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Scientific classification
Aplacophora[?] (solenogasters)
Polyplacophora (chitons)
Bivalvia (bivalves)
Scaphopoda[?] (tusk shells)
Gastropoda (snails and slugs)
Cephalopoda (squids, octopuses, etc)

The mollusks or molluscs are a large and diverse phylum of Animals, including a variety of familiar creatures, well-known for their decorative shells or as seafood. These range from tiny snails and clams to the octopus and squid (which are some of the most intelligent invertebrates). The giant squid is the largest invertebrate, and except for their larvae and some recently captured juveniles, has never been observed alive.

Mollusks are triploblastic[?] protostomes. The principal body cavity is a blood-filled hemocoel, with an actual coelom present but reduced to vestiges around the hearts, gonads, and metanephridia (kidney-like organs). The body is divided into a head, often with eyes or tentacles, a muscular foot and a visceral mass housing the organs. Covering the body is a thick sheet called the mantle, which in most forms secretes a calcareous shell.

Development passes through one or two trocophore stages, one of which (the veliger) is unique to the group. These suggest a close relationship between the mollusks and various other protostomes, notably the Annelids. Mollusk fossils are some of the best known and are found from the Cambrian onwards. There are eight living classes:

hypothetical                     Polyplacophora
ancestral                Monoplacophora
mollusk                   Gastropoda

Brusca & Brusca (1990) suggest that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, so indicated in the relationship diagram above.

Brusca & Brusca, 1990. Invertebrates. Sinauer Associates, Inc., Sunderland, Mass.

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