. Principles of economic zoo?logy. Zoology, Economic. GASTEROPODA 81 its shell partly exposed. Along the Atlantic coast people use the Mya or Ve7ius for their " ; Many hundred bushels are used every year for this purpose. The razor-shell clams have similar habits. They are concealed in vertical holes in the sand with the posterior end of the shell uppermost. They have a powerful club-shaped foot, and can dig so rapidly that unless one approaches very cautiously they escape from view. They seem to be sensitive to light and to the " jar " made by approaching footst
. Principles of economic zoo?logy. Zoology, Economic. GASTEROPODA 81 its shell partly exposed. Along the Atlantic coast people use the Mya or Ve7ius for their " ; Many hundred bushels are used every year for this purpose. The razor-shell clams have similar habits. They are concealed in vertical holes in the sand with the posterior end of the shell uppermost. They have a powerful club-shaped foot, and can dig so rapidly that unless one approaches very cautiously they escape from view. They seem to be sensitive to light and to the " jar " made by approaching footsteps. The borer (Pho'las) has its brittle but very hard shell marked like a file, with which it bores into the hardest rocks. The united siphons are longer than the rest of the body. Some forms are phosphorescent, emitting bluish-white light. The ship-worm (Tere'do) (Fig. 60), another borer, works into wood, doing much damage to ships in the tropics. The larva enters the wood when it is extremely small and enlarges the tunnel as it grows. The wood which it excavates is not used for food, but is carried off by the excretory siphon. Its food, which consists of microscopic organisms, is brought in by the currents. The amount of damage these borers do seems incredible. They completely honeycomb the hull of a wooden vessel. The best protection against them is the sheathing of the hull with copper. Palmetto is the best resistant among woods. The ship-worms caused the destruction of a dam in Holland, threatening destruction to the country. Their dis- persal is wide, since they are carried all over the world in the floating wood which they attack. CLASS II. GASTEROP'ODA These are asymmetric, usually univalve moUusks, and the head region bears either one or two pairs of tentacles. As in the snail (Fig. 61), the eyes are borne either at the bases or at the. Fig. 61.—A snail. (After Tenney.) tips of the tentacles. The shorter tentacles are probably organs of smell. The head contains the
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