{{Taxobox
| color = pink
| name = Bivalves
| image = Haeckel Acephala.jpg
| image_width = 250px
| image_caption = "Acephala" from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'', 1904
| regnum = [[Animal]]ia
| phylum = [[Mollusca]]
| classis = '''Bivalvia'''
| classis_authority = [[Carolus Linnaeus|Linnaeus]], 1758
| subdivision_ranks = [[Order (biology)|Orders]]
| subdivision = 
Subclass Anomalosdesmata
*[[Pholadomyoida]]
Subclass Heterodonta - [[clam]]s, zebra mussels
*†[[Cycloconchidae]]
*[[Hippuritoida]]
*†[[Lyrodesmatidae]]
*[[Myoida]]
*†[[Redoniidae]]
*[[Veneroida]]
Subclass Paleoheterodonta 
*[[Trigonioida]]; see Superfamily [[Trigoniacea]]
*[[Unionoida]] - freshwater mussels
Subclass Protobranchia
*[[Nuculoida]]
*†[[Praecardioida]]
*[[Solemyoida]]
Subclass Pteriomorphia - oysters, mussels
*[[Arcoida]]
*[[Mytiloida]]
*[[Ostreoida]]
*[[Pterioida]]
}}

'''Bivalves''' are [[mollusc]]s belonging to the class '''Bivalvia'''. They typically have two-part [[animal shell|shell]]s, with both parts being more or less [[symmetry|symmetrical]].  The class has 30,000 [[species]], including [[scallop]]s, [[clam]]s, [[oyster]]s and [[mussel]]s.  Other names for the class include '''Bivalva''', '''Pelecypoda''', and '''Lamellibranchia'''.

Bivalves are exclusively aquatic; they include both marine and freshwater forms.

Bivalves lack a [[radula]] and feed by siphoning and filtering small particles from water.  Some bivalves are [[Fauna (animals)#Epifauna|epifaunal]]: that is, they attach themselves to surfaces in the water, by means of a [[byssus]] or organic cementation. Others are [[Fauna (animals)#Infauna|infaunal]]: they bury themselves in sand or other sediments. These forms typically have a strong digging foot. Some bivalves can swim.

==Anatomy==

Bivalves are filter-feeding mollusks that use their gills to extract organic matter from the water in which they live. They have an open [[circulatory system]] that bathes the organs in [[hemolymph]]. [[Nephridia]] remove the excreted waste.

Bivalves are laterally compressed and have a shell composed of two valves. The valved shell makes them superficially similar to [[brachiopod]]s, but the construction of the shell is completely different in the two groups:  in brachiopods, the two valves are on the upper and lower surfaces of the body, while in bivalves, they are on the left and right sides.

==History==
Bivalves appeared late in the [[Cambrian explosion]] and came to dominate over [[brachiopod]]s during the [[Palaeozoic]]; indeed, by the [[Permian-Triassic extinction event|end-Permian extinction]], bivalves were undergoing a huge radiation in numbers while brachiopods (along with ~95% of all species) were devastated.
[[Image:Giant_clam.jpg|thumb|right|[[Giant clam]], ''Tridacna gigas''.]]

This raises two questions: how did the bivalves come to challenge the brachiopoda niche before the extinction event, and how did the bivalves escape the fate of extinction? Although inevitable biases exist in the fossil record and our documentation thereof, bivalves essentially appear to be better adapted to aquatic life. Far more sophisticated than the brachiopods, bivalves use an energetically-efficient ligament-muscle system for opening valves, and thus require less food to subsist. Furthermore, their ability to burrow allows for evasion of predators: buried bivalves feed by extending a [[siphon]] to the surface  (indicated by the presence of a [[palial sinus]], the size of which is proportional to the burrowing depth, and represented by their [[dentition]]). Additionally, bivalves became mobile: some developed spines for buoyancy, while others suck in and eject water to enable propulsion. This allowed bivalves to themselves become [[predator]]s.

With such a wide range of adaptations it is unsurprising that the shapes of bivalve shells vary greatly - some are rounded and globular, others are flattened and plate-like, while still others, such as the razor shell ''Ensis'', have become greatly elongated in order to aid burrowing byssonychia. The [[shipworm]]s of the family Teredinidae have elongated bodies, but the shell valves are much reduced and restricted to the anterior end of the body. They function as burrowing organs, allowing the animal to dig tunnels through wood.
[[Image:Valve-InternalView.png|thumb|left|Internal anatomy of the valve.]]

Some pelecypods are alive today in the Great Sea and the Black Sea.  Various Eurasian countries rely on them for food.

==External links==
*[http://www.ucmp.berkeley.edu/ Museum of Paleontology] - [[Palaeontology]] from the [[University of California, Berkeley]]

[[Category:Molluscs]]
[[Category:Bivalves]]
[[Category:Fossils]]

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