Mollusca is one of the most diverse
groups of animals on the planet, with at least 50,000 living species (and more
likely around 200,000). It includes such familiar organisms as snails,
octopuses, squid, clams, scallops, oysters, and chitons. Mollusca also includes
some lesser known groups like the monoplacophorans, a
group once thought to be extinct for millions of years until one was found in
1952 in the deep ocean off the coast of Costa Rica.
Molluscs are a clade of organisms
that all have soft bodies which typically have a "head" and a
"foot" region. Often their bodies are covered by a hard exoskeleton,
as in the shells of snails and clams or the plates of chitons.
Parts of almost every ecosystem in
the world, molluscs are extremely important members of many ecological
communities. They range in distribution from terrestrial mountain tops to the
hot vents and cold seeps of the deep sea, and range in size from 20-meter-long
giant squid to microscopic
aplacophorans, a
millimeter or less in length, that live between sand grains.
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