• Created by: naomi
  • Created on: 17-01-14 09:30

Phylum: Mollusca Class: Bivalva

Bivalves are invertebrate animals with the soft parts enclosed between two hinged CaCO3 VALVES

They are mostly marine animals although a small number of species live in fresh water of lakes and rivers

Some bivalves live attached to a fixed place on the sea floor but most are free living and are capable of slopw movement an burrowing into sediment

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Morphology: The shell

  • A bivalve has the soft parts of the body enclosed between two VALVES which are composed of calcium carbonate
  • the valves are called LEFT and RIGHT
  • the two valves are united along the HINGE
  • At the top of each valve is a beak-like protrusion called the UMBO or UMBONE which represents the early formed part of the shell
  • As the animal grow, additional increments of shell are laid down forming GROWTH LINES
  • sometimes RIBS radiating out from the umbo give additional ornamentation to the bivalve's shell
  • In some burrowing bivalves the two valves do not close completely leaving a GAPE between the shells
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Morphology: Shell Symmetry

  • most bivalves are bilaterally symmetrical or EQUIVALVE. The two valves are mirror images of each other
  • in a few species one valve is larger than the other and these are INEQUIVALVE. A well known example of inequivalce species in gryphea, or 'Devils toenail'
  • Each valve is usually asymmetrical about a line from the umbo to the ventral margin and is therefore INEQUILATERAL. (This can be contrasted with the symmetry of Brachiopods, where the place of symmetry bisects each valve). If you hold the shell with the umbo pointing away from you, the left valve is on your left, and the right is on your right.
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Morphology: Shell composition

The shell is made of two valves made of calcite with an organic layer on top

A cross section would show:

  • on the outside is the horny layer of PERIOSTRACUM
  • next, an inner layer of crystals of calcite or aragonite
  • finally, an innermost layer of NACRE (mother of pearl)
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Morphology: Teeth and sockets

  • on the inner surface of the valves is the hinge plate which is a thickening of the dorsal margin, just below the umbo
  • on each plate there are small projections called TEETH; which fit into small indentations called SOCKETS on the opposite hinge plate
  • DENTITION is the collective name for the teeth and sockets which act as guides ensuring that the two valves go exactly back into place when they close, making a secure tough fit
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Morphology: Opening and closing the shell

  • the LIGAMENT which consists of resilient conchiolin opens the valves. It may be external, lying above hinge plate or internal lying between the hinge plates
  • the valves are closed by the contraction of ADDUCTOR MUSCLES. When these muscles relax, the external ligament pulls the valves apart
  • the points where the posterior and anterior adductor muscles are attached to the valves are marked by MUSCLE SCARES
  • shells with only one muscle scar are MONOMYARIAN
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Morphology: Internal Shell Structures

  • a thin line runs from the anterior to the posterior adduvtor muscle scar, parallel to the ventral margin, this is the PALLIAL LINE. It marks where the mantle and soft parts were attached to the inside of the valve
  • bivalves which burrow in sediment have elongated tubular SIPHONS which extended from the burrow during feeding so that the pallial line is indented inwards to form a PALLIAL SINUS. This is always at the posterior end of the shell
  • bivalves have large GILLS which have been modified for filter feeding and have developed SIPHONS which connect the mantle cavity to the outer environment allowing them to burrow for safety
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