Mollusca

Mollusca is a <a href="/facts/Phylum/ELT3Jyax">phylum</a> of <a href="/facts/Protostome/XLtWYdt8">protostomic</a> <a href="/facts/Invertebrate/3CyqCVnN">invertebrate</a> <a href="/facts/Animal/4b8J33nd">animals</a>, whose members are known as molluscs or mollusks . Around 76,000 <a href="/facts/Extant_taxon/YK8uj7qU">extant</a> <a href="/facts/Species/oDg6b96r">species</a> of molluscs are recognized, making it the second-largest <a href="/facts/Animal/4b8J33nd">animal</a> phylum after <a href="/facts/Arthropoda/JKJLTFzp">Arthropoda</a>. The number of additional <a href="/facts/Fossil/by7NzIA1">fossil</a> species is estimated between 60,000 and 100,000, and the proportion of undescribed species is very high. Many taxa remain poorly studied.
Molluscs are the largest <a href="/facts/Marine_biology/3pNR9o6L">marine</a> phylum, comprising about 23% of all the named <a href="/facts/Marine_organism/TF5MGzFb">marine organisms</a>. They are highly diverse, not just in size and <a href="/facts/Anatomical/p8TCWJuV">anatomical</a> structure, but also in behaviour and <a href="/facts/Habitat/zo1DN2gA">habitat</a>, as numerous groups are <a href="/facts/Freshwater_mollusc/IlfW91WV">freshwater</a> and even <a href="/facts/Terrestrial_molluscs/rLCtwqKJ">terrestrial</a> species. The phylum is typically divided into 7 or 8 <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">taxonomic</a> <a href="/facts/Class_(biology)/a7MrgFu3">classes</a>, of which two are entirely <a href="/facts/Extinct/24p5NP2q">extinct</a>. <a href="/facts/Cephalopod/80GlMxHG">Cephalopod</a> molluscs, such as <a href="/facts/Squid/2FEvJ5wG">squid</a>, <a href="/facts/Cuttlefish/9AwRKA6M">cuttlefish</a>, and <a href="/facts/Octopus/z5uokKFF">octopuses</a>, are among the most <a href="/facts/Neurobiology/o5RV8Cas">neurologically advanced</a> of all <a href="/facts/Invertebrate/3CyqCVnN">invertebrates</a>—and either the <a href="/facts/Giant_squid/jNIw5vaP">giant squid</a> or the <a href="/facts/Colossal_squid/RIRss7u1">colossal squid</a> is the largest known extant invertebrate species. The <a href="/facts/Gastropod/rYPqm3PT">gastropods</a> (<a href="/facts/Snail/y94YHrPw">snails</a>, <a href="/facts/Slug/WYCTF9py">slugs</a> and <a href="/facts/Abalone/qcq58R6J">abalone</a>) are by far the most diverse class and account for 80% of the total classified molluscan species.
The four most universal features defining modern molluscs are a soft body composed almost entirely of <a href="/facts/Muscle/dmxrL8pH">muscle</a>, a <a href="/facts/Mantle_(mollusc)/GqSY3XB6">mantle</a> with a significant cavity used for breathing and <a href="/facts/Excretion/mspsTfnv">excretion</a>, the presence of a <a href="/facts/Radula/nQ0YeYCl">radula</a> (except for <a href="/facts/Bivalves/IQDxHVc8">bivalves</a>), and the structure of the <a href="/facts/Nervous_system/CDxRKU2U">nervous system</a>. 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, "<a href="/facts/Limpet/cAyIMhoT">limpet</a>-like" <a href="/facts/Gastropod_shell/HIIBd2oQ">shell</a> on top, which is made of <a href="/facts/Protein/Jxmagu3E">proteins</a> and <a href="/facts/Chitin/Fo1f8b86">chitin</a> reinforced with <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a>, 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 <a href="/facts/Coelomate/PD25dd6m">coelomates</a>, the <a href="/facts/Coelom/PD25dd6m">coelom</a> tends to be small.
The main body cavity is a <a href="/facts/Hemocoel/WXwudzzc">hemocoel</a> through which <a href="/facts/Blood/03zEulkj">blood</a> circulates; as such, their <a href="/facts/Circulatory_system/EGs6Y7sI">circulatory systems</a> are mainly <a href="/facts/Open_circulatory_system/WXwudzzc">open</a>. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded <a href="/facts/Mucus/hnlBuHAy">mucus</a> and microscopic, muscle-powered "hairs" called <a href="/facts/Cilia/C9l436HM">cilia</a> play various important roles. The generalized mollusc has two paired <a href="/facts/Ventral_nerve_cord/s7Q7toBB">nerve cords</a>, or three in <a href="/facts/Bivalve/IQDxHVc8">bivalves</a>. The <a href="/facts/Brain/Xo5ABfXl">brain</a>, in species that have one, encircles the <a href="/facts/Esophagus/XWa7WoWn">esophagus</a>. Most molluscs have <a href="/facts/Eye/h3lbYU55">eyes</a>, and all have sensors to detect chemicals, vibrations, and <a href="/facts/Somatosensory_system/0nhRsBnt">touch</a>. The simplest type of molluscan <a href="/facts/Reproductive_system/gL90a4cf">reproductive system</a> relies on <a href="/facts/External_fertilization/PB4mUeJx">external fertilization</a>, but more complex variations occur. Nearly all produce <a href="/facts/Egg/2a1McdtR">eggs</a>, from which may emerge <a href="/facts/Trochophore/bSNMnxRv">trochophore</a> <a href="/facts/Larvae/GKo2FIew">larvae</a>, more complex <a href="/facts/Veliger/A8W8PlDN">veliger</a> 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, <a href="/facts/Cephalopod/80GlMxHG">cephalopods</a>, and bivalves in the <a href="/facts/Cambrian/jIfzKtod">Cambrian</a> period, 541–485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral <a href="/facts/Lophotrochozoa/sz5UzpHC">Lophotrochozoa</a> and of their diversification into the well-known living and <a href="/facts/Fossil/by7NzIA1">fossil</a> forms are still subjects of vigorous debate among scientists.

Molluscs have been and still are an important food source for humans. Toxins that can accumulate in certain molluscs under specific conditions create a risk of food poisoning, and many jurisdictions have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably <a href="/facts/Pearl/7ombxKgg">pearls</a>, <a href="/facts/Mother_of_pearl/a5ek6xm4">mother of pearl</a>, <a href="/facts/Tyrian_purple/gwCmPKbz">Tyrian purple</a> dye, and <a href="/facts/Sea_silk/WX6pancq">sea silk</a>. Their shells have also been <a href="/facts/Shell_money/cNmuTTJd">used</a> as <a href="/facts/Money/3BYGU6C8">money</a> in some preindustrial societies.
A handful of mollusc species are sometimes considered hazards or pests for human activities. The bite of the <a href="/facts/Blue-ringed_octopus/Cun08eH0">blue-ringed octopus</a> is often fatal, and that of <a href="/facts/Enteroctopus_dofleini/jJRxrUrd">Enteroctopus dofleini</a> causes <a href="/facts/Inflammation/Ura86MIy">inflammation</a> that can last over a month. Stings from a few species of large tropical <a href="/facts/Cone_shell/2s30wDxM">cone shells</a> of the family Conidae can also kill, but their sophisticated, though easily produced, venoms have become important tools in <a href="/facts/Neurological/TXYEMSYI">neurological</a> research. <a href="/facts/Schistosomiasis/qXtvvvUo">Schistosomiasis</a> (also known as bilharzia, bilharziosis, or snail fever) is transmitted to humans by 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 <a href="/facts/Ecosystem/kVE8mSmR">ecosystems</a>.

Mollusca open-in-new

Mollusca