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 

 * '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, Tr achy car dhim. (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.) 



