Dissection Guide and Worksheet for the Clam (Mussel)
Mollusks are soft-bodied invertebrates. They have a muscular foot and a mantle. In most mollusks, the mantle secretes ahard shell. In this investigation you will observe the external and internal structures of a representative mollusk--the clam or fresh-water mussel. Clams are pelecypods, or bivalves, and have a two-part hinged shell. Clams are found in fresh water in streams, ponds, and lakes. They also are very common burrowed into the mud of ocean mud flats. Clams are often used for food.
Clams belong to the phylum Mollusca. Molluscs (Latin molluscus, "soft"), as the origin of the name suggests, are soft-bodied animals having an internal or external shell. Included in the phylum are snails, oysters, slugs, clams, octopuses, and squids. Most molluscs are bilaterally symmetrical (have a left and right side) and have well-developed respiratory, excretory, circulatory, and digestive systems. Some may have a calcareoushell surrounding the body mass.
Molluscs are similar to annelids in their development. Both have trochophore larvae. Molluscs differ from annelids, however, in the absence of segmentation. Further, the coelom, so prominent in the annelids, is greatly reduced in the molluscs and is generally restricted to an area surrounding the heart.
Most molluscs are slow moving, but the bodies of several species have been highly modified for rapid locomotion. Although primarily marine organisms, some molluscs are found in fresh water (clams and snails) and on land (snails and slugs).
The molluscs are characterized by having three main body regions: a head-foot, which is the sensory and locomotive part of the body; a visceral mass containing the excretory, digestive, and circulatory organs; and the mantle, which secretes the shell. The gills, which function in respiration, are located inside the mantle.
