Mollusca

Mollusca
Temporal range: Cambrian Stage 2–Recent
Tonicella-lineata.jpg
Tonicella lineata, a polyplacophoran or chiton, anterior end towards the right
Scientific classification edit
Kingdom:Animalia
Superphylum:Lophotrochozoa
Phylum:Mollusca
Linnaeus, 1758
Classes

See text.

Diversity[1]
85,000 recognized living species.
Cornu aspersum (formerly Helix aspersa) – a common land snail

Mollusca is the second largest phylum of invertebrate animals. The members are known as molluscs or mollusks[note 1] (k/). Around 85,000 extant species of molluscs are recognized.[2] The number of fossil species is estimated between 60,000 and 100,000 additional species.[3]

Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 8 or 9 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species.

The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small.The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults.The coelomic cavity is reduced.They have an open circulatory system and kidney-like organs for excretion.

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

Molluscs have been and still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies.

Mollusc species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.

Etymology

The words mollusc and mollusk are both derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia (lit. "the soft ones"; < μαλακός malakós "soft"), which he applied inter alia to cuttlefish.[4][5] The scientific study of molluscs is accordingly called malacology.[6]

The name Molluscoida was formerly used to denote a division of the animal kingdom containing the brachiopods, bryozoans, and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs. As it is now known these groups have no relation to molluscs, and very little to one another, the name Molluscoida has been abandoned.[7]

Other Languages
Afrikaans: Weekdiere
Alemannisch: Weichtiere
አማርኛ: ዛጎል ለበስ
العربية: رخويات
aragonés: Mollusca
asturianu: Mollusca
Avañe'ẽ: Syrymbe
azərbaycanca: Molyusklar
تۆرکجه: مولیوسکلار
বাংলা: মলাস্কা
Bân-lâm-gú: Nńg-thé tōng-bu̍t
башҡортса: Моллюскылар
беларуская: Малюскі
беларуская (тарашкевіца)‎: Малюскі
български: Мекотели
bosanski: Mehkušci
brezhoneg: Blotvil
català: Mol·luscs
Cebuano: Kinhason
čeština: Měkkýši
Cymraeg: Molwsg
dansk: Bløddyr
Deutsch: Weichtiere
eesti: Limused
Ελληνικά: Μαλάκια
español: Mollusca
Esperanto: Moluskoj
euskara: Molusku
فارسی: نرم‌تنان
føroyskt: Lindýr
français: Mollusca
Gaeilge: Moileasc
galego: Moluscos
한국어: 연체동물
हिन्दी: मोलस्का
hornjoserbsce: Mjechkuše
hrvatski: Mekušci
Ido: Molusko
Bahasa Indonesia: Moluska
interlingua: Mollusca
íslenska: Lindýr
italiano: Mollusca
עברית: רכיכות
Basa Jawa: Moluska
ქართული: მოლუსკები
Kinyarwanda: Ikinyamushongo
Kiswahili: Moluska
Кыргызча: Моллюск
Latina: Mollusca
latviešu: Gliemji
Lëtzebuergesch: Weechdéieren
lietuvių: Moliuskai
Limburgs: Weikdere
Lingua Franca Nova: Molusco
magyar: Puhatestűek
македонски: Мекотели
മലയാളം: മൊളസ്ക
Bahasa Melayu: Moluska
Mìng-dĕ̤ng-ngṳ̄: Niōng-tā̤ dông-ŭk
Nederlands: Weekdieren
日本語: 軟体動物
Napulitano: Mollusca
Nordfriisk: Wokdiarten
norsk: Bløtdyr
norsk nynorsk: Blautdyr
occitan: Mollusca
ଓଡ଼ିଆ: ସନ୍ଧିପଦ
oʻzbekcha/ўзбекча: Mollyuskalar
ਪੰਜਾਬੀ: ਮੋਲਸਕਾ
پنجابی: مولسکا
Plattdüütsch: Weekdeerter
polski: Mięczaki
português: Moluscos
română: Moluscă
Runa Simi: Llamp'uka
русский: Моллюски
Scots: Mollusca
Simple English: Mollusc
slovenčina: Mäkkýše
slovenščina: Mehkužci
српски / srpski: Мекушци
srpskohrvatski / српскохрватски: Mekušci
svenska: Blötdjur
Tagalog: Mollusca
tarandíne: Mollusca
తెలుగు: మొలస్కా
tetun: Molusku
Türkçe: Yumuşakçalar
українська: Молюски
vèneto: Mołusco
West-Vlams: Molluuskn
Winaray: Mollusca
吴语: 软体动物
ייִדיש: מאלוסקן
粵語: 軟體動物
中文: 软体动物