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Brachiopods first
appeared in the fossil record in the
period known as the Cambrian, which began
570 million years ago, and are still
around, but much less abundant. They
are shelled marine creatures, resembling
bivalve molluscs at first glance.
However, apart from the fact that they
both have two shells, they are very
different - brachiopods are not even
molluscs.
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The shell of a
brachiopod has two halves, or valves,
which fit together like a pair of hands,
but one slightly inside the other. The
larger valve is called a pedicle valve,
and at the pointier end (called the beak
or umbo) there is a small hole. In life,
a long fleshy stalk (or pedicle)
protuded from this hole and anchored the
animal to the sea bed. Scientists divide the phylum brachiopoda
into two classes. The earliest and most
primitive belong to the class
"Inarticula", and cannot move
their two valves relative to each other.
The other class, "Articulata",
have a sort of hinge for doing just that.
This adaptation allowed brachiopods to
survive underground in burrows or fixed
to rocks.
The smaller valve
(or brachial valve) contains a rounded
organ called a lophophore. This organ
traps microscopic food from the sea water
in tiny hairs called cilia.
See also:
bivalves
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