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 the 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 hremal. 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.— Unto 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, A; 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 

 water, but the spurting of water 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 



