ing secreted and carried to the food grooves bordering the gills, along which the food-laden strands are carried to the mouth.
Our common Atlantic Oyster and those in France are frequently found with green gills. The “green oysters” of Marennes, France, are famous for their supposed medicinal qualities. Americans are inclined to shy from “green oysters,” because they fear the color may be a sign of spoilage. Oysters feeding upon the small diatom, Navicula ostrearia, digest these single-celled plants and absorb from them large quantities of blue pigment. In the tissues of the oyster’s gills the pigment appears in the form of a sickly but quite harmless green. Occasionally, however, our oysters may take on a general greenish tint, not due to diatoms but to an increase in the amount of copper in the tissues. Such oysters have a rather brassy taste.
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Figure 12. Siphons of bivalves projecting above the sand bottom. Mya (b) is a suspension feeder, the others deposit feeders. a, Tellina and Macoma; b, Mya; c, Gari; d, Donax; e, Trachycardium. (After C. M. Yonge 1949.)
The clam has considerable choice in what it wishes to eat, and it can reject undesirable particles of sand or oversized pieces of food. The gills and the two fleshy palps, or flaps guarding the mouth, help in sorting out the right-sized organisms. Acceptable food is taken into the funnel-shaped mouth, passed through a short esophagus and enters the stomach. Inside the stomach, a further selection of food may take place with indigestible matter being passed on immediately through the intestine. The best food passes from the stomach into the digestive gland where it is broken down chemically and absorbed into the blood stream.
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Figure 13. Fecal pellets of mollusks are characteristic in shape and may aid in identification of genera and species. (After H. B. Moore 1931.)
