They can be found inhabiting trees, gardens, freshwater ponds and streams, estuaries, tidal pools, beaches, the continental shelf, and the deep ocean.
Some mollusks are excellent swimmers, others crawl or burrow in mud and sand.
The number of living species of mollusks has been estimated to range from 50,000 to 130,000. The mollusks are invertebrates, unsegmented, and most exhibit bilateral symmetry. Most mollusks can be described as free-living, multicellular animals that possess a true heart, and that have a calcareous exterior skeleton that covers at least the back or upper surface of the body. This exterior skeleton provides support for a muscular foot and the internal body organs, including the stomach mass. A thin flap of tissue called the mantle surrounds the internal organs of most mollusks, and it is this mantle that secretes the animal's shell.