Molluscs Fossil range: Ediacaran or Cambrian - Recent
Caribbean Reef Squid, Sepioteuthis sepioidea
Scientific classification
Kingdom: Animalia Superphylum: Lophotrochozoa Phylum: Mollusca Linnaeus, 1758
Diversity
c. The Ediacaran Period (ˌiːdiˈækərən named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with The Caribbean Reef Squid ( Sepioteuthis sepioidea) also known as just the Reef Squid is a small (20 cm Torpedo -shaped Squid with Fins The Lophotrochozoa (ləˌfɒtroʊkəˈzoʊə "crest-bearing animals" are one of three major groupings of Protostome animals Carl Linnaeus (Latinized as Carolus Linnaeus, also known after his ennoblement as, May 23 new style (13 May old style 1707 who laid the foundations for 200,000 species[1]
Classes
Aplacophora Bivalvia Caudofoveata Cephalopoda Gastropoda † Helcionelloida Monoplacophora Polyplacophora † Rostroconchia Scaphopoda † Tentaculita

Molluscs (British spelling) or mollusks (American spelling) are animals belonging to phylum Mollusca. A class is the Taxonomic rank in the Biological classification of organisms in Biology below phylum and above order. Bivalves are Molluscs belonging to the class Bivalvia. They have two-part shells and typically both valves are symmetrical along the hinge line Caudofoveata is a small class of the phylum Mollusca, also known as Chaetodermomorpha. The cephalopods ( Greek plural (kephalópoda "head-feet" are the Mollusc class Cephalopoda characterized by The class Gastropoda or the gastropods, also previously known as gasteropods, or univalves, and more commonly known as Snails Monoplacophora, meaning “bearing one plate” is a class of shelled Mollusks These organisms were known only from the Fossil record ranging from the Chitons are small to large primitive marine Mollusks in the class Polyplacophora. The Rostroconchia is a class of Extinct Mollusks dating from the early Cambrian to the late Permian. The tusk shells or scaphopods are a class of marine Mollusks which vary in size from very small to medium sized British English or UK English ( BrE, BE, en-GB) is the broad term used to distinguish the forms of the English language used in the Phonology North American English regional phonology In many ways compared to English English, North American English is conservative in its Phonology. A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. The word mollusc is derived from the French mollusque, which originated from the Latin molluscus, meaning thin-shelled, from mollis, soft^[2]. French ( français,) is a Romance language spoken around the world by 118 million people as a native language and by about 180 to 260 million people Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. The scientific study of molluscs is known as malacology. Science (from the Latin scientia, meaning " Knowledge " or "knowing" is the effort to discover, and increase human understanding Malacology is the branch of Invertebrate Zoology which deals with the study of Mollusks the second-largest Phylum of animals in terms of described

There are around 100,000 extant species within the phylum^[3] with an estimated 70,000 extinct species^[4]. Extant is a term commonly used to refer to Taxa (such as Species, genera or families) that are still in existence (living In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. They range widely in size from micromolluskan snails and clams to larger organisms such as the Colossal Squid, believed to be the world's largest invertebrate. A micromollusk is a descriptive term for a shelled Mollusk which is extremely small even at full adult size The Colossal Squid ( Mesonychoteuthis hamiltoni, from Greek mesos (middle onychos (claw and teuthis (squid sometimes called An invertebrate is an Animal lacking a Vertebral column. The group includes 98% of all animal Species — all animals except those in the Chordate Molluscs are typically divided into ten taxonomic classes, of which two are entirely extinct^[3][4]. Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos A class is the Taxonomic rank in the Biological classification of organisms in Biology below phylum and above order. In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. Class Cephalopoda molluscs such as squid, cuttlefish and octopus are among the most neurologically-advanced of all invertebrates^[3]. The cephalopods ( Greek plural (kephalópoda "head-feet" are the Mollusc class Cephalopoda characterized by Squid are marine Cephalopods of the order Teuthida, which comprises around 300 species Cuttlefish are marine animals of the order Sepiida belonging to the Cephalopoda class (which also includes Squid, Octopuses The Neurobiology is the study of cells of the Nervous system and the organization of these cells into functional circuits that process information and mediate behavior

The majority of mollusc species live in marine environments, and many of them are found intertidally, in the shallow subtidal and on the continental shelf. The continental shelf is the extended perimeter of each Continent and associated Coastal plain, which is covered during interglacial periods such Pelagic species of octopus and squid live throughout the water column of the ocean, and some species of clam and limpet live in the abyssal depths of the oceans around hot hydrothermal vents. Any water in the sea that is not close to the bottom is in the pelagic zone. A water column is a conceptual column of water from surface to bottom sediments A hydrothermal vent is a Fissure in a planet's surface from which geothermally heated Water issues However, not all molluscs are marine: the bivalve and gastropod classes also contain freshwater species, and the gastropods additionally have representatives that live on land, the land snails and slugs. Bivalves are Molluscs belonging to the class Bivalvia. They have two-part shells and typically both valves are symmetrical along the hinge line The class Gastropoda or the gastropods, also previously known as gasteropods, or univalves, and more commonly known as Snails The word snail is a Common name that can be used for almost all members of the Molluscan class Gastropoda which have coiled shells in the Slug is a common non-scientific word which is often applied to any Gastropod mollusk whatsoever that has a very reduced shell a small internal shell

A number of species of molluscs are valued by humans as seafood, or for their decorative shells. Seafood is any Sea Animal or Seaweed that is served as Food, or is suitable for eating particularly saltwater animals such A seashell, also known as a sea shell, is the Common name for a hard protective outer layer a shell or in some cases a " test " that was created Edible species include various clams, snails, squid and octopuses.

Anatomy

The shell of the tiger top snail, Calliostoma tigris, from New Zealand. Calliostoma tigris tigris is a large Subspecies of sea Snail, a marine Gastropod Mollusc.

Molluscs are triploblastic protostomes and many demonstrate bilateral symmetry. Triploblasty is a condition of the Blastula in which there are three primary Germ layers the Ectoderm, Mesoderm, and Endoderm. Protostomia (from the Greek: mouth first) are a Taxon of Animals Together with the Deuterostomes and a few smaller phyla "Bilateral symmetry" redirects here For bilateral symmetry in mathematics see Reflection symmetry. The principal body cavity is a blood-filled hemocoel. By the broadest definition a body cavity is any fluid filled space in a Multicellular organism. A hemocoel is a cavity or series of spaces between the organs of organisms with Open circulatory systems like most Arthropods and Mollusks A combination They have a true coelom (eucoelom); any coelomic cavities have been reduced to vestiges around the hearts, gonads, and metanephridia (kidney-like organs). By the broadest definition a body cavity is any fluid filled space in a Multicellular organism. The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic The gonad is the organ that makes Gametes The gonads in males are the Testes and the gonads in Females are the Ovaries. Nephridia are invertebrate organs which function similarly to Kidneys They remove Metabolic wastes from an animal's body The kidneys are complicated organs that have numerous biological roles The body is often divided into a head, with eyes or tentacles, a muscular foot, and a visceral mass housing the organs. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Tentacles can refer to the elongated flexible organs that are present in some animals especially Invertebrates and sometimes to the hairs of the leaves of some insectivorous In Anatomy, a viscus (ˈvɪskəs ( Plural: viscera /ˈvɪsərə/ is an internal organ of an animal (including humans in particular an internal

Molluscs have a mantle, which is a fold of the outer skin lining the shell, and a muscular foot that in most species is used for locomotion. The mantle is an important part of the anatomy of Molluscs It is the dorsal body wall which covers the visceral mass In most molluscs the mantle secretes a calcium carbonate external shell. Calcium carbonate is a Chemical compound with the Chemical formula Ca[[Carbon C]] O 3 In the majority of marine molluscs the gill or gills absorbs oxygen from the water. A gill is an anatomical structure found in many aquatic organisms Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

All species of the phylum Molluscs have a complete digestive tract that starts from the mouth and runs to the anus. The mouth, buccal cavity, or oral cavity is the first portion of the Alimentary canal that receives food and begins digestion by mechanically breaking up The anus is an opening at the opposite end of an Animal 's Digestive tract from the Mouth. Many have a feeding structure, the radula, mostly composed of chitin. The radula is a toothed Chitinous ribbon typically used for scraping cutting and chewing food before it enters the Esophagus. Chitin ( C 8 H 13 O 5 N)n (ˈkaɪtən is a long-chain Polymer of a N-acetylglucosamine This radula is a feature only found in molluscs. Radulae are very diverse within the Mollusca, ranging from structures used to scrape algae off rocks, to the harpoon-like structures of cone snails. Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms The cone snails or cone shells, sometimes simply known as "cones" (family Conidae) are a Taxonomic family of medium-sized to large sophisticated Cephalopods (squid, octopuses, cuttlefish) also possess a chitinous beak. The cephalopods ( Greek plural (kephalópoda "head-feet" are the Mollusc class Cephalopoda characterized by Squid are marine Cephalopods of the order Teuthida, which comprises around 300 species The Cuttlefish are marine animals of the order Sepiida belonging to the Cephalopoda class (which also includes Squid, Octopuses Unlike the closely related annelids, molluscs lack body segmentation. The annelids, collectively called Annelida (from Latin anellus "little ring" are a large phylum of Animals comprising

Development passes through one or two trochophore stages, one of which, (the veliger), is unique to the group. A trochophore (ˈtrɒkəfɔər also spelled trocophore) is a type of marine Larva with several bands of Cilia. A veliger is the free-swimming Planktonic Larva of many kinds of marine and fresh-water Gastropod Molluscs as well as a number of These larval stages suggest a close relationship between the molluscs and various other protostomes, notably the Annelids. The annelids, collectively called Annelida (from Latin anellus "little ring" are a large phylum of Animals comprising

Molluscs, because of their shells, have left an excellent fossil record, and are found from the Cambrian onwards. The Cambrian is a geologic period and system that began about Ma (million years ago at the end of the Proterozoic eon and ended about Ma with The oldest fossil species seems to be Odontogriphus omalus, found in the Burgess Shale. Odontogriphus (literally "toothed riddle" is a Genus of soft-bodied animals known from middle Cambrian. See also Burgess shale type fauna The Burgess Shale is famous for the exceptional preservation of the fossils found within it in which the soft parts are preserved It lived about 500 million years ago.

The giant squid, which until recently had not been observed alive in its adult form,^[5] is one of the largest invertebrates; however the colossal squid is even larger. The giant squid ( Genus: Architeuthis) is a deep-ocean dwelling Squid in the family Architeuthidae, represented by The Colossal Squid ( Mesonychoteuthis hamiltoni, from Greek mesos (middle onychos (claw and teuthis (squid sometimes called

Classification

There are ten classes of molluscs; eight of the classes have living representatives, the other two classes are known only from fossils. A class is the Taxonomic rank in the Biological classification of organisms in Biology below phylum and above order. More than 250,000 species of mollusc are recognized and named. Snails (Gastropoda) account for about 80% of living mollusc diversity. ^[6]

Evolution

Main article: Evolution of Mollusca

It is believed that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, as indicated in the relationship diagram to the right^[8]. Caudofoveata is a small class of the phylum Mollusca, also known as Chaetodermomorpha. Neomeniomorpha is a subclass of Aplacophora, the other subclass being Chaetodermomorpha. Chitons are small to large primitive marine Mollusks in the class Polyplacophora. Monoplacophora, meaning “bearing one plate” is a class of shelled Mollusks These organisms were known only from the Fossil record ranging from the The class Gastropoda or the gastropods, also previously known as gasteropods, or univalves, and more commonly known as Snails Abalone (from Spanish Abulón) are medium-sized to very large edible sea Snails marine Gastropod Mollusks in the The name Limpet is used for many kinds of mostly saltwater but also freshwater Snails specifically those that have a simple shell which is more or less broadly conical A conch (pronounced in the USA as "konk" or "konch" ˈkɒŋk or /ˈkɒntʃ/ is one of a number of different Species A nudibranch (pronounced (BrE or (AmE is a member of one Suborder of soft-bodied shell-less marine Opisthobranch Gastropod Mollusks Sea hares ( Aplysia species and related genera are very large sea Slugs with a soft internal shell made of protein Sea butterflies, also known as Thecosomata or flapping Snails, are a Taxonomic suborder of Pelagic swimming sea snails The word snail is a Common name that can be used for almost all members of the Molluscan class Gastropoda which have coiled shells in the Slug is a common non-scientific word which is often applied to any Gastropod mollusk whatsoever that has a very reduced shell a small internal shell The cephalopods ( Greek plural (kephalópoda "head-feet" are the Mollusc class Cephalopoda characterized by Squid are marine Cephalopods of the order Teuthida, which comprises around 300 species The Cuttlefish are marine animals of the order Sepiida belonging to the Cephalopoda class (which also includes Squid, Octopuses Nautilus (from Greek ναυτίλος, 'sailor' is the common name of any marine creatures of the Cephalopod family Nautilidae, the sole Bivalves are Molluscs belonging to the class Bivalvia. They have two-part shells and typically both valves are symmetrical along the hinge line Clam is a word which can be used for all some or only a few Species of Bivalve Mollusks the word is a Common name which has The common name oyster is used for a number of different groups of Bivalve Mollusks most of which live in marine habitats or Brackish water. A scallop (ˈskɒləp or /ˈskæləp/ is a marine Bivalve Mollusk of the family Pectinidae. The common name mussel is used for members of several different families of Clams or Bivalve Molluscs, from both saltwater and freshwater habitats The tusk shells or scaphopods are a class of marine Mollusks which vary in size from very small to medium sized The Rostroconchia is a class of Extinct Mollusks dating from the early Cambrian to the late Permian. In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. Latouchella is an extinct Genus of Mollusc from the Tommotian epoch of what is now Siberia. The current understanding of molluscan phylogeny is shown below

In this phylum's level of organization, organ systems from all three primary germ layers can be found:

1. Nervous system (with brain)
2. Excretory system (nephridium or nephridia)
3. Circulatory system (open circulatory system - except cephalopods which have a closed system)
4. Respiratory system (gills or lungs)

All major molluscan groups possess a skeleton, though it has been lost through evolution in some members of the phylum. A germ layer is a collection of cells formed during animal Embryogenesis. The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain Nephridia are invertebrate organs which function similarly to Kidneys They remove Metabolic wastes from an animal's body This is an article about the rock music band "Circulatory System" A gill is an anatomical structure found in many aquatic organisms lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive It is probable that the pre-Cambrian ancestor of the molluscs had calcium carbonate spicules embedded in its mantle and outer tissues, as is the case in some modern members. The Precambrian ( Pre-Cambrian) is an informal name for the supereon comprising the eons of the Geologic timescale that came before the current

The skeleton, if present, is primarily external and composed of calcium carbonate (aragonite or calcite). The snail shell or gastropod shell is perhaps the best known molluscan shell, but many pulmonate and opisthobranch snails have secondarily reduced and internalized shells, or have lost the shell completely. The gastropod shell is a shell which is part of the body of a gastropod or Snail. The bivalve or clam shell consists of two pieces (valves), articulated by muscles and an elastic hinge. The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonoids and nautiloids, and still possessed by Nautilus today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.

The first definitive evidence for molluscs comes from an early Cambrian radula,^[9] but the Ediacaran organism Kimberella is held by some to be an ancestral mollusc. Kimberella is a Genus of Fossils known only from rocks of the Ediacaran period and only one Species, Kimberella quadrata

Dangerous molluscs

Common Octopus (Octopus vulgaris)

A very small minority of molluscs can represent a serious risk to humans under the wrong circumstances. The Common Octopus ( Octopus vulgaris) is the most studied of all Octopus species

All octopuses are venomous^[10] but only a few species pose a significant threat to humans, such as octopuses in the genus Haplochlaena which have a very poisonous bite. The blue-ringed octopuses ( Genus Hapalochlaena) are three or four small Octopus Species that live in Tide pools in A few of the larger tropical cone snail species have a very poisonous sting. The cone snails or cone shells, sometimes simply known as "cones" (family Conidae) are a Taxonomic family of medium-sized to large sophisticated These bites and stings can sometimes be fatal.

Some people are severely allergic to shellfish as a food item. However, even for people without these allergies, clams can sometimes be risky to eat. When there is a "red tide", or other blooms of noxious plankton, or when there are high concentrations of bacteria in the water from sewage run-off, bivalves such as clams and mussels can temporarily become very problematic as a food source. "Red tide" is a common name for a phenomenon known as an Algal bloom, an event in which estuarine marine or fresh water algae accumulate rapidly in the Water Plankton consist of any drifting Organisms ( Animals Plants Archaea, or Bacteria) that inhabit the Pelagic zone of This is because bivalves are filter-feeders, and thus they can concentrate toxins from floating microorganisms within their tissues.

The traditional idea that the giant clam can trap the leg of a person between its valves, thus drowning them, has been shown to be a myth. In Japanese cuisine, Geoduck (mirugai is sometimes referred to as "giant clam"

Despite its name, the disease molluscum contagiosum is caused by a virus, and is not connected with molluscs in any way. Molluscum contagiosum ( MC) is a viral infection of the Skin or occasionally of the Mucous membranes. A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable

See also

• Important publications on molluscs

References

General references

External links

• - 210,000 mollusca pictures.
• Hardy's Internet Guide to Marine Gastropods
• Molluscs in captivity