

General features

Size range and diversity of structure






Distribution and abundance









Natural history

Reproduction and life cycles





Ecology and habitats


Retention of the larval anchoring byssus into adult life has freed many bivalves from soft substrates, allowing them to colonize hard surfaces. This has also been achieved by cementation, as, for example, in oysters.

There are no pelagic bivalves, except for Planktomya hensoni, which is still benthic as an adult but has an unusually long planktonic larval stage. Some bivalves can swim, albeit weakly, when removed from the sediment, as can some file shells. True swimming is, however, seen only in the family Pectinidae (scallops) but is used mostly as an escape reaction.

Many representatives of the superfamily Galeommatoidea are commensal, a few are parasitic, and both have thus become miniaturized. Most bivalves are found in coastal seas, but their diversity is greatest on continental landmasses, where large rivers create suitable deltaic habitats and the continental shelf is broad. Except on tropical ones with coral reefs, few bivalves are found on islands.

Of the various subclasses, two are most important ecologically: the Heterodonta are modern burrowers that include cockles, clams, shipworms, and giant clams and feed primarily on suspended material. In contrast, the Pteriomorphia, an older group that is epibyssate (that is, anchored to rocks) dominates hard substrates. The subclass is made up of oysters, mussels, jingle shells, and others. Some of their older representatives are endobyssate (that is, anchored to material within a burrow or dugout), exposing their evolutionary history. Most of these two classes occupy a wide diversity of subhabitats, with simple reproductive strategies, external fertilization, and planktonic larvae to effect wide dispersion. They apportion the shallow-water marine domain virtually everywhere. The Palaeoheterodonta (a group that includes the unionids) are exclusively freshwater species, but all have significantly more complicated life cycles.

The Palaeotaxodonta (or Protobranchia) are coastal and deepwater detrivores, always infaunal. They share this diversity of habitat with the Anomalodesmata, which have radiated along two lines: shallow-water species that are highly specialized, are hermaphroditic, occupy narrow niches, and have a short planktonic stage and deep-sea species that are even more specialized, most being predators.

Most bivalves are primary consumers, typically exploiting organic material. The two dominant bivalve subclasses are high in the diet of many predators. Some 60 million years ago great adaptive radiation, notably in the Bivalvia, took place with a similar radiation in predatory crustaceans, starfishes, and snails. It is thought that such predation pressure effectively drove the Bivalvia underground with the resultant evolution of many antipredation devices on the shell—spines, ridges, and teeth—or of the habit of burrowing to great depths. On coral reefs a similar pressure led to deep boring into the fabric of the coral and the evolution of a host–borer intimacy.


Unlike in other molluscan groups, locomotion in bivalves is used only when dislodgement occurs or as a means to escape predation.

The bivalve foot, unlike that of gastropods, does not have a flat creeping sole but is bladelike (laterally compressed) and pointed for digging. The muscles mainly responsible for movement of the foot are the anterior and posterior pedal retractors. They retract the foot and effect back-and-forth movements. The foot is extended as blood is pumped into it, and it is prevented from overinflating by concentric rings of circular, oblique, and longitudinal muscle fibres, which also help to direct pedal extension and permit fine mobility.

During burrowing, the foot is greatly extended anteriorly from between parted shell valves. Taking a grip on the substratum, typically by dilation of the tip, the pedal retractors pull the shell downward. This is accompanied by sharp closure of the shell valves, forcing water out of the mantle cavity into the burrow, helping to fluidize the sediment, and making movement through it more efficient. So effective is this mechanism that fast burrowers, when removed from the sediment, can swim short distances.

Food and feeding

The primitive bivalve was almost certainly a detritivore (consumer of loose organic materials), and the modern palaeotaxodonts still pursue this mode of life. The posterior leaflike gills serve principally for respiration; feeding is carried out by the palp proboscides, which collect surface detritus.

The vast majority of other bivalves feed on the plant detritus, bacteria, and algae that characterize the sediment surface or cloud coastal and fresh waters. The gills have gradually become adapted as filtering devices called ctenidia. The primitive posterior respiratory gills have enlarged and moved to lie lateral to the body as paired folds, or demibranchs. Further increases in surface area have been achieved by folding the platelike gill lamellae into plicae. Each lamella comprises vertical rows of filaments upon the outer head of which are complex arrays of cilia that create a flow of water through the gill, form a filtration barrier, and transport retained particles to food grooves in the dorsal axes or ventral margins of the ctenidia. Bound in mucus, the food is transported to the mouth via the labial palps, where further selection occurs (see below Internal features).

Two groups of bivalves have exploited other food sources. These are the shipworms (family Teredinidae) and giant clams (family Tridacnidae). Shipworms are wood borers and are both protected and nourished by the wood they inhabit. They possess ctenidia and are capable of filtering food from the sea. When elongating the burrow, they digest the wood as well. In the Tridacnidae, symbiotic zooxanthellae (minute algal cells) are contained within the mantle tissue. The relationship between clam and algae is probably mutually beneficial, the algae having access to the dissolved waste products of the clam and the clam benefiting from the nutritional value of either culled zooxanthellae or their metabolic products.

A few bivalves are parasitic—e.g., species of Entovalva, which live either in the esophagus or upon the body of sea cucumbers (Holothuroidea), and the larvae (glochidia) of freshwater Unionidae, which parasitize fish.

The most exotic adaptations of the basic bivalve feeding plan are found in two groups of deepwater bivalves. These are scallops of the genus Propeamussium and the various deepwater families of the Anomalodesmata. In Propeamussium what appear to be typical ctenidia are present in the mantle cavity, but on closer examination these prove to be wholly atypical in that the filament heads are internal. The ctenidia are incapable of filtering. The gut is minute, and detected prey is sucked into the mantle cavity by an inrush of water when the valves open. The food is then pushed into the mouth with the foot.

Many deepwater Anomalodesmata have modified the typical bivalve ctenidium into a septum—the “septibranch” ctenidium—that creates pressure changes within the mantle cavity and produces sudden inrushes of water, carrying prey into a funnellike inhalant siphon (Cuspidaria). Food is then pushed into the mouth by the palps and foot. Others evert the inhalant siphon, like a hood, over the prey (Poromya and Lyonsiella). Prey items include small bottom-dwelling crustaceans, polychaete worms, and larvae of other benthic animals.


The greatest affinity of bivalves is with coral reefs. Indo-Pacific, but not Caribbean, reefs are the habitat of giant clams, Tridacna. Dead corals are bored by representatives of the Gastrochaenidae, living corals by species of Lithophaga. A greater degree of intimacy between living coral and bivalve borer is now known, some species associating with a single coral.

Similarly with wood borers: piddocks (Pholadidae) are more common in hardwoods, while shipworms (Teredinidae) favour softwoods. In the degradation of wood in the sea, a variety of species may colonize it with time and with depth.

One group of bivalves, the superfamily Galeommatoidea, form highly intimate relationships with other marine invertebrates, particularly on soft shores and coral reefs. Typically less than 10 millimetres (0.4 inch) long, most are commensal; i.e., they form an association in which there is no detriment to the host and exploit it for protection, food, and respiratory currents. On soft shores they share the burrows of polychaete worms and crustaceans, sometimes attaching to the body of the host.

Form and function

General features

The bivalve body comprises a dorsal visceral mass and a ventral foot, which is enclosed within a thin mantle, or pallium. The mantle secretes from its outer surface a shell divided into left and right valves. Between the body and mantle is the mantle cavity, within which hang the left and right gills, or ctenidia. The ctenidia are divided into two demibranchs, inner and outer, each in turn comprising inner and outer lamellae. Anteriorly, the ctenidia unite with paired (left and right) labial palps, which are food-sorting organs. The mantle margin can be fused at various places leaving medial apertures anteriorly for the extension and retraction of the locomotory foot and, in most bivalves, posteriorly to create inhalant and exhalant apertures that may be formed into siphons of variable length according to habitat. Foot and siphons can be withdrawn between the shell valves into the mantle cavity for protection.

External features

The bivalves occupy a wide variety of habitats and, as a consequence, deviate widely from the basic body plan. The shell form is an obvious adaptation to the environment. Shells of many modern burrowers are ornamented and coloured, and those of near-surface-dwelling cockles are thick and radially ribbed. These adaptations stabilize the animal in the substrate and may confer some degree of protection against predators. Such bivalves are slow burrowers. In contrast, the shells of deep-burrowing species are thin and nonornamented. They are often brightly coloured, as in the Tellinidae. The shell is laterally compressed and thus more bladelike, but the adductor muscles are still of similar size (the isomyarian form). Such structural features adapt the animal for rapid movement through the sand; long siphons project to the surface above. Deep burrowing has been achieved by a different mechanism in the razor shells (e.g., the family Solenidae), where the anterior region of the shell is reduced and the posterior enormously elongate. Because of their short siphons, Ensis and Solen live close to the sediment surface, but, with the lateral compression of their polished shells, they are among the most proficient burrowers. Other bivalves—e.g., Mya (family Myidae)—live at great depths but do not burrow rapidly. The shell is largely unornamented and wider to accommodate the greatly elongated siphons, which can be retracted deeply within its borders.

Rock and wood boring are also specialized consequences of burrowing—the evolution of borers proceeded from the habitation of stiff muds or from nestling within crevices. Mechanical borers tunnel anterior end first; that face of the shell having a sculpture of spines. Borers derived from a nestling epibyssate ancestor are chemical borers that produce a calcium carbonate chelating secretion from the mantle margin. In such cases the shell is typically smooth, although calcareous encrustations on the posterior shell protect the borer from aperture-attacking predators. Reduction of the anterior adductor (the anisomyarian form) creates a triangular-shaped shell, as in the buried fan shell Pinna (Pinnidae) and the mussels (Mytilidae) of rocky coasts. Although such bivalves lack ornamentation, the shell is typically thick and dark.

Loss of the anterior adductor creates a shell with a circular outline, left and right valves being either equal or unequal. In some, lateral flattening and byssal attachment allows occupation of narrow crevices. Cementation by either valve is a further consequence of the loss of the anterior adductor muscle (the monomyarian form). Subsequent freedom from attachment, as in the scallops (Pectinidae), is associated with an almost circular outline, flat upper and cup-shaped lower valves, a deep radial sculpture, and, typically, bright coloration (Pecten).

Internal features










The shell

The bivalve shell is made of calcium carbonate embedded in an organic matrix secreted by the mantle. The periostracum, the outermost organic layer, is secreted by the inner surface of the outer mantle fold at the mantle margin. It is a substrate upon which calcium carbonate can be deposited by the outer surface of the outer mantle fold. The number of calcareous layers in the shell (in addition to the periostracum), the composition of those layers (aragonite or aragonite and calcite), and the arrangement of these deposits (e.g., in sheets, or foliate) is characteristic for different groups of bivalves. Middorsally an elastic ligament creates the opening thrust that operates against the closing action of the adductor muscles. The ligament typically develops either externally (parivincular) or internally (alivincular) but comprises outer lamellar, and inner fibrous, layers secreted by the mantle crest. The ligament type is generally characteristic of each bivalve group. The hinge plate with ligament also possesses interlocking teeth to enforce valve alignment and locking, when closed, to prevent shear. Many variations in teeth structure occur.

The mantle and musculature

The mantle lobes secrete the shell valves; the mantle crest secretes the ligament and hinge teeth. Growth takes place at the margins, although increases in thickness take place everywhere. The mantle is withdrawn between the shell valves by mantle retractor muscles; their point of attachment to the shell being called the pallial line.

The musculature comprises two (dimyarian) primitively equal (isomyarian) adductor muscles; the anterior and the posterior. The anterior of these may be reduced (anisomyarian; heteromyarian) or lost (monomyarian). Only very rarely is the posterior lost and the anterior retained.

Internal to the adductors are paired anterior and posterior pedal retractor muscles. Where the anterior adductor muscle is reduced, so are the anterior pedal retractors. In highly active burrowers, paired anterior pedal protractors and pedal elevator muscles occur—for example, the family Trigonioidea.

In byssally attached bivalves, pedal retractors are reduced and byssal retractors serve to pull the animal down in closer opposition to the rock surface. In oysters, commensurate with the extreme reduction of the foot, pedal retractors are lost. This is also the case in swimming scallops.

The nervous system and organs of sensation

The nervous system is simple and the head is completely absent, reflecting the sedentary habit. In primitive bivalves (e.g., Palaeotaxodonta) there are four pairs of ganglia—cerebral, pleural, pedal, and visceral. In all other bivalves the cerebral and pleural ganglia are fused into two cerebropleural ganglia, located above and on either side of the esophagus. The pedal ganglia are in the base of the foot, and the visceral ganglia are located under the posterior adductor muscle. Nerve fibres arising from the cerebropleural ganglia extend to the pedal and visceral ganglia. In some bivalves with long siphons, there are accessory siphonal ganglia, and in many swimming bivalves the visceral ganglia are much enlarged, presumably to coordinate complex swimming actions.

Again reflecting the sedentary life, sensory functions are largely taken over by the posterior mantle margins and typically comprise tentacles developed from the middle mantle folds that are mechanoreceptors and chemoreceptors. Scallops (family Pectinidae) have complex eyes with a lens and retina. In other bivalves, eyes are simple ciliated cups, although some variation is possible. In the predatory deepwater septibranchs the inhalant siphon, which captures food, is surrounded by tentacles that have vibration-sensitive papillae for detecting the movements of prey.

Situated close to the pedal ganglia but with direct connections to the cerebropleural ganglia are a pair of statocysts, which comprise a capsule of ciliated sense cells. In the lumen is either a single statolith or numerous crystalline statoconia. Their points of contact with the surrounding cilia yield information about the animal’s orientation. Additionally, most bivalves with or without eyes have light-sensitive cells that respond to shadows. Below the posterior adductor muscle an osphradium has been identified in some bivalves that may monitor water flow and quality.

The digestive system and nutrition

The bivalve digestive system comprises a complex stomach and associated structures but an otherwise simple intestine. The various types of stomach have been used to erect an alternative classification. Digestion typically takes place in two phases: extracellular in the stomach and intracellular in the digestive diverticula, opening laterally from the stomach wall. Transport of food particles is effected by cilia, creating an array of tracts and sorting areas within the stomach. The principal organ of extracellular digestion is the crystalline style. It is rotated in its sac by cilia; the head, projecting into the stomach, grinds against a part of the stomach wall lined by a chitinous gastric shield. As it rotates, it dissolves, releasing enzymes and initiating primary extracellular digestion of the mucus-bound food. Products of this process are passed in a fluid suspension into large embayments and thence into the digestive diverticula, where intracellular digestion takes place. Waste material is consolidated in the midgut and rectum and expelled as firm fecal pellets from an anus opening into the exhalant stream. Feeding and digestion are highly coordinated, typically regulated by tidal and diurnal cycles.

The excretory system

Blood is forced through the walls of the heart into the pericardium. From there it passes into the kidneys where wastes are removed, producing urine. The paired kidneys (nephridia) are looped with an opening into the pericardium and another into the suprabranchial chamber. The kidneys may be united. Bivalves also possess pericardial glands lining either the auricles of the heart or the pericardium; they serve as an additional ultrafiltration device.

The respiratory system

In the primitive bivalves the paired gills are small and located posteriorly. The gills in all other bivalves (save septibranchs, which have lost their gills) are greatly enlarged and possess a huge surface area. While the gills are thought to serve a respiratory function, respiratory demands are low in these mostly inactive animals, and, since the body and mantle are both bathed in water, respiration probably takes place across these surfaces as well. Such a mechanism has been demonstrated for a few bivalves, most notably freshwater species that are exposed to occasional drought. In such species, drying induces slight shell gaping posteriorly, the mantle margins exposing themselves to air. For most intertidal bivalves (which are alternately exposed to wetting and drying), respiration all but ceases during the drying phase.

The vascular system

The heart, enclosed in a pericardium, comprises a medial ventricle with left and right auricles arising from it. Blood oxygenated within the ctenidia flows to the auricles and from there to the ventricle, where it is pumped into anterior and posterior aortas. The blood then enters hemocoelic spaces in the mantle and visceral mass and returns to the heart via the ctenidia or the kidneys. The blood serves both to transport oxygen and metabolic products to tissues deep within the body and as a hydrostatic skeleton (for example, in the extension of the foot during locomotion and siphons during feeding). There are amoeboid corpuscles, but, except in a few bivalves, no hemoglobin or other respiratory pigment occurs.

The reproductive system

The reproductive system is simple and comprises paired gonads. These gonads discharge into the renal duct in primitive bivalves but open by separate gonopores into the suprabranchial chamber in more modern bivalves. Typically, the sexes are separate, but various grades of hermaphroditism are not uncommon. Eggs and sperm are shed into the sea for external fertilization in most bivalves, but inhalation of sperm by a female permits a type of internal fertilization and brooding of young, usually within the ctenidia.

Features of defense and aggression

The most significant adaptation is the earliest division of the shell into two valves within which the animal was wholly contained. Slow components of the adductor muscle permit sustained adduction, while the interlocking hinge teeth prevent shear. In addition, the shell may be strongly ridged, forming an interlocking shell margin, and it may be concentrically ringed with spines or sharp ridges projecting outward. Posterior sense organs, including photophores and eyes, are developed around the siphons and mantle margins. Detection leads to withdrawal deep into the sediment by burrowing species. In such animals the shell is smooth and compressed. Scallops respond to predation by swimming; shallow-burrowing cockles can leap using the foot. In the razor clams the siphons can break off (autotomize) when bitten, to be regenerated later. Similarly, noxious secretions are produced by the similarly autotomizing long tentacles of the Limidae (file shells). The unique pallial organ of fan shells (family Pinnidae) produces a secretion of sulfuric acid when bitten.

Only the deepwater subclass Anomalodesmata (families Verticordiidae, Poromyidae, and Cuspidariidae) and the scallops are predators. Prey is captured either in the sudden rush of water into the mantle cavity or by the rapid eversion of the inhalant siphon.

Evolution and paleontology

The oldest known bivalves are generally believed to be Fordilla troyensis, which is best preserved in the lower Cambrian rocks of New York (about 510 million years old), and Pojetaia runnegari from the Cambrian rocks of Australia. Fordilla is perhaps ancestral to the pteriomorph order Mytiloida, Pojetaia to the Palaeotazodonta order Nuculoida.

By the Ordovician Period (488.3 million to 443.7 million years ago) most modern subclasses were represented by definable ancestors. The oldest Ordovician bivalves are, however, the subclass Palaeotaxodonta, which are thought to have given rise to the Cryptodonta by elongation. Modern assessment of their shell structure and body form, notably with the possession of posterior protobranch gills and with palp proboscides for deposit feeding in the Palaeotaxodonta, generally supports this view. An extinct subclass Actinodontia also arose in the Ordovician Period and may be represented today by the superfamily Trigonioidea (placed in the subclass Palaeoheterodonta), which are an aberrant group of the subclass Pteriomorphia. The remaining, more typical, members of the Pteriomorphia also arose at this time and persist today, still characteristically occupying a range of substrate types but with byssal attachment and a trend toward loss of the anterior adductor muscle. The common mussels (family Mytilidae) are thought to be derived from an extinct group, the family Modiomorphidae. The subclass Orthonotia also arose in the Ordovician Period and are the probable ancestors of the deep-burrowing razor shells (Solenoidea). The origins of the subclass Anomalodesmata are less clear, but they too arose in the Ordovician Period and may have links to the order Myoida, which presently includes deep-burrowing forms and borers. Representatives of the superfamily Lucinoidea are very different from all other bivalves, with an exhalant siphon only and an anterior inhalant stream. Some of these deposit feeders also possess, like the subclass Cryptodonta, sulfur-oxidizing bacteria in the ctenidia and are thought to have ancient origins, represented by the fossil Babinka. Babinka is itself interesting and is closely related either to Fordilla, one of the oldest bivalves, or to the ancestors of the molluscan class Tryblidia. Today the superfamily Lucinoidea is generally placed within the subclass Heterodonta, which is a younger group that traces back to the Paleozoic Era, when the first radiation of all bivalves took place.

The stamp of modernity was placed upon the Bivalvia in the Mesozoic Era (251 million to 65.5 million years ago), when virtually all families currently recognized were present. Throughout time, the fortunes of the subclasses have waxed and waned, with repeated modification of form allowing repeated diversification into different habitats. Similarity of habitat is matched by similarity in structure and form, allowing for various interpretations of the fossil record. It is clear, however, that most modern bivalves can trace their ancestry back a long way and that the inherent plasticity of the bivalve form is responsible for the success of a molluscan experiment in lateral compression of the shell.


No system of classification erected for the Bivalvia has been accepted by all. Paleontologists interpret bivalves on the basis of shell features, notably shell and ligament structure, arrangement of hinge teeth, and body form as interpreted from internal muscle scars.

Investigators of Holocene (11,700 years ago to the present) forms use other anatomic features, such as adductor muscle arrangement, the ctenidia and their junction with the labial palps, the extent and complexity of mantle fusion, stomach structure, and morphology of the hinge area to classify bivalves. Cluster analysis using many morphological features is effective with lower taxa but less so with higher taxonomic categories because of the many examples of parallel evolution from the basic bivalve plan. The triangular mussel form, for example, has evolved in representatives of virtually every subclass, resulting in similar morphologies. Shell microstructure and mineralogy evidence generally support paleontological conclusions that the class Bivalvia comprises six subclasses, recognizing, however, that some of these taxa may have more than one first ancestor (polyphyletic). In a group with a fossil history extending back to the Cambrian Period and occupying a wide range of aquatic habitats, this is not unexpected, particularly since the basic bivalved form permits repeated modification.

Annotated classification

Class Bivalvia
Laterally symmetrical; left and right calcareous shell valves; dorsal elastic hinge ligament; anterior and posterior adductor muscles; lateral paired filtering ctenidia surrounding the visceral mass; primitively burrowing by means of a muscular foot, but some crawl, some attach to rocks by byssal threads from the foot, some are cemented, and some bore into soft rocks, corals, and wood; some commensal, a few parasitic, and some deepwater species predatory; microphagous feeding; mostly marine, at all depths, also estuarine and freshwater; about 8,000 extant species.
Subclass Palaeotaxodonta (Protobranchia)
Numerous similar teeth along the hinge plate; isomyarian; unique shell microstructure of aragonitic composite prisms and internal nacre; posterior ctenidia comprising 2 divergent rows of flat, short, filaments; protobranch respiratory gill; food collected by labial palps; mostly near-surface-dwelling marine detritivores; considered to be the most primitive of living bivalves, if not the most ancient. About 500 living species.
Order Nuculoida
Equal shell valves with taxodont hinge teeth; isomyarian; posterior protobranch ctenidia; large labial palps usually with palp proboscides, which effect feeding; foot with flat sole; marine; unattached; infaunal. About 450 living species.
Subclass Cryptodonta
Hinge either weakly taxodont or edentulous; distinctive shell structure of aragonitic simple prisms and nacre internally; large posterior protobranch ctenidia; small labial palps; of primitive and ancient lineage; marine; unattached; infaunal.
Order Solemyoida
Shell valves equal and elongate, lacking hinge teeth, covered by a shiny periostracum; dimyarian or monomyarian; some with protobranch ctenidia containing symbiotic sulfur-oxidizing bacteria; minute palps; minute or absent gut; foot with flat sole; marginally papillate; marine; deep-burrowing; infaunal. About 35 species.
Subclass Pteriomorphia
Order Arcoida
Shell solid, elongate or circular-oval, often heavily ribbed; fibrous periostracum with simple crossed-lamellar outer layer and inner complex crossed-lamellar layer, thereby differing from all other pteriomorphs; dimyarian; hinge with vertical denticulations; ctenidia filibranch; mantle margin with uniquely divided outer fold; foot often byssate; marine; epibyssate; infaunal. About 150 species.
Order Trigonioida
Shell valves equal, trigonally oval, strongly ribbed; shell with outer aragonitic prismatic layer and inner nacre layers; strong hinge teeth transversely grooved; typically isomyarian, with pedal elevator and protractor muscles as well as retractors; ctenidia filibranch, without mantle fusions; powerful foot; marine; infaunal; living species confined to Australia. 5 species.
Order Mytiloida (common mussels)
Shell equivalve, rounded, elongate or triangular depending on habits; anisomyarian tending toward monomyarian; hinge edentulous; shell microstructure of outer calcitic fibrous prisms and inner nacre; ctenidia filibranch; mantle margin lacking fusions; foot creeping; typically byssate; marine, estuarine, rarely freshwater; endobyssate and epibyssate. About 250 species.
Order Pterioida (pearl oysters and fan shells)
Shell equivalve, variably shaped; anisomyarian but often monomyarian; shell structure of outer simple calcitic prisms and inner nacre; ctenidia pseudolamellibranch, often plicate (deeply folded); mantle margin lacking fusions; foot reduced; marine; endobyssate or epibyssate. About 100 species.
Order Limoida
Shell equivalve, ovally elongate, ribbed, often thin and transparent, with outer foliated calcite and inner crossed-lamellar aragonitic layers; hinge short and edentulous; monomyarian; ctenidia pseudolamellibranch, encircling the adductor; palps small and lips of mouth variably fused; mantle margins unfused and often red, with long autotomizing tentacles; some swim weakly; marine; epibyssate with byssus sometimes formed into a nest. About 125 species.
Order Ostreoida (oysters and scallops)
Shell valves unequal, variable, typically lacking hinge teeth; shell structure of foliated calcite, upper valve with outer prismatic calcite; most scallops with inner crossed-lamellar layers; dimyarian but most monomyarian; ctenidia pseudolamellibranch; mantle fusions lacking; foot often lost in adult; scallops capable of swimming; some deepwater scallops predatory; marine; epibyssate; cemented by lower or left valve or free. About 600 species.
Subclass Palaeoheterodonta
Characterized by equal shell valves with a variable hinge dentition; aragonitic shell with outer prismatic and inner layers of nacre; most approximately isomyarian; ctenidia eulamellibranch; mantle fusions lacking, especially ventrally; complicated life cycles; wholly freshwater; nonbyssate; infaunal. About 1,200 species.
Order Unionoida
Large, equivalve, varying from round to elongate and with equally variable sculpture; shell of outer prismatic layer and inner layers of nacre; hinge schizodont; dimyarian; ctenidia eulamellibranch with either 1 or both demibranchs functioning as an incubatory marsupium; ovoviviparous; parasitically larviparous; freshwater; some cemented and oysterlike; mostly infaunal. About 1,200 species.
Subclass Heterodonta
Shell highly variable; hinge plate teeth may be reduced or absent; shell comprises crossed-lamellar, complex crossed-lamellar, or prismatic layers, but never nacreous; primitively isomyarian but with wide range of adductor muscle configurations; ctenidia eulamellibranch; mantle margins extensively fused, particularly posteriorly, often to form long inhalant and exhalant siphons; mostly marine but also estuarine and freshwater; some epibyssate, some bore soft rocks and wood; generally infaunal. About 4,000 species.
Order Veneroida
Shell typically equivalve and of outer crossed-lamellar and inner complex crossed-lamellar layers; hinge comprises radiating cardinal and lateral teeth, often weakly developed; adductor muscles of varying proportions according to habit; ctenidia eulamellibranch, mantle margins extensively fused, often developed into long siphons; most are active burrowers with a large foot; some epibyssate; mostly marine, some estuarine and freshwater; includes the poorly known miniature commensals and parasites; widely divergent, accounting for more than 30 percent of the extant bivalves. Approximately 3,000 species.
Order Myoida
Shell typically thin, equivalve, comprising either 2 or 3 layers; hinge plate with cardinal dentition, often degenerate; approximately isomyarian but with much variation; boring forms develop accessory shell plates; ctenidia eulamellibranch, mantle margins extensively fused and covered in periostracum; small foot; marine deep burrowers with long siphons but also rock-and wood-boring. About 400 species.
Subclass Anomalodesmata
Characterized by highly variable shell, either equivalve or inequivalve, often gaping either posteriorly or anteriorly; hinge plate thickened and enrolled but generally edentulous; shell of two or three layers, the inner nacreous; typically isomyarian but with wide variation; ctenidia either eulamellibranch and plicate or septibranch; mantle margins extensively fused, often covered in periostracum; foot reduced; siphons of variable length; consistently hermaphroditic; marine; mostly burrowing; some epibyssate or cemented. About 450 species.
Order Pholadomyoida
Shell more or less equivalve but of widely divergent form; shell comprises aragonitic prisms and nacre or homogeneous structures; typically isomyarian; ctenidia eulamellibranch and plicate but many deepwater species are septibranch; extensive mantle fusions, reduced foot and pedal gape; siphons of variable length; shallow-water forms are burrowing, nestling, epibyssate, or cemented suspension feeders; some deepwater forms are predators with exotic modifications to the bivalve plan.

Critical appraisal

Generally, the classification scheme is accepted up to the level of family and even superfamily. The arrangement of higher categories is, however, still debated. Some authors, for example, combine the subclasses Palaeotaxodonta and Cryptodonta into a single group of primitive detrivores with protobranch gills. Differences in shell structure, however, argue against this. Similarly, the order Arcoida is separated by some from the subclass Pteriomorphia; shell structure again supports this, but other anatomic features do not. The order Trigonioida traditionally has been located within the subclass Palaeoheterodonta, but this has also been disputed, anatomic features suggesting instead an affinity with the subclass Pteriomorphia. This means that the subclass Palaeoheterodonta comprises only the order Unionoida, which has come to occupy the freshwater domain exclusively. Some authors would prefer to relocate the order Myoida from the subclass Heterodonta into the subclass Anomalodesmata, arguing that the edentulous shell, extensive mantle fusions, and deep-burrowing habit are characteristics shared with early ancestors of the order Pholadomyoida. The subclass Anomalodesmata, however, is itself possibly too narrowly demarcated, and some authorities would, for example, separate the deepwater carnivorous septibranchs from the shallow-water pholadomyoids into their own order, the Septibranchoida.

This lack of classificatory agreement is not unusual with regard to a group that has adopted a simple sedentary, filter-feeding mode of life. Simplification and parallel evolution will lead to similarity in form, structure, and function. Debate in creating classificatory trees and reconstructing the historical record is thus about the relative significance of the fossil shell record, as there is little information on tissue morphology, and the importance of morphological data obtained from living representatives.