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36 American Seashells
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 incHned to shy from
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'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.
\ I
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, Trachy cardhim. (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.
TELLINI\ CHLf\MYS MYTILUS VOLSELLA
Fi(;i!KK 13. Fecal pellets of niollusks are characteristic in shape and may aid in
identification of genera and species. (After H. B. Moore 193 1.)