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, cuttlefish and squid (which are considered the most intelligent invertebrates). There are some 70,000 described species within this phylum [1].
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.
Anatomy
Molluscs are triploblastic protostomes. The principal body cavity is a blood-filled hemocoel. It is unknown whether 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, octopuses, 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.
Classification
There are nine classes of mollusks, eight still living and one known only from fossils, These classes make contain the 250,000 and more species of mollusc:
- 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)
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Caudofoveata (?) |
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Aplacophora |
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Polyplacophora |
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Monoplacophora |
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Gastropoda |
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Cephalopoda |
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Bivalvia |
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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 circulatory system).
- Respiratory System (gills or lungs).
All major molluscan groups possess a skeleton, though it has been lost evolutionarily in some members of the phylum. It is probable that the pre-Cambrian ancestor of the molluscs had calcium carbonate spicules embedded in its mantle and outer tissues, as is the case in some modern members. The skeleton, if present, is primarily external and composed of calcium carbonate (aragonite or calcite). The snail or gastropod shell is perhaps the most well known molluscan shell, but many pulmonate and opistrobranch snails have internalized or altogether lost the shell secondarily. The bivalve or clam shell consists of two pieces (valves), articulated by muscles and an elastic hinge. The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonites and nautiloids, and still possessed by Nautilus today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.
A very typical mollusc is the curdmaene (also known as geurverspreider) that lives around the North Sea.
See also
References
