through the burrow to the surface when the tide is in, and draws into its shell the water containing animalcules and oxygen.
Oysters to the value of many millions of dollars are gathered and sold every year. The most valuable oyster fisheries of the United States are in Chesapeake Bay. The young oysters, or "spat," after they attach themselves to the bottom in shallow water, are transplanted. New oyster beds are formed in this way. The beds are sometimes strewn with pieces of rock, broken pottery, etc., to encourage the oysters to attach themselves. The dark spot in the fleshy body of the oyster is the digestive gland, or liver. The cut ends of the tough adductor muscles are noticeable in raw oysters. The starfish is very destructive in oyster beds.
Pearls are deposited by bivalves around some irritating particle that gets between the shell and the mantle. The pearl oyster furnishes most of the pearls; sometimes pearls of great value are obtained from fresh-water mussels in the United States. Name articles that are made partly or wholly of mother-of-pearl.
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Fig. 202.—A Snail.
l, mouth; vf, hf, feelers; e, opening of egg duct; fu, foot; ma, mantle; lu, opening to lung; a, vent.
Study of a Live Snail or Slug.—Is
its body dry or
moist? Do land
snails and slugs have lungs or gills? Why? How many pairs
of tentacles has it? What is their relative length and position?
The eyes are dark spots at bases of tentacles of snail and at the
tips of the rear tentacles of slug. Touch the tentacles. What
happens? Do the tentacles simply stretch, or do they turn inside
