AskDefine | Define mollusc

Dictionary Definition

mollusc n : invertebrate having a soft unsegmented body usually enclosed in a shell [syn: mollusk, shellfish]

User Contributed Dictionary

English

Alternative spellings

Etymology

From French mollusque, from New Latin Mollusca (phylum name), from Latin molluscus (thin-shelled), from mollis (soft); see .

Pronunciation

  • italbrac RP: , /ˈmɒləsk/, /"mQl@sk/
  • italbrac US: , /ˈmɑːləsk/, /"mA:l@sk/

Noun

  1. A soft-bodied invertebrate of the phylum Mollusca, typically with a hard shell of one or more pieces.
  2. A weak-willed person.

Translations

soft-bodied invertebrate of phylum Mollusca

References

pedialite Mollusca
  • American Heritage 2000
  • Dictionary.com
  • CancerWEB
  • WordNet 2003

Extensive Definition

Molluscs (British spelling) or mollusks (American spelling) are animals belonging to phylum Mollusca. The word mollusc is derived from the French mollusque, which originated from the Latin molluscus, meaning thin-shelled, from mollis, soft. The scientific study of molluscs is known as malacology.
There are around 100,000 extant species within the phylum with an estimated 70,000 extinct species. They range widely in size from micromolluskan snails and clams to larger organisms such as the Colossal Squid, believed to be the world's largest invertebrate. Molluscs are typically divided into ten taxonomic classes, of which two are entirely extinct is one of the largest invertebrates; however the colossal squid is even larger.

Classification

There are ten classes of molluscs; eight of the classes have living representatives, the other two classes are known only from fossils. More than 250,000 species of mollusc are recognized and named. Snails (Gastropoda) account for about 80% of living mollusc diversity.

Evolution

It is believed that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, as indicated in the relationship diagram to the right.
In this phylum's level of organization, organ systems from all three primary germ layers can be found:
  1. Nervous system (with brain)
  2. Excretory system (nephridium or nephridia)
  3. Circulatory system (open circulatory system - except cephalopods which have a closed system)
  4. Respiratory system (gills or lungs)
All major molluscan groups possess a skeleton, though it has been lost through evolution 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 shell 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.
The first definitive evidence for molluscs comes from an early Cambrian radula, but the Ediacaran organism Kimberella is held by some to be an ancestral mollusc.

Dangerous molluscs

A very small minority of molluscs can represent a serious risk to humans under the wrong circumstances.
All octopuses are venomous but only a few species pose a significant threat to humans, such as octopuses in the genus Haplochlaena which have a very poisonous bite. A few of the larger tropical cone snail species have a very poisonous sting. These bites and stings can sometimes be fatal.
Some people are severely allergic to shellfish as a food item. However, 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, or when there are high concentrations of bacteria in the water from sewage run-off, bivalves such as clams and mussels can temporarily become very problematic as a food source. This is because bivalves are filter-feeders, and thus they can concentrate toxins from floating microorganisms within their tissues.
The traditional idea that the giant clam can trap the leg of a person between its valves, thus drowning them, has been shown to be a myth.
Despite its name, the disease molluscum contagiosum is caused by a virus, and is not connected with molluscs in any way.

References

General references

  • Biology: The Unity and Diversity of Life
  • Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 202. Checklist, atlas of distribution and bibliography for the marine mollusca of Ireland. in. Marine Biodiversity in Ireland and Adjacent Waters. Ulster Museum. publication no. 8.
  • Ponder, Winston F. and Lindberg, David R. (Eds.) (2008) Phylogeny and Evolution of the Mollusca. Berkeley: University of California Press. 481 pp. ISBN 978-0520250925.

External links

mollusc in Arabic: رخويات
mollusc in Min Nan: Nńg-thé tōng-bu̍t
mollusc in Bulgarian: Мекотели
mollusc in Catalan: Mol·lusc
mollusc in Czech: Měkkýši
mollusc in Welsh: Molwsg
mollusc in Danish: Bløddyr
mollusc in German: Weichtiere
mollusc in Estonian: Limused
mollusc in Modern Greek (1453-): Μαλάκια
mollusc in Spanish: Mollusca
mollusc in Esperanto: Molusko
mollusc in Persian: نرم‌تنان
mollusc in French: Mollusca
mollusc in Korean: 연체동물
mollusc in Croatian: Mekušci
mollusc in Ido: Molusko
mollusc in Indonesian: Mollusca
mollusc in Interlingua (International Auxiliary Language Association): Mollusco
mollusc in Icelandic: Lindýr
mollusc in Italian: Mollusca
mollusc in Hebrew: רכיכות
mollusc in Latin: Mollusca
mollusc in Latvian: Moluski
mollusc in Luxembourgish: Weechdéieren
mollusc in Lithuanian: Moliuskai
mollusc in Hungarian: Puhatestűek
mollusc in Macedonian: Мекотели
mollusc in Dutch: Weekdieren
mollusc in Japanese: 軟体動物
mollusc in Norwegian: Bløtdyr
mollusc in Norwegian Nynorsk: Blautdyr
mollusc in Occitan (post 1500): Mollusca
mollusc in Polish: Mięczaki
mollusc in Portuguese: Moluscos
mollusc in Romanian: Moluscă
mollusc in Quechua: Llamp'u uywa
mollusc in Russian: Моллюски
mollusc in Simple English: Mollusc
mollusc in Slovak: Mäkkýše
mollusc in Slovenian: Mehkužci
mollusc in Serbian: Мекушци
mollusc in Finnish: Nilviäiset
mollusc in Swedish: Blötdjur
mollusc in Telugu: మొలస్కా
mollusc in Thai: หอย
mollusc in Vietnamese: Động vật thân mềm
mollusc in Turkish: Yumuşakçalar
mollusc in Ukrainian: Молюски
mollusc in Chinese: 软体动物
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