definition of mollusca

A phylum of the kingdom MetazoaMollusca have soft, unsegmented bodies with an anterior head, a dorsal visceral mass, and a ventral foot. Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. Fig. [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. The molluscs include many familiar animals, including clams, snails, slugs, and squid, as well as some less familiar animals, like tusk shells and chitons (Fig. Learn more. [11][15][26], The generalized mollusc is bilaterally symmetrical and has a single, "limpet-like" shell on top. Oysters, clams, and mussels, esp. [62] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago. Either way, this quiz on Spanish words for animals is for you. Although highly diverse, all members of the phylum share certain general features. [15], The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. Mollusca (n.). Medical Definition of Mollusca. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Example sentences containing Mollusca A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. [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. In abalones there are holes in the shell used for respiration and the release of egg and sperm, in the nautilus a string of tissue called the siphuncle goes through all the chambers, and the eight plates that make up the shell of chitons are penetrated with living tissue with nerves and sensory structures. 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". [37] Shelled molluscs therefore predate the earliest trilobites. Most material © 2005, 1997, 1991 by Penguin Random House LLC. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Many have a calcareous shell. 1. gastropods; bivalves; cephalopods; chitons 2. Budd, G. E. & Jensen, S. A critical reappraisal of the fossil record of the bilaterian phyla. 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. [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. Tyrian purple, made from the ink glands of murex shells, "fetched its weight in silver" in the fourth century BC, according to Theopompus. However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. [80] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree. ; García-Álvarez, O.L. Natural pearls form when a small foreign object gets stuck between the mantle and shell. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. The anus, in the part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. Many molluscs live in…. [65][66][67][68] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago. Most bivalves are filter feeders, which can be measured through clearance rates. Carnivorous molluscs usually have simpler digestive systems. Although molluscs are coelomates, the coelom tends to be small. They constitute the phylum Mollusca, a major group of animals known to have as many as 100,000 living species and more than 50,000 fossil forms. [49][52] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from 515 to 510 million years ago are fragments of a genuinely mollusc-like radula. [32] The foot carries a pair of statocysts, which act as balance sensors. [81] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question.[82]. [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. Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. [107], "Mollusk" redirects here. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. ii. : 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 marine … Definition of Mollusca. [50] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[74]. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of nonmarine species. Many mollusks live…. For those grazing, two feeding strategies are predominant. [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. Meronyms (members of "Mollusca"): mollusc; mollusk; shellfish (invertebrate having a soft unsegmented body usually enclosed in a shell). Originally: a heterogeneous group of mostly shell-less invertebrates, forming the second order of the class Vermes in Linnaeus's classification (Systema Naturae (ed. 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. [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. [8], 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. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Good evidence exists for the appearance of gastropods (e.g., Aldanella), cephalopods (e.g., Plectronoceras, ?Nectocaris) and bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian period, c. 500 million years ago, though arguably each of these may belong only to the stem lineage of their respective classes. [45] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. The genus Lingula (Mollusca) has also come down from remotest ages, having outlived all its earlier associates. adj., adj mollus´cous. Keep scrolling for more. [16] Cephalopoda such as squid, cuttlefish, and octopuses are among the neurologically most advanced of all invertebrates. Most mollusks have a hard shell that protects the body. (1955). class Scaphopoda; Scaphopoda (small class of bilaterally symmetrical marine forms comprising the tooth shells) While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells. 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. They also occur in freshwater and on land. Their venom is a complex mixture of toxins, some fast-acting and others slower but deadlier. [97], All octopuses are venomous,[98] but only a few species pose a significant threat to humans. 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. [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. Related words - Mollusca synonyms, antonyms, hypernyms and hyponyms. 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? 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