Malacology
Caudofoveata
Aplacophora
Polyplacophora - Chitons
Monoplacophora
Bivalvia - Bivalves
Scaphopoda - Tusk shells
Gastropoda - Snails and Slugs
Cephalopoda - Squids, Octopuses, etc.
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The
mollusks or
molluscs are 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 and
clams to the
octopus and
squid (which are considered 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, although the
Colossal Squid is likely to be even larger.
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.
Mollusks have a mantle, which is a shell-like outer cover, 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 of the phylum Mollusca have a complete digestive tract that starts from the mouth to the anus. Many have a
radula, mostly composed of
chitin, in the mouth, which allows then to scrape food from the surface by sliding back and forth. Mollusks also have a
coelom, made from cell masses, where all organs are suspended. Unlike Coelomates, mollusks lack body segmentation.
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 and one class, known only from fossils:
- 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 angel, sea butterfly, Sea Lemon; estimated 40,000 - 150,000 species)
- Class Cephalopoda (squids, octopuses, nautilus, cuttlefish; 786 species, all marine)
- Class † Important publications in mollusk
References
- Brusca & Brusca, 1990. Invertebrates, Sinauer Associates, Inc., Sunderland, Mass.
- Starr & Taggart, 2002. Biology: The Unity and Diversity of Life, Thomson Learning, inc., Pacific Grove, California.
The Mollusk is the title of a 1997 album by Ween.
