Mollusks are in a group containing between sixty and seventy thousand species-second only to insects. Although usually found in the sea, mollusks also live on land and in fresh water, and can be found all over the world. They can be found above the snow line in the Himalayas to ocean depths where water pressure is measured in tons per square inch. They can be found in scorching deserts and also in blocks of ice. They range in size from microscopic sea slugs to the sixty-foot giant squid. Generally speaking, mollusks are characterized by a soft, un-segmented body; a protective external shell; a muscular sole-like organ called the foot; a rasp-like "tongue" fitted with many rows of hooked teeth, and a fold of tissue that lines the shell and secretes the calcium carbonate with which the shell is built.
Animals, commonly referred as "Hermit Crabs", are sometimes found in shells as well. How does this happen? When a mollusk dies or is eaten, the shell becomes vacant. There are millions of crab-like creatures that roam the ocean floor, which are in search of mainly two things: a home and food. As they happen upon a shell, they will inspect it to see if it is empty. This is a risky thing for these creatures because they could be eaten by the mollusk if they get too close. After inspecting the shell and feeling that it is of adequate size, it will enter its new home. It will continue to dwell in this shell until it gets too big for it. When it gets too big, it will search for a new shell and repeat the entire process.