Information about mollusk

Molluscs
Fossil range: Ediacaran or Cambrian - Recent

Caribbean Reef Squid, Sepioteuthis sepioidea

Kingdom:	Animalia
Phylum:	Mollusca Linnaeus, 1758

Classes

Caudofoveata
Aplacophora
Polyplacophora
Monoplacophora
Bivalvia
Scaphopoda
Gastropoda
Cephalopoda
† Rostroconchia
† Helcionelloida
† ?Bellerophontida

The molluscs (British spelling) or mollusks (American spelling) are members of the very large and diverse phylum Mollusca. Molluscs include a wide variety of animals such as clams and snails, squid and octopus, which are well-known and valued by humans either as seafood or for their decorative shells. Molluscs live in a wide variety of habitats, in the oceans, on land and in freshwater.

There are some 112,000 species within this phylum.^[1] The scientific study of molluscs is called malacology.

Molluscs range from minute snails and clams to larger organisms such as squid, cuttlefish and octopus, which are among the most neurologically-advanced invertebrates^[2].

The vast majority of molluscs live in marine environments, and are found intertidally, in the shallow subtidal and on the continental shelf, although some species do live in the abyssal depths of the oceans around hot vents. Not all mollusks are marine: two taxomonic groups or classes, the bivalves and the gastropods, also contain freshwater species. Only the gastropods have representatives that live on land: the land snails and slugs.

Anatomy

Molluscs are triploblastic protostomes and many demonstrate bilateral symmetry. 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.

The shell of the tiger top snail, Calliostoma tigris, from New Zealand.

Molluscs have a mantle, which is a fold of the outer skin lining the shell, and a muscular foot that in most species is used for locomotion. In most molluscs the mantle secretes a calcium carbonate external shell. In the majority of marine mollusks the gill or gills absorbs oxygen from the water.

All species of the phylum Mollusca have a complete digestive tract that starts from the mouth and runs to the anus. Many have a feeding structure, the radula, mostly composed of chitin. This radula is a feature only found in molluscs. Radulae are very 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 larval stages suggest a close relationship between the molluscs and various other protostomes, notably the Annelids.

Molluscs, because of their shells, have left an excellent fossil record, and are found from the Cambrian onwards. The oldest fossil species seems to be Odontogriphus omalus, found in the Burgess Shale. It lived about 500 million years ago.

The giant squid, which until recently had not been observed alive in its adult form,^[3] is one of the largest invertebrates; however the colossal squid is even larger.

Classification

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

There are ten classes of molluscs, eight are still living, the others are known only from fossils. These classes make up 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 (sea snails with shells, such as abalone, limpets, conch, etc, and marine snails without a shell or with a reduced shell, such as nudibranchs, sea hares; sea angel, sea butterfly, sea lemon etc; land snails and slugs, freshwater snails, total estimated at 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)

Common Octopus (Octopus vulgaris)

Main article: Evolution of Mollusca

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 - except cephalopods which are closed).
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 best known molluscan shell, but many pulmonate and opisthobranch snails have secondarily reduced and internalized shells, or have lost the shell completely. 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 ammonoids 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.

Dangerous mollusca

A small minority of molluscs represent a serious risk to humans under certain circumstances; a few octopus species and a few large cone snail species have a very poisonous bite.

Some people are severely allergic to shellfish, but even for people without these allergies, clams can sometimes be risky to eat: when there is a "red tide", or other blooms of noxious plankton, bivalves such as clams and mussels can become poisonous; because they are filter-feeders they can concentrate chemicals from floating microorganisms within their tissues.

Despite its name, the disease molluscum contagiosum is caused by a virus and is not connected with molluscs in any way.

References

1. ^ Feldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
2. ^ Barnes, R. D. (1987) Invertebrate Zoology (Fifth Edition), Saunders College Publishing, Philadelphia, USA. (pg. 456)
3. ^ Kubodera, T. & Mori, K. (2005) PDF Proceedings of the Royal Society B: Biological Sciences 272 (1581), 2583-2586.

General references

Brusca & Brusca (1990). Invertebrates. Sunderland, Mass.: Sinauer Associates.
Starr & Taggart (2002). Biology: The Unity and Diversity of Life. Pacific Grove, California: Thomson Learning.
Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 202. Checklst, atlas of distribution and bibliography for the marine mollusca of Ireland. in. Marine Biodiversity in Ireland and Adjacent Waters. Ulster Museum. publication no. 8.

External links

Cryogenian

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The Cambrian is a major division of the geologic timescale that begins about 542 ± 1.0 Ma (million years ago) at the end of the Proterozoic eon and ended about 488.3 ± 1.7 Ma with the beginning of the Ordovician period (ICS, 2004).
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S. sepioidea

Binomial name
Sepioteuthis sepioidea
(Blainville, 1823)

Synonyms

Loligo sepiodea
Blainville, 1823
Sepioteuthis biangutata
Rang, 1837

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Scientific classification or biological classification is a method by which biologists group and categorize species of organisms. Scientific classification also can be called scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis.
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Carolus Linnaeus (Carl von Linné)

Carl von Linné, Alexander Roslin, 1775. Currently owned by and hanging at the Royal Swedish Academy of Sciences.
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class is the rank in the scientific classification of organisms in biology below Phylum and above Order.

For example, Mammalia is the class used in the classification of dogs, whose phylum is Chordata (animals with notochords) and order is Carnivora (mammals that eat meat).
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Caudofoveata

Families
See text.

Caudofoveata is a small class of the phylum Mollusca, also known as Chaetodermomorpha. The class is often combined with Solenogastres and termed Aplacophora.
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Aplacophora

Subclasses and families

Subclass Chaetodermomorpha (Caudofoveata)
Chaetodermidae
Falcidentidae

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Polyplacophora
Blainville, 1816

Families

See text.

Chitons are mollusks of the class Polyplacophora that live near the edge of the ocean in most of the world, but some species have been found in deep water.
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Monoplacophora
Odhner, 1940

Orders

Cyrtonellida
Tryblidiida
Pelagiellida

Monoplacophora is a class of mollusks thought to be extinct until April 1952, when a living animal was dredged up from deep marine sediments in the Middle
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Bivalvia
Linnaeus, 1758

Subclasses

Anomalosdesmata
Cryptodonta
Heterodonta
Paleoheterodonta
Palaeotaxodonta
Pteriomorphia
and see text
Bivalves are mollusks belonging to the class Bivalvia.
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Scaphopoda
Bronn, 1862

Orders

Dentaliida
Gadilida

The tusk shells are a class (biology) of marine mollusks which vary in size from very small to medium sized. The official name of this class is Scaphopoda, meaning "shovel-footed".
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Gastropoda
Cuvier, 1797

Subclasses

Eogastropoda (True Limpets and relatives)
Orthogastropoda

The gastropods, also previously known as gasteropods, or univalves
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Cephalopoda
Cuvier, 1797

Orders

Subclass Nautiloidea

†Plectronocerida
†Ellesmerocerida
†Actinocerida
†Pseudorthocerida
†Endocerida
†Tarphycerida
†Oncocerida

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Rostroconchia

The Rostroconchia is a class of extinct mollusks dating from the early Cambrian to the late Permian. They were initially thought to be bivalves, but were later given their own class.
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Bellerophontida
Ulrich & Scofield, 1897

Families
See text.
Bellerophontida or Bellerophontina are an order of extinct snail-like molluscs that appeared in the latest Cambrian and continued through to the Triassic.
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British English (BrE, BE, en-GB) is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere in the Anglophone world.
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American English (AmE, AE, AmEng, USEng, en-US), also known as United States English or U.S. English, is a set of dialects of the English language used mostly in the United States.
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phylum (Greek Φῦλον plural: Φῦλα phyla) is a taxon in the rank below kingdom and above class.
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clam is a kind of mollusc that has a shell divided into two pieces called valves, in other words, a clam is a bivalve mollusc.

The word "clam" has no real taxonomic significance in biology. However in the USA the word can sometimes be used to mean any bivalve mollusc.
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snail is loosely applied to almost all members of the molluscan class Gastropoda which have coiled shells in the adult stage.

The class Gastropoda is the second largest class of invertebrates, second only to the insects.
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For other uses, see Squid (disambiguation).

Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops
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Octopoda
Leach, 1818

Suborders

†Pohlsepia (incertae sedis)
†Proteroctopus (incertae sedis)
†Palaeoctopus (incertae sedis)
Cirrina
Incirrina
Synonyms

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Seafood is any sea animal or seaweed that is served as food or is suitable for eating, particularly seawater animals, such as fish and shellfish (including mollusks and crustaceans).
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shell is a hard, rigid outer layer, which has evolved in a very wide variety of different animals, including mollusks, sea urchins, crustaceans, turtles and tortoises, armadillos, etc.
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species is one of the basic units of biological classification. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring.
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Science (from the Latin scientia, 'knowledge'), in the broadest sense, refers to any systematic knowledge or practice.^[1] Examples of the broader use included political science and computer science, which are not incorrectly named, but rather named according to
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Malacology is the branch of invertebrate zoology which deals with the study of mollusks, the second-largest phylum of animals in terms of described species.^[1] One division of malacology, conchology, is devoted to the study of shelled mollusks.
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Mollusk

Anatomy

Classification

Dangerous mollusca

See also

References

General references

External links