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definition of mollusca

[1] Haszprunar in 2001 estimated about 93,000 named species,[18] which include 23% of all named marine organisms. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Estimates of the numbers of nonmarine molluscs vary widely, partly because many regions have not been thoroughly surveyed. Most have a well-developed head, which may bear sensory tentacles; in some, like the clam, the head is very reduced. Tyrian purple, made from the ink glands of murex shells, "fetched its weight in silver" in the fourth century BC, according to Theopompus. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it. The phylum is typically divided into 8 or 9 taxonomic classes, of which two are entirely extinct. & Rolán, E., ed. [32] The foot carries a pair of statocysts, which act as balance sensors. [54], However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China,[55][56] are thought to be early molluscs with rather snail-like shells. Image courtesy of Jan Delsing, Wikimedia Commons The phylum Mollusca is the second-largest animal phylum, with over 100,000 species. Although highly diverse, all members of the phylum share certain general features. Mollusk, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. In form Mollusca, with singular or plural agreement. A phylum of the kingdom MetazoaMollusca have soft, unsegmented bodies with an anterior head, a dorsal visceral mass, and a ventral foot. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. [50] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[74]. Snails are intermediate hosts for many parasitic flukes. Definition of molluscs in the Definitions.net dictionary. Molluscs are some of the most successful organisms on Earth, and after arthropods they are the most diverse group of invertebrates. The genus Lingula (Mollusca) has also come down from remotest ages, having outlived all its earlier associates. Online biomonitoring of bivalve activity, 24/7: This page was last edited on 11 January 2021, at 22:13. 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. [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. [46] However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated. Their venom is a complex mixture of toxins, some fast-acting and others slower but deadlier. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. Blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked,[96] but their venom kills 25% of human victims. Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Bivalve molluscs are used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments. See more. [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. In gastropods, it secretes mucus as a lubricant to aid movement. [36], Most molluscs are herbivorous, grazing on algae or filter feeders. Budd, G. E., and S. Jensen. Bivalvia, formerly called Lamellibranchiata, or Conchifera). Mollusca live two hundred fathoms down in the Norwegian seas. [29] pl.) (məˈlʌskə) noun. Shelled molluscs therefore predate the earliest trilobites. Meronyms (members of "Mollusca"): mollusc; mollusk; shellfish (invertebrate having a soft unsegmented body usually enclosed in a shell). [3] The number of fossil species is estimated between 60,000 and 100,000 additional species. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of the three functioning as the principal center of "thinking". Some shell "currencies" were not used for commercial transactions, but mainly as social status displays at important occasions, such as weddings. [24], Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. 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. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered nonmarine molluscs. The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. Molluscs, especially bivalves such as clams and mussels, have been an important food source since at least the advent of anatomically modern humans, and this has often resulted in overfishing. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. [12] The radula is unique to the molluscs and has no equivalent in any other animal. Some such as the scallops have eyes around the edges of their shells which connect to a pair of looped nerves and which provide the ability to distinguish between light and shadow. [71], Monoplacophorans("limpet-like", "living fossils"), Gastropods(snails, slugs, limpets, sea hares), Cephalopods(nautiloids, ammonites, octopus, squid, etc. An open circulatory system is present in some molluscs for the blood circulation through vessels and also some have closed circulatory systems to move blood through the body enclosed entirely in blood vessels. The coelomic cavity is reduced. Definition of Mollusca. 652) and including cephalopods, echinoderms, hydroids, slugs, and … An estimated 200 million people in 74 countries are infected with the disease – 100 million in Africa alone. Many mollusks live…. Some molluscs sting or bite, but deaths from mollusc venoms total less than 10% of those from jellyfish stings. 3.52. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia (the soft ones; < μαλακός malakós "soft"), which he applied inter alia to cuttlefish. Many molluscs live in…. One of the grand divisions of the animal kingdom, including the classes Cephalopoda, Gastropoda, PteropodaScaphopoda, and Lamellibranchiata, or Conchifera. [15] The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine. (1955). Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly,[99] and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated. [9], The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. (mŏl-lŭs′kă) A phylum of animals that includes the bivalves (mussels, oysters, clams), slugs, and snails. & Urgorri, V. (2018). Molluscs' mouths also contain glands that secrete slimy mucus, to which the food sticks. Pronunciation of Mollusca and its etymology. molluscum [mŏ-lus´kum] 1. any of various skin diseases marked by the formation of soft rounded cutaneous tumors. Eventually, the larva sinks to the seafloor and metamorphoses into the adult form. For millennia, molluscs have been a source of food for humans, as well as important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, and chemical compounds. Octopuses, oysters, snails, and squid are just a … [35] The development of molluscs is of particular interest in the field of ocean acidification as environmental stress is recognized to affect the settlement, metamorphosis, and survival of larvae. In addition to the debates about whether Kimberella and any of the "halwaxiids" were molluscs or closely related to molluscs,[48][49][52][53] debates arise about the relationships between the classes of living molluscs. mollusc definition: any animal that has a soft body, no spine, and is often covered with a shell. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. Attempts to control it by introducing the predatory snail Euglandina rosea proved disastrous, as the predator ignored Achatina fulica and went on to extirpate several native snail species, instead. Glory-of-the-sea snail (Conus gloriamaris) is prized by collectors for its ornate shell. [14](pp300–343, 367–403) The best natural pearls are produced by marine pearl oysters, Pinctada margaritifera and Pinctada mertensi, which live in the tropical and subtropical waters of the Pacific Ocean. [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. A number of species of molluscs can bite or sting humans, and some have become agricultural pests. 3.51 A). [11] About 80% of all known mollusc species are gastropods. Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs. [42] (An extensive list of molluscs with unusual feeding habits is available in the appendix of .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .id-lock-free a,.mw-parser-output .citation .cs1-lock-free a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited a,.mw-parser-output .id-lock-registration a,.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription a,.mw-parser-output .citation .cs1-lock-subscription a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}GRAHAM, A. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species. Their shells have also been used as a form of currency in some preindustrial societies. Molluscs (or mollusks) are an important phylum of invertebrate animals. [15] In bivalves, the foot is adapted for burrowing into the sediment;[32] in cephalopods it is used for jet propulsion,[32] and the tentacles and arms are derived from the foot. The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). noun An invertebrate of a large phylum which includes snails, slugs, mussels, and octopuses. [100], All species of cone snails are venomous and can sting painfully when handled, although many species are too small to pose much of a risk to humans, and only a few fatalities have been reliably reported. : 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. Either way, this quiz on Spanish words for animals is for you. Many taxa remain poorly studied.[5]. Most molluscs have only one pair of gills, or even only a singular gill. The radula primarily functions to scrape bacteria and algae off rocks, and is associated with the odontophore, a cartilaginous supporting organ. [45] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. The group includes the gastropods (snails, slugs, etc), bivalves (clams, mussels, etc), and cephalopods (cuttlefish, octopuses, etc) [76] For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm. Related words - Mollusca synonyms, antonyms, hypernyms and hyponyms. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life. The marine … A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar.[15]. adj., adj mollus´cous. Most molluscs have muscular mouths with radulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. the phylum comprising the mollusks. [12] The blood of these molluscs contains the respiratory pigment hemocyanin as an oxygen-carrier. Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood. CS1 maint: bot: original URL status unknown (, "Phylogeny and taxonomy of the Kermia–Pseudodaphnella (Mollusca: Gastropoda: Raphitomidae) genus complex: A remarkable radiation via diversification of larval development", "Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons", "First-ever observations of a live giant squid in the wild", "The global decline of nonmarine mollusks", 10.1641/0006-3568(2004)054[0321:TGDONM]2.0.CO;2, "A comparative study of the ultrastructure and mineralogy of calcified land snail eggs (Pulmonata: Stylommatophora)", "How the Pacific Oyster Responds to Ocean Acidification: Development and Application of a Meta-Analysis Based Adverse Outcome Pathway", "Xenophyophores (Protozoa, Sarcodina) in the diet of, "Coexistence of hydroid-eating Nudibranchs: Do feeding biology and habitat use matter? [79], However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded. Most mollusk species grow a hard shell for … Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. [65][66][67][68] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago. they have foot along with ventricle structure for locomotion. As now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned. [62] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago. [33], Most molluscs' circulatory systems are mainly open. 1. molluscum [n] - See also: molluscum. mollusk. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string". ; Guerra, A.; Moreira, J.; Pérez, J.; Rolán, E.; Troncoso, J.S. Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 million years ago. mollusk meaning: 1. any animal that has a soft body, no spine, and is often covered with a shell. [69] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event. Although molluscs are coelomates, the coelom tends to be small. Biol. 2, McClure's Magazine, Vol 31, No 2, June 1908. [17] In 2009, Chapman estimated the number of described living mollusc species at 85,000. [38] Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls,[39] whereas dorid nudibranchs and some Vetigastropoda feed on sponges[40][41] and others feed on hydroids. [51], There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms. [26][14](p291–292) Two of the commonly recognized "classes" are known only from fossils.[16]. Many have a calcareous shell. Volborthella's classification is uncertain. [14](pp300–343)[59][60], Volborthella, some fossils of which predate 530 million years ago, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. In forms having only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell. mollusc (plural molluscs) A soft-bodied invertebrate of the phylum Mollusca, typically with a hard shell of one or more pieces. Mollusca (n.) "division of invertebrate animals with soft, unsegmented bodies, no jointed legs, and commonly covered by hard shells," 1797, from Modern Latin mollusca, chosen by Linnaeus as the name of an invertebrate order (1758), from neuter plural of Latin molluscus "thin-shelled," from mollis "soft," from PIE root *mel-(1) "soft." Tentaculita may also be in Mollusca (see Tentaculites). [61] The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. It is a major group in the animal kingdom as regards the number which occu­pies next to arthropods. Keep scrolling for more. [75] The diagram on the right summarizes a phylogeny presented in 2007 without the annelid worms. Most material © 2005, 1997, 1991 by Penguin Random House LLC. Mollusca (prop. ), Aplacophorans(spicule-covered, worm-like), The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. 1790–1800;

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