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38 septibranch clams (Poromya and Cuspidaria). In order to make up for the loss of efficient food-gathering gills, the palps near the mouth have become

very specialized. In the Nucula Nut Clams, a pair of strong, muscular, contractile organs serve as food gatherers. These proboscides are very flexible, moving about freely in all directions. Food material is picked up by the tip and is carried swiftly down a large groove in the proboscis to a palp pouch and then to the stomach by means of minute cilia. E. S. Morse very aptly described the action of these appendages in our Atlantic Nut Clam, Nucula proxima:

"Without seeing the behaviour of these appendages it is difficult to appreciate the remarkable action of these feeding organs. The graceful movements of these beautiful and translucent appendages, exceeding the length of the shell, sweeping rapidly the bottom of the dish in which they are confined, or even turned back and feeding on the surface of the shell, are a most curious and interesting sight."

Oddly enough, the gills of the bivalves are not primarily used for respiration, despite their conspicuous size. As has been noted, their main function is in connection with feeding. Some experts deny their role as respiratory organs entirely, claiming that the mantle with its extremely effective blood supply serves as the main place of oxygen and carbon dioxide exchange. It has been found that blood coming from the mantle to the heart is completely charged with oxygen received from the sea water. Undoubtedly, however, the gills do absorb oxygen to some extent. Indirectly, the gills are extremely useful in respiration, since they produce the all-important currents which bring in oxygen and remove carbon dioxide dissolved in the water.