[75] The diagram on the right summarizes a phylogeny presented in 2007 without the annelid worms. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. 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 ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). Mollusca is considered to the second-largest phylum of animals after Arthropoda and are invertebrates. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly The People’s Choice 2020 Word Of The Year: 2020 Was A $#@#%%$@! The Dictionary.com Word Of The Year For 2020 Is …. [1] Haszprunar in 2001 estimated about 93,000 named species,[18] which include 23% of all named marine organisms. It contains the body organs. Most bivalves are filter feeders, which can be measured through clearance rates. Mollusks have soft bodies, and their bodies are not divided into rings like the segmented worms called annelids . Start studying Definitions for Mollusca. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Most mollusk species grow a hard shell for … Keep scrolling for more. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. (figuratively) A weak-willed person. Most material © 2005, 1997, 1991 by Penguin Random House LLC. One of the grand divisions of the animal kingdom, including the classes Cephalopoda, Gastropoda, PteropodaScaphopoda, and Lamellibranchiata, or Conchifera. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora. [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. Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. [26][14](p291–292) Two of the commonly recognized "classes" are known only from fossils.[16]. Molluscs (or mollusks) are an important phylum of invertebrate animals. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. 2016: The origin of the animals and a "Savannah" hypothesis for early bilaterian evolution. Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. [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. Trigo, J.E. [10], Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. Are you learning Spanish? The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. Some feed on microscopic, filamentous algae, often using their radula as a 'rake' to comb up filaments from the sea floor. Does “Bimonthly” Mean Twice A Month Or Every Two Months? Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable. 652) and including cephalopods, echinoderms, hydroids, slugs, and … [15], At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so it acts as a bobbin, winding the mucus string onto itself. Journal of Molluscan Studies. a comic character, usually masked, dressed in … Their venom is a complex mixture of toxins, some fast-acting and others slower but deadlier. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it. "[92], Mollusc shells, including those of cowries, were used as a kind of money (shell money) in several preindustrial societies. [15] The whole soft body of bivalves lies within an enlarged mantle cavity. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills. The large group of animals called mollusks live on land and in both freshwater and salt water. 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. Many have a calcareous shell. Mollusca live two hundred fathoms down in the Norwegian seas. [21], Molluscs have more varied forms than any other animal phylum. [45] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. adj., adj mollus´cous. Classification into higher taxa for these groups has been and remains problematic. molluscum [mŏ-lus´kum] 1. any of various skin diseases marked by the formation of soft rounded cutaneous tumors. The Most Surprisingly Serendipitous Words Of The Day. 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. ii. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. The shell is secreted by a mantle covering the upper surface. 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. Molluscs are some of the most successful organisms on Earth, and after arthropods they are the most diverse group of invertebrates. [12], The mantle cavity, a fold in the mantle, encloses a significant amount of space. [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. Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Molluscs have been and still are an important food source for anatomically modern humans. They are particularly useful since they are sessile so that they are representative of the environment where they are sampled or placed. Their population status or structure, physiology, behaviour or the level of contamination with elements or compounds can indicate the state of contamination status of the ecosystem. Eventually, the larva sinks to the seafloor and metamorphoses into the adult form. Although molluscs are coelomates, the coelom tends to be small. The anus, a pair of osphradia (chemical sensors) in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity. A number of species of molluscs can bite or sting humans, and some have become agricultural pests. They also occur in freshwater and on land. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, 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 underside consists of a single muscular "foot". Information and translations of molluscs in the most comprehensive dictionary definitions resource on … 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. 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. Or do you just have an interest in foreign languages? [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. & Rolán, E., ed. Other Words from mollusk Example Sentences Learn More about mollusk. There are more than 100,000 species, or types, of mollusk. [14](pFront endpaper) About 200,000 living species in total are estimated,[1][20] and 70,000 fossil species,[13] although the total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today. 2. molluscum contagiosum. [12], The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Bivalvia, formerly called Lamellibranchiata, or Conchifera). ; García-Álvarez, O.L. [37] Image courtesy of Jan Delsing, Wikimedia Commons The phylum Mollusca is the second-largest animal phylum, with over 100,000 species. Numerous molluscs also live in freshwater and terrestrial habitats. [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. In the 1990s, this species reached the West Indies. [63] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.[64]. mollusk. 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. mollusc definition: any animal that has a soft body, no spine, and is often covered with a shell. [70] Even so, bivalves remain abundant and diverse. 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. 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. [12], In some forms the shell contains openings. [36], Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. 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. An estimated 200 million people in 74 countries are infected with the disease – 100 million in Africa alone. 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. 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. [107], "Mollusk" redirects here. & Urgorri, V. (2018). In gastropods, it secretes mucus as a lubricant to aid movement. The underside of the animal consists of a single muscular "foot". Mollusc species can also represent hazards or pests for human activities. Learn more. 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 gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species. It is a major group in the animal kingdom as regards the number which occu­pies next to arthropods. Molluscs are found in nearly all freshwater and marine environments, and some are found also on land. n. 1. gastropods; bivalves; cephalopods; chitons 2. [3] The number of fossil species is estimated between 60,000 and 100,000 additional species. Most are encased in a protective calcareous shell. Definition of mollusk. Meaning of Mollusca with illustrations and photos. Budd, G. E., and S. Jensen. [57], Some analyses of helcionellids concluded these were the earliest gastropods. [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. The group includes the gastropods (snails, slugs, etc), bivalves (clams, mussels, etc), and cephalopods (cuttlefish, octopuses, etc) [50] In one particular branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans; but this is difficult to reconcile with the embryological origins of spicules. [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. [87], Other luxury and high-status products were made from molluscs. Mollusca: 1 n gastropods; bivalves; cephalopods; chitons Synonyms: phylum Mollusca Type of: phylum (biology) the major taxonomic group of animals and plants; contains classes 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. Other characteristics that commonly appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species. Troncoso, J.S., Trigo, J.E. What does molluscs mean? A mollusk is a kind of animal with a soft body. For those grazing, two feeding strategies are predominant. Hypernyms ("Mollusca" is a kind of...): phylum ((biology) the major taxonomic group of animals and plants; contains classes). Mollusk definition, any invertebrate of the phylum Mollusca, typically having a calcareous shell of one, two, or more pieces that wholly or partly enclose the soft, unsegmented body, including the chitons, snails, bivalves, squids, and octopuses. [11] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. Budd, G. E. & Jensen, S. A critical reappraisal of the fossil record of the bilaterian phyla. Mollusca (n.). 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." Pronunciation of Mollusca and its etymology. The anus, in the part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. 1790–1800;
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