Mollusca
From EvoWiki
The molluscs (British spelling) or mollusks (American spelling) are the large and diverse phylum Mollusca, which includes a variety of familiar animals well-known for their decorative shells or as seafood. These range from tiny snails, clams, and abalone to squid, cuttlefish and the octopus (which is considered the most intelligent invertebrate). There are some 112,000 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 possible that the colossal squid is even larger. The scientific study of molluscs is called malacology.
Anatomy
Molluscs are triploblastic protostomes. The principal body cavity is a blood-filled hemocoel. They have a true coelom (eucoelom); any coelomic cavities have been reduced to vestiges around the hearts, gonads, and metanephridia (kidney-like organs). The body is often divided into a head, 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 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, octopodes, cuttlefish) also possess a chitinous beak. 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. The oldest fossil seems to be Odontogriphus omalus, found in the Burgess Shale. It lived about 500 million years ago (although there are possible mollusk fossils such as Kimberella dating from the Ediacaran period however these possible mollusk fossils from the Ediacaran period are highly disputed and have benn interpreted in a variety of other ways.
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Classification
There are nine classes of molluscs, eight still living, the others known only from fossils. These classes make up the 250,000 and more species of mollusc:
- 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 (squid, octopodes, nautilus, cuttlefish; 786 species, all marine)
- Class † Rostroconchia (fossils; probably more than 1,000 species; probable ancestors of bivalves)
- Class † Helcionelloida (fossils; snail-like creatures such as Latouchella)
References
- Feldkamp, Susan (2002). Modern Biology. United States: Holt, Rinehart, and Winston. pp. 725
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