Anodonta Cygnea

Anodonta Cygnea, the large fresh-water mussel, affords a good type of the structure of the bivalved mollusca. Its shell consists of two equal valves, which articulate on a hinge line, the ligament of which keeps them open when the animal is dead. In a dissection the foot and gills are the parts that first attract attention by their size; the former is a triangular muscular organ, by which the animal crawls into the mud in which it is usually half buried. The gills are a pair of flaps composed of many lamellae, or thin plates. The mouth is just above the foot, below the anterior of the two strong muscles by which the shell is closed; from the mouth passes an oesophagus, which leads to a stomach, and this to the intestine; the anus is at the posterior or narrower end of the shell. The water that aerates the gills circulates through two tubes, which form a siphon; both openings of this are at the posterior end of the shell. The heart is a three-chambered organ just below the hinge line, and over a renal organ. The nervous system consists of three centres or ganglia united by nerve-cords. Pearls are sometimes formed in the shell from the innermost layer. Fisheries for them have been worked in England since the time of the Romans.