Imagine you found a creature lying on the beach, and wanted to determine if it were a mollusk or an annelid. What would you look for, and how would you know which it was?

Molluscs are amazingly diverse, with 110,000 named species, second only to the arthropods among all phyla of animals. Molluscs include such familiar creatures as clams, oysters, snails, and octopi. They share a distant common ancestor with the annelid worms, an evolutionary heritage suggested by their larval form, called a trochophore larva, found in all molluscs and in certain marine annelids called polychaete worms. It's hard to imagine that a clam could be a close cousin to the earthworm, because most familiar molluscs have a highly modified body type. The ancestral mollusc probably resembled a chiton, a flattened worm like animal protected by a dorsal shell.

Both molluscs and annelids probably evolved from free-living flatworms. Both flatworms and molluscs are triploblastic, bilaterally symmetric, and cephalized. But molluscs have developed a true coelom, an internal body cavity enclosed by mesodermal membranes. The coelom in molluscs, however, is strangely reduced to a small space around the heart, sometimes called a hemocoel. Is this the rudimentary beginning of the more elaborate coelom found in higher animals? Or did ancestral molluscs abandon a more active life style for a sedentary existence (like the clam), no longer needing a fully developed coelom? Primitive molluscs also show a rudimentary type of segmentation, an important feature of the annelid worms. Are molluscs descended from annelids, or is it the other way around? The evolution of this group remains a source of great controversy.

Molluscs are mostly aquatic, and are named from the Latin , meaning "soft". Their soft bodies are enclosed in a hard shell made of calcium carbonate (CaCO3), which functions as an exoskeleton. This shell is secreted by a thin sheet of tissue called the mantle, which encloses the internal organs like a glove. The mantle creates a small empty space called a mantle cavity, which is modified for different functions in different groups of molluscs.