National Geographic : 1969 Mar
Feasting night and day, the oyster grows on a diet of minute plankton (right). Cilia-lined gills pull in an average of 31/2 gallons of water (blue arrows) an hour, absorbing oxygen and trapping plankton (red dots). After coating bits of food with mucus, cilia fan them along the gill edges to the palps, or sorting flaps, and thence to the mouth. In the stomach, a crystalline, knoblike "style" unique to certain mollusks and the only known rotating mechanism in animals-releases enzymes and stirs food particles to aid digestion. Wastes leave through the anus, carried out by the dis charge of water from the gills. With a snap of its shells, the oyster also disposes of rejected food. THE MANTLE, a fleshy pad lining each valve, cushions the organs. At shell edge the mantle divides into three lobes: The inner one regulates intake of water; the middle lobe senses light; the outer lobe and entire mantle spread a calcium-laden material to build and repair the shell. RIGHT VALVE MANTLE THE ADDUCTOR, a two-part muscle at the body's center, controls movement. The quick muscle snaps shells shut. Then the catch muscle takes over and holds valves closed, permitting survival on coastalflats between tides and on the journey to the dinner table. QUICK CATCH MUSCLE MUSCLE i I MIDDLE LOBE INNER LOBE FUSION POINT sets up a barrierbetween the gill, or intake, chamber and the cloacal, or output, chamber. boiling whirlpools of the Strait of Messina, we brought back not only Pinna nobilis but an other shell associated with the pomp of an cient royalty.* This was Murex brandaris, a shell that launched a thousand ships and helped determine the course of ancient empire. From this three-inch mollusk and the relat ed Trunculariopsis,Phoenician artisans of the cities of Tyre and Sidon made the splendid purple dye that has been the color of royalty ever since. They pushed across the Mediter ranean in search of them. Fishermen trapped vast quantities of the mollusks in plaited boxes baited with cockles, the little leaping clams that murex likes to eat. The Phoenicians crushed the murex shells, extracted the mantles, salted them, and ex posed them to the sun for two or three days. Poured into a kettle and covered with water, the mantles simmered for 10 days over a low fire. The result was a clear broth that changed in sunlight to bright yellow, through shades of green to blue, and finally to a permanent brilliant magenta. Cloth dyed with this beautiful and endur ing purple cost $10,000 to $12,000 a pound. Only the rich and the mighty were allowed to wear it, and the phrase "born to the purple" became a fixture. Indo-Pacific Holds Varied Treasures The Mediterranean lies within one of the four regions into which malacologists divide the world of marine mollusks (map, pages 396-7). But by far the most fabulous for the diversity of its shell-bearing animals is the Indo-Pacific, a vast reach of water extending from the Red Sea and eastern coast of Africa across the Indian Ocean, and into the Pacific beyond Hawaii and Easter Island. The shores of India, of all Australia, Ma laysia, Indonesia, the South Sea islands *See "Fishing in the Whirlpool of Charybdis," by Paul A. Zahl, NATIONAL GEOGRAPHIC, November 1953.