Internal morphology of brachiopods

The internal anatomy of articulated brachiopods can be divided simply into two sections. The pedicle, main internal organs, and muscles are packed together at the back of the shell whilst the lophophore dominates the mantle cavity in the central and anterior area (Fig. 6.1a). Food particles are trapped by the lophophore's cilia, and passed back to the mouth and into the digestive tract. Waste products are emitted as small pellets from the anterior of the shell.

The pedicle is the primary method of attachment. Depending on the mode of life of the species it varies from being a thick muscular stalk to a series of thin threads. In rare cases, a species loses its pedicle, closes the pedicle opening with plates of calcite and rests freely on the sea bed. Rhynchonelliform brachiopods, which possess teeth and sockets, have two sets of muscles positioned close to the hinge area at the posterior of the shell (Fig. 6.1a). Adductor muscles, attached perpendicularly to the interiors of the dorsal and ventral valve, pull the two valves together when contracted and close the shell. Diductor muscles are attached obliquely to the ventral valve, just outside the adductors, and to the cardinal process of the dorsal valve. They pull on the dorsal hinge area when contracted and cause the shell to open. Brachiopods without a mechanical hinge have a more complex musculature that extends further into the shell and reduces shearing between the valves.

The lophophore dominates the mantle cavity and is responsible for collecting suspended food from sea water. It generates currents to draw water into the shell, and removes food with sticky, fine tentacles. In brachiopods, the lophophore is developed as a pair of tentacle-bearing arms. The overall shape varies from a simple horseshoe to complicated folded configurations. It is an immovable, fixed structure held in place by the pressure within the brachiopod shell or supported by skeletal elements in the dorsal valve (Fig. 6.1b).

Pedicle: used to attach the animal to the sea floor

Diductor muscle: contracts to open the shell

Pedicle: used to attach the animal to the sea floor

Diductor muscle: contracts to open the shell

Posterior

Adductor muscle: contracts to close the valves

Dorsal

Anterior

Ventral

Stomach Mouth Mantle: sheets of tissue which line the shell

Fig. 6.1 Brachiopod internal morphology: (a) soft tissue, and (b) hard parts.

Lophophore: food gathering and respiratory mechanism. Two arms (brachia) fringed with cilia generate currents that bring in food and oxygen

Dorsal

Posterior

Adductor muscle: contracts to close the valves

Anterior

Ventral

Stomach Mouth Mantle: sheets of tissue which line the shell

Muscle scars: can be used to identify certain species

Cardinal process: diductor muscle attachment site

Cardinal process: diductor muscle attachment site

Brachiopod Internal Structure

Brachidium: lophophore support

Ventral valve

Dorsal valve

Brachidium: lophophore support

Ventral valve

Dorsal valve

Fig. 6.1 Brachiopod internal morphology: (a) soft tissue, and (b) hard parts.

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Responses

  • Melilot
    Are Lophophore soft or hard tissue?
    6 years ago

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