surface of the pillar delight both artist and scientist. is thrown into folds. These also grow as In building his houses and furnishings, man does the shell, and the intertwining curves and is learning to choose simple, efficient designs. screw threads, seen by X-ray, add greatly to Shells show us how Nature was doing this the beauty. long before the first man appeared on earth.* For efficient design as well as beauty, I think the mollusk shell is unexcelled. Look at the See, "Shells Take You Over World Horizons," by Rutherford Platt, NATIONAL GEOGRAPHIC MAGAZINE, wrinkles spiraling around the Scotch bonnet, July, 1949. INDEX FOR JULY-DECEMBER, 1954, VOLUME READY Index for Volume CVI (July-December, 1954) of the NATIONAL GEOGRAPHIC MAGAZINE will be mailed upon request to members who bind their copies as works of reference.