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79-420: Diasoma is a proposed clade of mollusks uniting the classes Scaphopoda and Bivalvia . Whether scaphopods and bivalves are each other's closest living relatives among mollusks is disputed, leaving the monophyly of Diasoma in doubt. Diasoma was originally proposed on morphological grounds by Bruce Runnegar and John Pojeta Jr., in 1974. The name means "through-body", referring to the relatively straight gut with

158-405: 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

237-435: 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

316-494: A coiled shell, the whorls spiral. The central axis passes through the columella, the central pillar of the shell. Among proposed roles invoked for the variability of shells during evolution include mechanical stability, defense against predators and climatic selection. The shells of some gastropods have been reduced or partly reduced during their evolution . This reduction can be seen in all slugs , in semi-slugs and in various other marine and non-marine gastropods. Sometimes

395-435: 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

474-418: 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

553-403: 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

632-574: A molecular phylogenetic analysis published by Hao Song and colleagues in 2023 supports the monophyly of Diasoma. The extinct rostroconchs , a group possibly ancestral to the Scaphopoda, are also considered to belong to Diasoma. Song and colleagues inferred that Diasoma originated approximately 520 million years ago, during the Cambrian period , and considered the earlier fossil genera Anabarella , Watsonella , and Mellopegma to be members of

711-627: A mouth at the anterior end and anus at the posterior end, contrasting with gastropods and cephalopods , in which the gut is more curved and the mouth and anus are usually much closer together. The grouping was accepted by many studies in the 1980s and 1990s, but a phylogenetic analysis of 18s rDNA conducted by Gerhard Steiner and Hermann Dreyer in 2003 found scaphopods to be more closely related to cephalopods than bivalves. A 2020 phylogenetic analysis by Kevin Kocot and colleagues found scaphopods to be more closely related to gastropods than bivalves. However,

790-399: 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

869-407: 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,

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948-465: A rare or abnormal sinistral one. Sinistrality arose independently 19 times among marine gastropods since the start of the Cenozoic . This left-handedness seems to be more common in freshwater and land pulmonates. But still the dextral living species in gastropods seem to account for 99% of the total number. The chirality in gastropods appears in early cleavage ( spiral cleavage ) and the gene NODAL

1027-678: 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

1106-399: A ridge or shelf next to the suture ( Clavilithes ). An outer (lateral) emargination or notch, sometimes prolonged into a slit occurs in certain types (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.), and the progressive closing of this slit may give rise to a definitely marked slit band. In some cases the slit is abandoned and left as a hole ( Fissurellidae ), or by periodic renewal as

1185-424: 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

1264-496: 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

1343-411: A simple Mendelian recessive. In photographs or illustrations, a gastropod shell can be shown oriented in a number of standard ways: The shell begins with the larval shell, the (usually) minute embryonic whorls known as the protoconch , which is often quite distinct from the rest of the shell and has no growth lines. From the protoconch, which forms the apex of the spire , the coils or whorls of

1422-456: 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

1501-399: A small minority of species and genera are virtually always sinistral (left-handed), and a very few species (for example Amphidromus perversus ) show a mixture of dextral and sinistral individuals. There occur also aberrantly sinistral forms of dextral species and some of these are highly sought by shell collectors. If a coiled gastropod shell is held with the spire pointing upwards and

1580-403: A succession of holes ( Haliotis ). The outer emargination is often only indicated by the reflected course of the lines of growth on the shell. On the inside of the outer lip, various ridges or plications called lirae are sometimes found, and these occasionally may be strong and tooth-like ( Nerinea ). Similar ridges or columellar plicae or folds are more often found on the inner lip, next to

1659-415: 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

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1738-437: 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

1817-529: 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

1896-402: 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

1975-510: Is involved. A more recent study (2013) correlates the asymmetric coiling of the shell by the left-right asymmetric expression of the decapentaplegic gene in the mantle. In a few cases, both left- and right-handed coiling are found in the same population. Sinistral mutants of normally dextral species and dextral mutants of normally sinistral species are rare but well documented occurrences among land snails in general. Populations or species with normally mixed coiling are much rarer, and, so far as

2054-555: Is known, are confined, with one exception, to a few genera of arboreal tropical snails. Besides Amphidromus , the Cuban Liguus vittatus (Swainson), Haitian Liguus virgineus (Linnaeus) (family Orthalicidae ), some Hawaiian Partulina and many Hawaiian Achatinella (family Achatinellidae ), as well as several species of Pacific islands Partula (family Partulidae ), are known to have mixed dextral-sinistral populations. A possible exception may concern some of

2133-491: Is layered upon the periostracum as the snail grows. Upper image: Dorsal view, showing whorls and apex Central image: Lateral view showing the profile of the shell Lower image: Basal view showing umbilicus in the centre. Gastropod shell morphology is usually quite constant among individuals of a species. Controlling variables are: Some of these factors can be modelled mathematically and programs exist to generate extremely realistic images. Early work by David Raup on

2212-449: 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

2291-433: 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

2370-612: 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

2449-420: 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

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2528-430: Is secreted by, and attached to, the upper surface of the posterior part of the foot. The operculum is of very variable form in the different groups of snails that possess one. The terminology used to describe the shells of gastropods includes: The overall shape of the shell varies. For example, three groups can be distinguished based on the height – width ratio: The following are the principal modifications of form in

2607-410: Is the maximum measurement of the shell at right angles to the central axis. Both terms are only related to the description of the shell and not to the orientation of the shell on the living animal. The largest height of any shell is found in the marine snail species Syrinx aruanus , which can be up to 91 cm. The central axis is an imaginary axis along the length of a shell, around which, in

2686-554: 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

2765-487: 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

2844-405: 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)

2923-462: 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

3002-441: The mantle . The calcareous central layer, ostracum, is typically made of calcium carbonate (CaCO 3 ) precipitated into an organic matrix known as conchiolin . The outermost layer is the periostracum which is resistant to abrasion and provides most shell coloration. The body of the snail contacts the innermost smooth layer that may be composed of mother-of-pearl or shell nacre, a dense horizontally packed form of conchiolin, which

3081-593: The olive and Terebra , are smooth, elongated, and lack elaborate sculpture, in order to decrease resistance when moving through sand. On land, high-spired forms are often associated with vertical surfaces, whereas flat-shelled snails tend to live on the ground. A few gastropods, for instance the Vermetidae , cement the shell to, and grow along, solid surfaces such as rocks, or other shells. Most gastropod shells are spirally coiled. The majority (over 90%) of gastropod species have dextral (right-handed) shells, but

3160-400: 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

3239-582: The European clausiliids of the subfamily Alopiinae . They are obligatory calciphiles living in isolated colonies on limestone outcrops. Several sets of species differ only in the direction of coiling, but the evidence is inconclusive as to whether left- and right-handed shells live together. Soos (1928, pp. 372–385) summarized previous discussions of the problem and concluded that the right- and left-handed populations were distinct species. Others have stated that these populations were not distinct, and

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3318-548: The analog computer also revealed many possible combinations that were never adopted by any actual gastropod. Some shell shapes are found more often in certain environments, though there are many exceptions. Wave-washed high-energy environments, such as the rocky intertidal zone, are usually inhabited by snails whose shells have a wide aperture, a relatively low surface area, and a high growth rate per revolution. High-spired and highly sculptured forms become more common in quiet water environments. The shell of burrowing forms, such as

3397-412: 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

3476-412: The aperture more or less facing the observer, a dextral shell will have the aperture on the right-hand side, and a sinistral shell will have the aperture on the left-hand side. This chirality of gastropods is sometimes overlooked when photographs of coiled gastropods are "flipped" by a non-expert prior to being used in a publication. This image "flipping" results in a normal dextral gastropod appearing to be

3555-672: 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

3634-418: The columella or central spiral twist. These may be oblique or normal to the axis of coiling (horizontal), few or numerous, readily seen, or far within the shell so as to be invisible except in broken shells. When the axis of coiling is hollow (perforate spire) the opening at the base constitutes the umbilicus . The umbilicus varies greatly in size, and may be wholly or in part covered by an expansion or callus of

3713-423: The diasome stem group . Mollusks 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

3792-412: The first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals may appear by intercalation between the primary ones, and generally are absent in the young shell, except in some highly accelerated types. Tertiary spirals are intercalated between the preceding groups in more specialized species. Ribs are regular transverse foldings of the shell, which generally extend from

3871-421: 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

3950-422: 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

4029-407: The gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side. Most authors figure the shells with the apex of the spire uppermost. In life, when the soft parts of these snail are retracted, in some groups the aperture of the shell is closed by using a horny or calcareous operculum , a door-like structure which

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4108-429: The gastropod shell. Detailed distinction of the shape can be: The most frequently used measurements of a gastropod shell are: the height of the shell, the width of the shell, the height of the aperture and the width of the aperture. The number of whorls is also often used. In this context, the height (or the length) of a shell is its maximum measurement along the central axis. The width (or breadth, or diameter)

4187-414: The inner lip ( Natica ). Many Recent shells, when the animal is alive or the shell is freshly empty, have an uppermost shell layer of horny, smooth, or hairy epidermis or periostracum , a proteinaceous layer which sometimes is thick enough to hide the color markings of the surface of the shell. The periostracum, as well as the coloration, is only rarely preserved in fossil shells. The apertural end of

4266-419: The later ones. When an angulation occurs, the space between it and the suture above it constitutes the area known as the "shoulder" of the shell. The shoulder angle may be smooth or keeled, and may sometimes have nodes or spines. The most simple form of sculpture of the gastropod shell consists of longitudinal ridges, and/or transverse ridges. Primary spirals may appear in regular succession on either side of

4345-475: 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

4424-468: 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

4503-422: 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 ,

4582-936: 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 Gastropod shell The gastropod shell has three major layers secreted by

4661-524: 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,

4740-411: 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

4819-498: The previous condition of an uncoiled limpet -like shell. In large enough quantities, gastropod shells can have enough of an impact on environmental conditions to affect the ability of organic remains in the local environment to fossilize. For example, in the Dinosaur Park Formation , fossil hadrosaur eggshell is rare. This is because the breakdown of tannins from local coniferous vegetation would have caused

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4898-548: 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

4977-482: The question is far from settled. The Peruvian clausiliid, Nenia callistoglypta Pilsbry (1949, pp. 216–217), also has been described as being an amphidromine species. The genetics of reverse coiling in a rare dextral mutant of another clausiliid, Alinda biplicata (Montagu), has been studied by Degner (1952). The mechanism is the same as in Radix peregra (Müller), with the direction of coiling determined by

5056-411: The reduction of the shell is associated with a predatory way of feeding. Some taxa lost the coiling of their shell during evolution. According to Dollo's law , it is not possible to regain the coiling of the shell after it is lost. Despite that, there are few genera in the family Calyptraeidae that changed their developmental timing ( heterochrony ) and gained back ( re-evolution ) a coiled shell from

5135-440: 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

5214-613: 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

5293-499: The shell gradually increase in size. Normally the whorls are circular or elliptical in section. The spire can be high or low, broad or slender, according to the way the coils of the shell are arranged, and the apical angle of the shell varies accordingly. The whorls sometimes rest loosely upon one another (as in Epitonium scalare ). They also can overlap the earlier whorls such that the earlier whorls may be largely or wholly covered by

5372-410: The shell may be simple or variously modified. An outer and an inner (columellar) lip are generally recognized. These may be continuous with each other, or may be divided by an anterior notch. This, in some types ( Fusinus , etc.) it is drawn out into an anterior siphonal canal , of greater or lesser length. An upper or posterior notch is present in certain taxa, and this may result in the formation of

5451-415: 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

5530-412: 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

5609-435: 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

5688-467: The surface of the shell (see Platyceras ). When a row of spines is formed at the edge or outer lip of the shell during a resting period, this feature sometimes remains behind as a varix as in ( Murex ) and many of the Ranellidae . Varices may also be formed by simple expansion of the outer lip, and a subsequent resumption of growth from the base of the expansion. The aperture or peristome of

5767-455: The suture to suture. They are usually spaced uniformly and crossed by the spirals. In specialized types, when a shoulder angle is formed, they become concentrated as nodes upon this angle, disappearing from the shoulder above and the body below. Spines may replace the nodes in later stages. They form as notches in the margin of the shell and are subsequently abandoned, often remaining open in front. Irregular spines may also arise on various parts of

5846-499: 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

5925-423: 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

6004-402: 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

6083-520: 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

6162-417: 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

6241-437: 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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