The mollusks are members of the large and diverse phylum Mollusca,
which includes a variety of familiar creatures well-known for their
decorative shells or as seafood. These range from tiny snails, clams,
and abalone to the octopus, cuttlefish and squid (which are considered
the most intelligent invertebrates). There are some 70,000 described
species within this phylum.
The giant squid, which until recently had not been observed alive
in its adult form, is the largest invertebrate; although it is likely
that the Colossal Squid is even larger. The scientific study of
mollusks is called malacology.
The mollusk's body is often divided into a head, with eyes or tentacles,
a muscular foot and a visceral mass housing the organs. Mollusks
have a mantle, which is a fold of the outer skin lining the shell,
and a muscular foot that is used for motion. Many mollusks have
their mantle produce a calcium carbonate external shell and their
gill extracts oxygen from the water and disposes waste.
All species have a complete digestive tract that starts from the
mouth to the anus. Many have a feeding structure, the radula, mostly
composed of chitin. Radulae are diverse within the Mollusca, ranging
from structures used to scrape algae off rocks, to the harpoon-like
structures of cone snails. Cephalopods (squid, octopuses, cuttlefish)
also possess a chitinous beak. Unlike the closely related annelids,
mollusks lack body segmentation.
Development passes through one or two trochophore 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
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