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38 American Seash ells
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
Figure 14. The pair of proboscides in the Nut Clams, Nucula, sweep up food and
transport it to the mouth. The gills are not used in gathering food as in the majority
of clams, a, X5; b and c, ends of the proboscides. Xio. (After K. Hirasaka 1927.)
very specialized. In the Nucula Nut Clams, a pair of strong, muscular, con-
tractile 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.
HOW THEY BREATHE
Oddly enough, the gills of the bivalves are not primarily used for respira-
tion, 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 com-
pletely 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.