SALT AND FRESH-WATER CLAMS. 25 
forward transverse muscle. It will be seen by the position 
of the mouth, that the so-called head of the clam is not the 
head at all. One may call it the tail with more propriety, 
though it is simply two tubes united together, projecting 
from behind for the purposes before mentioned. On each 
side of the mouth are a pair of lappets or palpi; these prob- 
ably assist in directing the minute currents of food into the 
mouth. The mouth opens almost directly into an irregularly 
shaped stomach. The intestine, after several turns in the 
abdomen, passes along the back, going directly through the 
heart, and terminates above the posterior muscle. Fig. 7, 
plate 1, represents the heart as seen from above. This con- 
sists of a ventricle (v) and two aurieles (A), one on each 
side, which takes the blood from the gills. The gills are two 
in number, and hang from below the back, on each side of 
the abdomen. The thickened portion of the base of the 
tubes, commonly called the shoulder, are muscles to draw in 
the tubes. Space will not allow us to enter farther into the 
anatomy of the clam. We may add, however, that nearly 
all bivalves are organized in a similar way. We give a 
transverse section of a fresh-water mussel to show the vari- 
ous organs. (See the plate and explanation.) 
The clam is used for food in Europe, Asia and America. 
Jeffrey says, “it forms one of the numerous articles of 
Chinese diet, being brought to market after having been 
boiled for a long time, and cooked with a seasoning of which 
onions are a base. The people call it Tsega.” Fabricius 
states that in Greenland the clam is eaten by the walrus, 
Arctic fox, and birds. 
In the fresh-water clam, instead of two long tubes covered 
by one sheath as in the sea-clam, we have two short tubes, one 
only being separate, the other merging into the mantle, which 
is open throughout; though by reference to the plate it will 
be seen that the tubes bear a general resemblance to those of 
the sea-clam. In the fresh-water clam the elastic substance 
opening the shells is outside, and pulls them apart when the 
AMER. NATURALIST, VOL. III. 4 
