Brachiopods and pelecypods

Brachiopods and pelecypods

Brachiopods is a phylum that includes organisms having a shell consists of two unequal valves (inequivalves) but pelecypods is a class of phylum Mollusca that includes organisms having mostly two equal valves (equivalves).

Phylum BrachiopodaClass PelecypodaPhylum Mollusca

- The shell is composed of CaCO3 in

pelecypods (calcareous), it is also

calcareous in most brachiopods although

some have chitnophosphatic shell.

Chemical composition of shell

Chemical composition of shell

- The two valves articulate along a hinge line, which carries teeth and sockets.

-They Close and open by muscles.

- In brachiopods the larger valves contains teeth and the smaller valve contains sockets.

- In pelecypods each valves carries teeth and sockets.

- The mantle edge may be modified into tubes or siphons that are able to draw water and

food into the gills and expel it. Typically the two tubes are located posteriorly and are

designated the incurrent (inhalent) and excurrent (exhalent) siphons.

- When the scars of two adductor muscles occur the bivalve is termed dimayarian. If the two

scars are about the same size, the bivalve is isomyarian, in contrast to the anisomyarian in

which the anterior scar is distinctly smaller.

- Some bivalves like oysters have one adductor muscle scar, these are termed monomyarian.

Muscle Scars

The earliest part of the valves is called

beak and the rounded part adjacent to

the beak is called umbo.

The shell is bilaterally symmetrical with

the plane of symmetry passing

between the valves in pelecypods but

bisects each valve in brachiopods.

Shell

Line of symmetry

Q. Compare between Brachiopods and Pelecypods?

Brachiopods Pelecypods - Inequivalved. - Equilateral. - Valves pedicle and Brachial. - Plane of symmetry across

valves and through beaks. - Pedicle opening present. - Teeth in one valve, sockets

in the opposite (except in inarticulate).

- Valves open and close by muscles.

I. Equivalved. II. Inequilateral. III. Valves right and left. IV. Plane of symmetry

between valves. V. No pedicle opening. VI. Teeth and sockets (if

present) in each valve. VII. Valves open by

ligament or resilium.

Byssus: is an organ composed of chitinous fibers, which attach the pelecypod shell to the substratum. Brachiopods attach themselves by pedicle.

Pelecypods attached by byssusBrachiopods attached by pedicle

- Oysters are pelecypods with two

unequal valves, the larger valve is the

left valve and is cemented to the

substrate. The smaller valve acts as a

lid or operculum.

- They have only one muscle scar

(monomyarian).

- There are neither teeth nor sockets.

- The beak is strongly curved.

Oysters

-During Cretaceous oysters formed banks reaching more

than 5 meters in height and extending for several

kilometers.

- They are still living today and live in aggregates

cemented together.

Ex. G. Ostrea, G. Exogyra.

Oysters

- Rudists have also two unequal valves.

The larger valve is the right valve, it has

a conical shape and is fixed to the

substrate.

- The smaller valve is the left one, which

acts as a lid.

- They have pachydont dentition.

- They died out (extinct) at the end of

Cretaceous.

Rudistids