Mollusk

The mollusks or molluscs are a large                  Mollusks
and diverse phylum of Animals,
including a variety of familiar              Scientific classification

creatures, well-known for their         Kingdom: Animalia
decorative shells or as seafood. These
range from tiny snails and clams to     Phylum:  Mollusca

the octopus and squid (which are some                 Classes
of the most intelligent                 Caudofoveata
invertebrates). The giant squid is the  Aplacophora (solenogasters)
largest invertebrate, and, except for   Polyplacophora (chitons)
their larvae and some recently          Monoplacophora
captured juveniles, has never been      Bivalvia (bivalves)
observed alive.                         Scaphopoda (tusk shells)
                                        Gastropoda (snails and slugs)
Mollusks are triploblastic              Cephalopoda (squids, octopuses, etc)
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:

   * Class Caudofoveata (deep-sea wormlike creatures; 70 known species)
   * Class Aplacophora (solenogasters, deep-sea wormlike creatures; 250
     species)
   * Class Polyplacophora (chitons; 600 species, rocky marine shorelines)
   * Class Monoplacophora (deep-sea limpet-like creatures; 11 living
     species)
   * Class Bivalvia (also Pelecypoda) (clams, oysters, scallops, mussels;
     8,000 species)
   * Class Scaphopoda (tusk shells; 350 species, all marine)
   * Class Gastropoda (snails and slugs, limpets, sea hares; sea butterfly,
     Sea Lemon; 40,000 species)
   * Class Cephalopoda (squids, octopuses, nautilus, cuttlefish; 650
     species, all marine)

             Ê             Ê     Ê ÊÊÊ      ÊÊÊÊÊÊCaudofoveata
             Ê             Ê     Ê ÊÊÊ       ÊÊÊÊÊÊAplacophora
             hypothetical  Ê Ê ÊÊÊ ÊÊÊ    ÊÊÊÊÊÊPolyplacophora
             ancestral     Ê     Ê ÊÊÊ    ÊÊÊÊÊÊMonoplacophora
             mollusk       Ê     Ê ÊÊÊÊÊÊ     ÊÊÊÊÊÊGastropoda
             Ê             Ê     Ê ÊÊÊÊÊÊ    ÊÊÊÊÊÊCephalopoda
             Ê             Ê     Ê ÊÊÊÊÊÊ       ÊÊÊÊÊÊBivalvia
             Ê             Ê     Ê ÊÊÊÊÊÊ     ÊÊÊÊÊÊScaphopoda

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.

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