66 
ZOOLOGY. 
equivalve, or with both valves alike, but not equilateral, one end (the 
anterior) being distinguishable from the other or posterior, the clam 
burrowing into tlie mud by the anterior end, that containing the 
mouth of the mollusk. The hinge is situated directly over the heart, 
and is therefore dorsal or haemal. On the interior of the shells are 
the two round "muscular impressions" made by the two adductor 
muscles and the ''pallial impression," parallel to the edge of the 
shell, made by the thickened edge of the mantle. On carefully open- 
ing the shell, by dividing the two adductor muscles, and laying the 
animal on one side in a dissecting trough filled with water, and re- 
moving the upper valve, the mantle or body-walls will be disclosed; 
the edge is much thickened, while within, the mantle where it covers 
the elliptical rounded body is very thin. The so-called black head, 
or siphon, is divided by a partition into two tubes, the upper, or 
Fig. 73.— CTnio complanatus, fresh-water mussel, partly buried in the sand, the 
siphonal openings above the level of the river-bottom. 
that on the hinge or dorsal side, being excurrent; the lower and 
larger being incurrent — a current of sea-water laden with minute 
forms of lite passing into it. Each orifice is surrounded with a circle 
of short tentacles. This siphon protrudes through a slit in the man- 
tle-edge, and is very extensible, as seen in Fig. 72, J.; it is extended, 
when the clam is undisturbed, from near the bottom of its hole to 
the level of the sea-bottom. In the fresh -water mussel ( Unto, Fig. 73) 
the two siphonal openings are above the level of the sandy bottom 
of the water, when the mussel is ploughing its way through the sand 
with its tongue-shaped foot, which is a muscular organ attached to the 
body mass. In the foot is an orifice for the passage in and out of 
wmter, but the spurting of w^ater from the clam's hole, observed in 
walking over the flats, is the stream thus ejected from the siphon. The 
inflowing currents of water pass from the inner end of the muscular 
