[Illustration: FIG. 91.--Teredo, a boring shell.] The teredo or shipworm (Fig. 91) is called a worm because it secretes a limy shell, but it is really a bivalve shell open at both ends, a shell which with one exception causes more destruction than all other marine animals combined. Instinctively it bores into wood, forming an irregular tunnel and lining it with a delicate coating of carbonate of lime. Some years ago I visited on the outer Florida Reef, an old wreck which was newly buried in the sand and partly exposed at low tide. The timbers of the vessel looked strong and able to stand many a storm, yet with a blow of my hand I broke through the planking. The interior was completely honeycombed by the teredo, so that it was a maze of tubes. At this place the life of a pile of yellow pine was a year and a half; in other words, after being exposed to the teredo for this length of time, it was useless. On the Pacific, at Avalon Bay, the piles last about two years, being rapidly destroyed, even though soaked in various poisonous fluids and coated with tar. Many thousands of dollars have been expended in experiments with devices to outwit the teredo, but without avail, and they are the greatest menace to navigation and piers to-day, making their way into hulls, despite the copper sheathing. In the mud banks of the waters of Sumatra, teredos are found which attain a length of six feet, with tubes four inches in diameter. [Illustration: FIG. 92.--Mactra: _a_, foot; _b_, _c_, siphons.] The shells are famous for their beauty, the polished valves and their marvelous tints presenting attractive combinations. The common mactra (Fig. 92), the cockle (Fig. 93) with its deep radiations, the gorgeous pectens of the South, the splendid pearl-bearing shells, all tell a wonderful story of the resources of nature, and emphasize the fact that the smallest and most inconspicuous animals vie with the larger forms in beauty of shape and color. [Illustration: FIG. 93.--Cockle.] XII. THE UNIVALVES [Illustration: FIG. 94.--Group of shells: 1, Cymbium; 2, Cerithium; 3, Voluta; 4, Cardium; 5, Phorus; 6, Murex; 7, Vermetus; 8, Trochus; 9, Pholas; 10, Turritella.]