The brachiopods are a large group of exclusively marine organisms with
shells composed of two pieces called _valves_ (fig. 13). These valves
are usually composed of calcareous or phosphatic material and enclose
and protect the soft parts of the brachiopod animal. The soft parts are
composed of muscles, the _mantle_ (which secretes the valves),
digestive, respiration, reproductive, and excretory organs, and the
tentacle-bearing _lophophore_.
In adult life the brachiopod is attached to the sea bottom by means of a
fleshy stalk called the _pedicle_ (fig. 14), and this is usually
extruded through a hole (the _pedicle foramen_) which is located in the
_ventral_ or _pedicle_ valve. The upturned area which is usually present
on the pedicle valve is called the _beak_. The other valve, known as the
_dorsal_ or _brachial_ valve, is usually the smaller of the two (fig.
13b). The two valves are opened by means of muscles, and since death
results in relaxation of these muscles, fossil brachiopods are typically
found with valves closed.
Brachiopods vary greatly in size and shape and exhibit a wide variety of
ornamentation, such as spines, ribs, nodes, and other structures. They
are abundant fossils in many of the Paleozoic rocks of Texas but are
relatively rare in Mesozoic and Cenozoic formations.
The phylum has been divided into two subclasses, the Inarticulata and
the Articulata. This classification is based upon the nature of the
_hinge-line_—the edge of the shell where the two valves articulate.
[Illustration: Fig. 13. Morphology and principal parts of articulate
brachiopods.]
a
Pedicle foramen
Hinge line
b
Pedicle valve
Beak
Brachial valve