63-459: A squid ( pl. : squid) is a mollusc with an elongated soft body, large eyes, eight arms , and two tentacles in the orders Myopsida , Oegopsida , and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting these criteria). Like all other cephalopods , squid have a distinct head, bilateral symmetry , and a mantle . They are mainly soft-bodied, like octopuses , but have
126-405: 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 . 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, " limpet -like" shell on top, which
189-435: A 'rake' to comb up filaments from the sea floor. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts. Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing
252-518: A cloud of ink to distract predators. Squid are used for human consumption with commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific and elsewhere. They are used in cuisines around the world, often known as " calamari ". Squid have featured in literature since classical times, especially in tales of giant squid and sea monsters . Squid are members of
315-471: A cone-shaped posterior region known as the "visceral hump". The mollusc shell is reduced to an internal, longitudinal chitinous "pen" in the functionally dorsal part of the animal; the pen acts to stiffen the squid and provides attachments for muscles. On the functionally ventral part of the body is an opening to the mantle cavity, which contains the gills (ctenidia) and openings from the excretory, digestive and reproductive systems . An inhalant siphon behind
378-435: A group comprising cephalopods. Molluscus is used in classical Latin as an adjective only with nux ( nut ) to describe a particular type of soft nut. The use of mollusca in biological taxonomy by Jonston and later Linnaeus may have been influenced by Aristotle 's τὰ μαλάκια ta malákia (the soft ones; < μαλακός malakós "soft"), which he applied inter alia to cuttlefish . The scientific study of molluscs
441-418: A hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells. The shell consists of three layers: the outer layer (the periostracum ) made of organic matter, a middle layer made of columnar calcite , and an inner layer consisting of laminated calcite, often nacreous . In some forms the shell contains openings. In abalones there are holes in the shell used for respiration and
504-403: A maximum of 120,000 species. The total number of described species is difficult to estimate because of unresolved synonymy . 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 . The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of
567-399: A number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control
630-407: A pair of statocysts , which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. 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. In bivalves,
693-678: A rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords , or three in bivalves . The brain , in species that have one, encircles the esophagus . Most molluscs have eyes , and all have sensors to detect chemicals, vibrations, and touch . The simplest type of molluscan reproductive system relies on external fertilization , but more complex variations occur. Nearly all produce eggs , from which may emerge trochophore larvae , more complex veliger larvae, or miniature adults. The coelomic cavity
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#1732779915831756-424: 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 pearls , mother of pearl , Tyrian purple dye, and sea silk . Their shells have also been used as money in some preindustrial societies. A handful of mollusc species are sometimes considered hazards or pests for human activities. The bite of
819-496: A shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs ) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate ), except the outermost layer, which in almost all cases is all conchiolin (see periostracum ). Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora . While most mollusc shells are composed mainly of aragonite , those gastropods that lay eggs with
882-456: A singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. 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. If the osphradia detect noxious chemicals or possibly sediment entering
945-558: A site for muscle attachment. The cuttlebone or sepion of the Sepiidae is calcareous and appears to have evolved afresh in the Tertiary . Squid are soft-bodied molluscs whose forms evolved to adopt an active predatory lifestyle. The head and foot of the squid are at one end of a long body, and this end is functionally anterior , leading the animal as it moves through the water. A set of eight arms and two distinctive tentacles surround
1008-470: A small internal skeleton in the form of a rod-like gladius or pen, made of chitin . Squid diverged from other cephalopods during the Jurassic and occupy a similar role to teleost fish as open water predators of similar size and behaviour. They play an important role in the open water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts
1071-415: Is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton . The blood of these molluscs contains the respiratory pigment hemocyanin as an oxygen -carrier. The heart consists of one or more pairs of atria ( auricles ), which receive oxygenated blood from the gills and pump it to
1134-2041: Is accepted that Sepiidae cuttlefish are a kind of squid, then the squids, excluding the vampire squid, form a clade as illustrated. Orders are shown in boldface; all the families not included in those orders are in the paraphyletic order "Oegopsida", except Sepiadariidae and Sepiidae that are in the paraphyletic order "Sepiida", Nautilus [REDACTED] Octopoda [REDACTED] Vampyroteuthidae (vampire squid) [REDACTED] Cranchiinae (glass squid A) [REDACTED] Cycloteuthidae Psychroteuthidae (glacial squid) [REDACTED] Onychoteuthidae (hooked squid) [REDACTED] Taoniinae (glass squid B) [REDACTED] Architeuthidae (giant squid) [REDACTED] Lepidoteuthidae (Grimaldi scaled squid) [REDACTED] Octopoteuthidae (octopus squid) [REDACTED] Ancistrocheiridae (sharpear enope squid) [REDACTED] Lycoteuthidae (firefly squid) [REDACTED] Pyroteuthidae (fire squid) [REDACTED] Bathyteuthidae [REDACTED] Ommastrephidae (flying squid) [REDACTED] Pholidoteuthidae [REDACTED] Gonatidae (armhook squid) [REDACTED] Chiroteuthidae (whip-lash squid) [REDACTED] Sepiolida (bobtail squid) [REDACTED] Sepiadariidae (pyjama and bottletail squid) [REDACTED] Chtenopterygidae [REDACTED] Thysanoteuthidae [REDACTED] Enoploteuthidae [REDACTED] Brachioteuthidae [REDACTED] Neoteuthidae Histioteuthidae (cock-eyed squid) [REDACTED] Batoteuthidae (bush-club squid) Mastigoteuthidae (whip-lash squid) [REDACTED] Joubiniteuthidae (Joubin's squid) [REDACTED] Magnapinnidae (bigfin squid) [REDACTED] Spirulida (ram's horn squid) [REDACTED] Myopsida (neritic squid) Loliginidae [REDACTED] Sepiidae (cuttlefish) [REDACTED] Idiosepiidae (pygmy squid) [REDACTED] Crown coleoids (the common ancestor of octopuses and squid) diverged in
1197-437: Is accordingly called malacology . 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. As now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned. The most universal features of
1260-529: Is an endemic snail species of the Eastern Alps . There is strong evidence for self-fertilization in the easternmost snail populations of this species. The most basic molluscan larva is a trochophore , which is planktonic and feeds on floating food particles by using the two bands of cilia around its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through
1323-402: Is associated with the odontophore , a cartilaginous supporting organ. The radula is unique to the molluscs and has no equivalent in any other animal. Molluscs' mouths also contain glands that secrete slimy mucus , to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string". At the tapered rear end of
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#17327799158311386-449: Is made of proteins and chitin reinforced with calcium carbonate , 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 coelomates , the coelom tends to be small. The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open . The "generalized" mollusc's feeding system consists of
1449-433: Is one of the largest invertebrates , surpassed in weight but not in length by the colossal squid . Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of non-marine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004
1512-667: Is reduced. They have an open circulatory system and kidney-like organs for excretion. Good evidence exists for the appearance of gastropods, cephalopods , and bivalves in the Cambrian period, 541–485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil 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
1575-420: Is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot". The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs. The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat , to sea, fresh water or air. The cavity
1638-554: 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 ecosystems . The words mollusc and mollusk are both derived from the French mollusque , which originated from the post-classical Latin mollusca , from mollis , soft, first used by J. Jonston (Historiæ Naturalis, 1650) to describe
1701-487: The IUCN Red List of Threatened Species included nearly 2,000 endangered non-marine molluscs. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of non-marine species. Because of the great range of anatomical diversity among molluscs, many textbooks start
1764-405: The blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Stings from a few species of large tropical cone shells of the family Conidae can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis, or snail fever)
1827-462: The coelom , a small cavity that surrounds the heart, into which they shed ova or sperm . The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization . Some molluscs use internal fertilization and/or are hermaphrodites , functioning as both sexes; both of these methods require more complex reproductive systems. C. obtusus
1890-404: The foot of the ancestor is modified into a complex set of appendages around the mouth. The sense organs are highly developed and include advanced eyes similar to those of vertebrates . The ancestral shell has been lost, with only an internal gladius , or pen, remaining. The pen, made of a chitin-like material, is a feather-shaped internal structure that supports the squid's mantle and serves as
1953-422: The ram's horn squid of the monotypic order Spirulida . The vampire squid ( Vampyroteuthis infernalis ), however, is more closely related to the octopus than to any squid. The cladogram , not fully resolved, is based on Sanchez et al., 2018. Their molecular phylogeny used mitochondrial and nuclear DNA marker sequences; they comment that a robust phylogeny "has proven very challenging to obtain". If it
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2016-400: The ventricle , which pumps it into the aorta (main artery ), which is fairly short and opens into the hemocoel. 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 . A pair of metanephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from
2079-412: The anus. New tissue grows in the bands of mesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. 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. Eventually, the larva sinks to
2142-411: The arms and tentacles in some species, but their function is unclear. The two tentacles are much longer than the arms and are retractile. Suckers are limited to the spatulate tip of the tentacle, known as the manus . In the mature male, the outer half of one of the left arms is hectocotylised – and ends in a copulatory pad rather than suckers. This is used for depositing a spermatophore inside
2205-672: The body structure of molluscs are a mantle with a significant body cavity used for breathing and excretion , and the organization of the nervous system. Many have a calcareous shell. Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. The following are present in all modern molluscs: Other characteristics that commonly appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to
2268-456: The class Cephalopoda , subclass Coleoidea . The squid orders Myopsida and Oegopsida are in the superorder Decapodiformes (from the Greek for "ten-legged"). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from squids and differ recognizably in their gross anatomical features. They are the bobtail squid of order Sepiolida and
2331-480: The coleoids remained in marine environments. The ancestral coleoid was probably nautiloid-like with a strait septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the cuttlefishes , the squids and the octopuses . Squid have differentiated from the ancestral mollusc such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been
2394-569: The food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion , and largely locate their prey by sight. They are among the most intelligent of invertebrates , with groups of Humboldt squid having been observed hunting cooperatively . They are preyed on by sharks , other fish, sea birds, seals and cetaceans , particularly sperm whales . Squid can change colour for camouflage and signalling . Some species are bioluminescent , using their light for counter-illumination camouflage, while many species can eject
2457-421: The foot is adapted for burrowing into the sediment; in cephalopods it is used for jet propulsion, and the tentacles and arms are derived from the foot. Most molluscs' circulatory systems are mainly open , except for cephalopods , whose circulatory systems are closed . Although molluscs are coelomates , their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity
2520-422: 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 . The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of
2583-399: The funnel draws water into the mantle cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion. In this form of locomotion, water is sucked into the mantle cavity and expelled out of the funnel in a fast, strong jet. The direction of travel is varied by the orientation of the funnel. Squid are strong swimmers and certain species can "fly" for short distances out of
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2646-592: The late Paleozoic ( Mississippian ), according to fossils of Syllipsimopodi , an early relative of vampire squids and octopuses. True squid diverged during the Jurassic, but many squid families appeared in or after the Cretaceous . Both the coleoids and the teleost fish were involved in much adaptive radiation at this time, and the two modern groups resemble each other in size, ecology, habitat, morphology and behaviour, however some fish moved into fresh water while
2709-475: The living molluscs. In 2009, Chapman estimated the number of described living mollusc species at 85,000. Haszprunar in 2001 estimated about 93,000 named species, which include 23% of all named marine organisms. Molluscs are second only to arthropods in numbers of living animal species — far behind the arthropods' 1,113,000 but well ahead of chordates ' 52,000. About 200,000 living species in total are estimated, and 70,000 fossil species, although
2772-421: The mantle cavity of a female. A ventral part of the foot has been converted into a funnel through which water exits the mantle cavity. The main body mass is enclosed in the mantle, which has a swimming fin along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and internal. The visceral mass, which is covered by a thin, membranous epidermis , forms
2835-468: The mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart. Molluscs use intracellular digestion . 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 radula primarily functions to scrape bacteria and algae off rocks, and
2898-422: The most neurologically advanced of all invertebrates —and either the giant squid or the colossal squid is the largest known extant invertebrate species. The gastropods ( snails , slugs and abalone ) 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 muscle ,
2961-456: The mouth; each appendage takes the form of a muscular hydrostat and is flexible and prehensile, usually bearing disc-like suckers. The suckers may lie directly on the arm or be stalked. Their rims are stiffened with chitin and may contain minute toothlike denticles. These features, as well as strong musculature, and a small ganglion beneath each sucker to allow individual control, provide a very powerful adhesion to grip prey. Hooks are present on
3024-929: The number of classes of molluscs; for example, the table below shows seven living classes, and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora . Two of the commonly recognized "classes" are known only from fossils. Brachiopods Bivalves Monoplacophorans ("limpet-like", "living fossils") Scaphopods (tusk shells) Gastropods ( snails , slugs , limpets , sea hares ) Cephalopods ( nautiloids , ammonites , octopus , squid , etc.) Aplacophorans (spicule-covered, worm-like) Polyplacophorans (chitons) Wiwaxia Halkieria † Orthrozanclus † Odontogriphus Vampyroteuthidae Too Many Requests If you report this error to
3087-524: The oceans, from the seashores to the abyssal zone , but some form a significant part of the freshwater fauna and the terrestrial ecosystems . Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes . About 80% of all known mollusc species are gastropods. Cephalopoda such as squid , cuttlefish , and octopuses are among the most neurologically advanced of all invertebrates. The giant squid , which until recently had not been observed alive in its adult form,
3150-411: The part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems. 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. The cephalic molluscs have two pairs of main nerve cords organized around
3213-548: The proportion of undescribed species is very high. Many taxa remain poorly studied. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms . They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat , as numerous groups are freshwater and even terrestrial species. The phylum is typically divided into 7 or 8 taxonomic classes , of which two are entirely extinct . Cephalopod molluscs, such as squid , cuttlefish , and octopuses , are among
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#17327799158313276-440: 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 body of a mollusc has a ventral muscular foot, which is adapted to different purposes (locomotion, grasping the substratum, burrowing or feeding) in different classes. The foot carries
3339-613: The seafloor and metamorphoses into the adult form. While metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult. 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. Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as
3402-415: 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. The sorting process is by no means perfect. Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus, in
3465-412: 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. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it. The particles are sorted by yet another group of cilia, which send
3528-435: 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. The depiction is visually rather similar to modern monoplacophorans . The generalized mollusc is an unsegmented, bilaterally symmetrical animal and has a single, " limpet -like" shell on top. The shell
3591-499: The three functioning as the principal center of "thinking". 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 simplest molluscan reproductive system relies on external fertilization , but with more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to
3654-423: The total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today. Molluscs have more varied forms than any other animal phylum . They include snails , slugs and other gastropods ; clams and other bivalves ; squids and other cephalopods ; and other lesser-known but similarly distinctive subgroups. The majority of species still live in
3717-402: The urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity. 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. Most molluscs have only one pair of gills, or even only
3780-520: The usual fashion, but its diet includes protists such as the xenophyophore Stannophyllum . Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls, whereas dorid nudibranchs and some Vetigastropoda feed on sponges and others feed on hydroids . (An extensive list of molluscs with unusual feeding habits is available in the appendix of GRAHAM, A. (1955). "Molluscan diets" . Journal of Molluscan Studies . 31 (3–4): 144. .) Opinions vary about
3843-417: The water over their gills. Most bivalves are filter feeders, which can be measured through clearance rates. Research has demonstrated that environmental stress can affect the feeding of bivalves by altering the energy budget of organisms. Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in
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#17327799158313906-407: The water. Mollusc Mollusca is a phylum of protostomic invertebrate animals , whose members are known as molluscs or mollusks ( / ˈ m ɒ l ə s k s / ). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda . The number of additional fossil species is estimated between 60,000 and 100,000, and
3969-437: Was at the rear in the earliest molluscs, but its position now varies from group to group. 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) known as "Organs of bojanus" and gonads (reproductive organs) are in the mantle cavity. The whole soft body of bivalves lies within an enlarged mantle cavity. The mantle edge secretes
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