Mollusks are among the best known of the invertebrates. Most of us have walked along the shore of a lake or ocean collecting their shells. Mollusca is the largest marine phylum, comprising about 23% of all the named marine organisms. Phylum Mollusca includes clams, oysters, octopods, snails, slugs, and the largest of all the invertebrates, the giant squid. Numerous mollusks live in freshwater and terrestrial habitats. This phylum contains more than fifty thousand living species and thirty-five thousand fossil species (second only to the arthropods in number) have been described.

Although most mollusks are marine, many snails and clams live in fresh water, and some species of snails and slugs inhabit the land. According to past research, mollusks probably evolved early in the history of the protostomes, soon after the evolution of the coelom but before the origin of the segmented body that is characteristic of annelids and arthropods. Most mollusks share certain basic characteristics, although mollusks vary widely in outward appearance. Molluscs are highly diverse, not only in size and in anatomical structure, but also in behavior and in habitat. Molluscs have such a varied range of body structures that it is difficult to find defining characteristics that apply to all modern groups. The two most universal features are a mantle with a significant cavity used for breathing and excretion and the structure of the nervous system. The "generalized" mollusk's feeding system consists of a rasping "tongue" called a radula and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The brain, in species that have one, encircles the esophagus. Most mollusks have eyes, and all have sensors that detect chemicals, vibrations and touch. A striking feature of mollusks is the use of the same organ for multiple functions.

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Common characteristics among mollusks:
1. A broad, flat, muscular foot, located ventrally, which is used for locomotion.
2. The body organs (viscera) concentrated as a visceral mass located above the foot.
3. A soft body, usually covered by a dorsal shell composed mainly of calcium carbonate.
4. A mantle, a thin sheet of tissue that covers the visceral mass and usually contains glands that secrete a shell. The mantle generally overhangs the visceral mass, forming a mantle cavity that contains gills and other structures.
5. The coelom is generally reduced to small compartments around certain organs, including the heart and excretory organs (metanephridia). The main body cavity is typically a hemocoel, a space containing blood).
6. A rasplike structure called a radula, which is a belt of teeth in the mouth region. (The radula is not present in clams or their relatives, or in other suspension feeders.)

Fast facts!
1. Their shells have also been used as money in some pre-industrial societies.
2. A striking feature of mollusks is the use of the same organ for multiple functions. For example: the heart and kidneys are important parts of the reproductive system as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity, which is important for excretion and reproduction.
3. The words mollusc and mollusk are both derived from the French word mollusque, which originated from the Latin word molluscus, from mollis, meaning soft.
4. Molluscus was itself an adaptation of Aristotle's "the soft things", which he applied to cuttlefish .
5. The scientific study of molluscs is known as malacolgy.
6. The most abundant metallic element in mollusks is calcium.
7. About 80% of all known mollusc species are gastropods (snails and slugs)

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