Health & Medical Eating & Food

Why Do Most Mullusks Have Shells?

    Shell Formation

    • A thin layer of tissue called the mantle covers the soft body. Its cells transform calcium, absorbed from water or food, into the mineral calcium carbonate, which forms the hard shell. Shells often grow quickly and, according to the Yale-New Haven Teachers Institute, "A shell with a crack can be repaired within a few days."

    Shell Types

    • Mollusks with shells are called either univalve, meaning one part, or bivalve, meaning two parts. The common garden snail, with one shell, is a univalve. The snail withdraws its body into the shell if it senses danger. Clams, scallops and oysters are examples of bivalve mollusks, which have two shells joined together at the base by a muscle. The shells are usually spread apart but the muscle pulls them together if the animal needs protection.

    Shell Features

    • Univalve shells are often coiled, such as those seen on snails. These shells spiral outward in one direction as they grow larger. Bivalve shells have yearly growth rings that show the animal's approximate age, similar to the rings on a tree stump.

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