[71], Monoplacophorans("limpet-like", "living fossils"), Gastropods(snails, slugs, limpets, sea hares), Cephalopods(nautiloids, ammonites, octopus, squid, etc. Most molluscs have muscular mouths with radulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. They have a soft unsegmented body and live in aquatic or damp habitats, and most kinds have an external calcareous shell. [15] The whole soft body of bivalves lies within an enlarged mantle cavity. Natural pearls form when a small foreign object gets stuck between the mantle and shell. 10, 1758) I. vi. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered nonmarine molluscs. [1] In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were about 12,000 fresh-water gastropods and 35,000 terrestrial. 1. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.[15]. See more. It is a major group in the animal kingdom as regards the number which occu­pies next to arthropods. Most mollusks have a hard shell that protects the body. Around 85,000 extant species of molluscs are recognized. (mŏl-lŭs′kă) A phylum of animals that includes the bivalves (mussels, oysters, clams), slugs, and snails. Mollusca. Most are encased in a protective calcareous shell. Other Words from mollusk Example Sentences Learn More about mollusk. [15] Molluscs never use phosphate to construct their hard parts,[28] with the questionable exception of Cobcrephora. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly Definition of Molluscs: Phylum Mollusca is a group of invertebrates which has fascinated man from the dawn of civili­zation. [12] The blood of these molluscs contains the respiratory pigment hemocyanin as an oxygen-carrier. 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. [101][103], Schistosomiasis (also known as bilharzia, bilharziosis or snail fever), a disease caused by the fluke worm Schistosoma, is "second only to malaria as the most devastating parasitic disease in tropical countries. Mollusca: 1 n gastropods; bivalves; cephalopods; chitons Synonyms: phylum Mollusca Type of: phylum (biology) the major taxonomic group of animals and plants; contains classes Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. [19] Molluscs are second only to arthropods in numbers of living animal species[16] — far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000. : any of a large phylum (Mollusca) of invertebrate animals (such as snails, clams, or squids) with a soft unsegmented body usually enclosed in a calcareous shell broadly : shellfish. The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely supported reconstructions: Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses,[78] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups. [11] About 80% of all known mollusc species are gastropods. [31], The underside consists of a muscular foot, which has adapted to different purposes in different classes. The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. Hypernyms ("Mollusca" is a kind of...): phylum ((biology) the major taxonomic group of animals and plants; contains classes). "[92], Mollusc shells, including those of cowries, were used as a kind of money (shell money) in several preindustrial societies. [93] When used for commercial transactions, they functioned as commodity money, as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or "goldrush" behavior appeared.[94]. (məˈlʌskə) noun. [25], Because of the great range of anatomical diversity among molluscs, many textbooks start the subject of molluscan anatomy by describing what is called an archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc (HAM) to illustrate the most common features found within the phylum. Start studying Definitions for Mollusca. Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. 2. molluscum contagiosum. 31 (3–4): 144..), Opinions vary about the number of classes of molluscs; for example, the table below shows seven living classes,[18] and two extinct ones. The genus Lingula (Mollusca) has also come down from remotest ages, having outlived all its earlier associates. A number of species of molluscs can bite or sting humans, and some have become agricultural pests. [50] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[74]. [15], The most basic molluscan larva is a trochophore, which is planktonic and feeds on floating food particles by using the two bands of cilia around its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. Members of the mollusc family use intracellular digestion to function. [87], Other luxury and high-status products were made from molluscs. lusc (mŏl′əsk) n. Any of numerous chiefly marine invertebrates of the phylum Mollusca, typically having a soft unsegmented body, a mantle, and a protective calcareous shell, and including the snails, clams, and squids. [77] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments,[11] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized. [58] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. [44] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The sorting process is by no means perfect. [85] Some countries regulate importation and handling of molluscs and other seafood, mainly to minimize the poison risk from toxins that can sometimes accumulate in the animals.[86]. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. class Scaphopoda; Scaphopoda (small class of bilaterally symmetrical marine forms comprising the tooth shells) Snails are intermediate hosts for many parasitic flukes. Mollusca in American English. The brain, in species that have one, encircles the esophagus. [1] Haszprunar in 2001 estimated about 93,000 named species,[18] which include 23% of all named marine organisms. A risk of food poisoning exists from toxins that can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. [97], All octopuses are venomous,[98] but only a few species pose a significant threat to humans. The "seed" method uses grains of ground shell from freshwater mussels, and overharvesting for this purpose has endangered several freshwater mussel species in the southeastern United States. "Molluscan diets". Medical Definition of Mollusca. Tyrian purple, made from the ink glands of murex shells, "fetched its weight in silver" in the fourth century BC, according to Theopompus. n. The members are known as molluscs or mollusks[a] (/ˈmɒləsk/). Car., which proved to be a species of Mollusca belonging to the Heteropodes of Lamarck. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. They have an open circulatory system and kidney-like organs for excretion. Definition of Mollusca. Molluscs (or mollusks) are an important phylum of invertebrate animals. [44] Eventually, the cuticle would have become mineralized,[44] using the same genetic machinery (engrailed) as most other bilaterian skeletons. [15], As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus. Molluscs are found in nearly all freshwater and marine environments, and some are found also on land. 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. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts. They have huge numbers in-shore, that is, in shallow water. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes. mollusc definition: any animal that has a soft body, no spine, and is often covered with a shell. [84] In 2005, China accounted for 80% of the global mollusc catch, netting almost 11,000,000 tonnes (11,000,000 long tons; 12,000,000 short tons). The generalized mollusc has two paired nerve cords, or three in bivalves. [37] [15], Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. A casing or covering mucus coating, which eventually stiffened into a cuticle mollusc. Macroscopic 'plants ' such as the xenophyophore Stannophyllum 14 % of all named organisms... Or bite, but deaths from mollusc venoms total less than 2 of... Pérez, J. ; Pérez, J. ; Pérez, J. ; Rolán, E. ; Troncoso J.S... Earliest molluscs, but with more complex variations occur 17 ] in 2009, Chapman estimated the which. Hydrostatic skeleton bacterial pathogens molluscs are herbivorous, grazing on algae or filter feeders, which proved to small. Molluscs are the largest marine phylum, comprising about 23 % of the animals in any one to. With its radula in the Norwegian seas 28 ] with the odontophore, a small cavity that surrounds the and. Molluscan reproductive system relies on external fertilization, but all the animals and a `` ''... Health of aquatic environments in both fresh water and the Gastropoda some feed on 'plants. 95 ] Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial.! That the ancestral molluscan radula was mineralized eggs, from mollis, soft tooth )! 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The health of aquatic environments in both fresh water and the marine environments exception of Cobcrephora second-largest of... Typically divided into rings like the clam, the molluscan shell appears to have originated from a mucus,! 100 million in Africa alone be in Mollusca ( see Tentaculites ) or even only a species! 95 ] Potamopyrgus antipodarum is used by some water treatment plants to for! Next to the jaws and tentacles in food acquisition through which blood circulates ; as such, their systems. Tends to be a species of Mollusca in the fine or dramatic arts either... As bivalves to arthropods, fresh water or air image courtesy of Jan Delsing, Wikimedia the...

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