Phylum Mollusca Mollusks are soft-bodied animals with a specialized layer of epidermal cells called a mantle that secretes a shell. Mollusks produce many kinds of external shells - some mollusks have only a remnant internal shell, and others have no shell at all. This phylum's diversity of 100,000 species is surpassed only by arthropods and probably nematodes. Although mollusks are coelomate, their coelom is often reduced to a small chamber surrounding the heart. Open circulatory systems have a few large vessels and a heart, but no smaller vessels and capillaries.
Some mollusks also have a differentiated head.
The mantles of the left and right valves join posteriorly to form a ventral incurrent siphon and a dorsal excurrent siphon that direct water through the clam. When a clam burrows in sediment the siphons extend to the water. THe space between the mantle and visceral mass is the mantle cavity. Water flows into the mantle cavity through the incurrent siphon over the gills, and then dorsally into a space between the visceral mass where gills attach to the mantle. Water exits this chamber posteriorly through the excurrent siphon.