Mollusca

For the 1997 album by Ween, see The Mollusk.

Template:Taxobox begin Template:Taxobox begin placement Template:Taxobox regnum entry Template:Taxobox subregnum entry Template:Taxobox branch entry Template:Taxobox superphylum entry Template:Taxobox phylum entry
Linnaeus, 1758 Template:Taxobox end placement Template:Taxobox section subdivision Caudofoveata
Aplacophora
Polyplacophora - Chitons
Monoplacophora
Bivalvia - Bivalves
Scaphopoda - Tusk shells
Gastropoda - Snails and Slugs
Cephalopoda - Squids, Octopuses, etc.
Template:Taxobox end The molluscs or mollusks 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, clams, and abalone to the octopus and squid (which are considered the most intelligent invertebrates). 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 molluscs is called malacology.

Molluscs are triploblastic protostomes. The principal body cavity is a blood-filled hemocoel. They have a true coelom; however, it is 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.

Molluscs have a mantle, which is a fold of the outer skin lining the shell, and a muscular foot that is used for motion. Many molluscs 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 them to scrape food from the surface by sliding back and forth. Molluscs also have a coelom in which the organs are suspended. Unlike the closely related annelids, molluscs 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 molluscs and various other protostomes, notably the Annelids. Mollusc 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); now generally recognized as a subclass of Aplacophora.
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 (nudibranchs, 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 † Rostroconchia (fossils; probably more than 1,000 species; probable ancestors of bivalves)

					Caudofoveata (?)
					Aplacophora
hypothetical					Polyplacophora
ancestral					Monoplacophora
mollusc					Gastropoda
					Cephalopoda
					Bivalvia
					Scaphopoda

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.

In this Phylum's level of organization, organ systems from all three primary germ layers can be found:

Nervous System (with brain).
Excretory System (nephridium or nephridia).
Circulatory System (open).
Respiratory System (gills or lungs).

No skeletal system can be found.

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.

Mollusc Mollusc Mollusks 1999

See also

References