The soft parts in the living organism include the muscular pedicle, circulatory and digestive systems

The soft parts in the living organism include the muscular pedicle, circulatory and digestive systems, genital glands and the arm like pair of ciliated lophophore.

The lophophore (also known as brachia) are used to gather food.

The soft parts of the animal are enclosed within a hard exoskeleton known as shell, which is secreted by a thin skin like mantle membrane. In fact the soft parts are enclosed in a cavity formed by the mantle, known as mantle cavity. The shell is divided into two unequal parts known as valves.

The valves are equilateral and are placed in dorso-ventral position in the animal. The ventral valve is comparatively larger and more convex then the dorsal valve. Both the valves are drawn posteriorly and produced into a beak like structure known as umbo (umbones).

In the posterior part of the animal the valves are joined with each other along a line below their umbones, called hinge line. The valves open along the anterior margin. The hinge line may be straight or curved.

The valves are held together either by teeth and socket present along their hinge line or by muscles. The forms with teeth and socket are grouped as Articulate and those with muscles are known as Inarticulate. In case of articulate brachiopods the hinge consists of two small curved processes or teeth on the ventral valve, which fit into the corresponding sockets on the dorsal valve.

In a few cases, the dental systems on the ventral valve are supported by small calcareous plates known as dental plates. In some shells a triangular area is found in between hint line and umbo, known as cardinal area or hinge area. The area may be ii both the valves or in one valve only.

In majority cases a small circuit opening is found on the umbo of the ventral valve. This opening is called pedicle opening. In a few cases this opening is found shared by both I valves. In the living brachiopods, a stout fibrous muscular structure know as pedicle comes out through the pedicle opening with the help of which the animal attaches itself to the substratum. Because of presence of the pedicle opening, the ventral valve is called as pedicle valve. Since the brachia are attached to the dorsal valve, it is also known as| brachial valve.

The valves are opened and closed by two sets of muscles known as J divaricator and adductor muscles which connect the valves at their interior surface. The scars of these muscles in fossils are described as muscular I impressions. The brachia are attached to the brachial valve or the dorsal valve by means of brachial skeleton.

The brachial skeleton may be i.e. the form of two curved hooks, known as cure. Other forms of brachial skeleton may be looped ribbon or spirally coiled shaped. The shape and size of the shells vary widely.

It may be biconvex, Plano-convex or concavo-convex in shape. In length the shells vary from less than 3 cm to a length of 30 – 40 cm. A line joining the middle point of the anterior and posterior margins divides the valves into two equal halves making the valves equilateral.

The length of the shell is measured from the umbo to the anterior margin and width from lateral to lateral margin while the thickness from the centre of one valve to the centre of the other on their outer surface.

The external surface the shells are decorated with various types of ornamentation like concentric and radial growth lines (or rings), radiating ribs, tubercles, spines as well as small elevations (or ridges) and depressions. In some cases the lateral margin on both the sides of the hinge line are drawn into ear like structure while in others the anterior margin of both valves are crenulated or dentate providing a better grip when the valves are closed.

Brachiopods constitute an important group of fossilized animals in terms of their utility. These are marine invertebrates and live in shallow seas between shore line and 200 m depth. They had worldwide distribution and their fossils are preserved in abundance both in Paleozoic and Mesozoic rocks.

Presently more than 200 types of both articulate and inarticulate are found living in different parts of the earth. These are mostly sessile benthos living at the bottom of the sea attached to the underwater rocks. A few types, however, are found floating on the surface of the water.

The study of Brachiopod fossils preserved in different geological formations indicates that they came to live on the surface of the earth during the early part of the Paleozoic era. More than 30,000 types of Brachiopod fossils are found preserved in the sedimentary rocks deposited from Cambrian period till the recent times.

Those living during Cambrian period were mostly inarticulate. With the advance of geological time the number of articulates increased with a spectacular decrease in the number of inarticulate. Gradually their number increased during ordovician and Silurian, reaching a climax towards Devonian.

Most of the Brachiopod group perished leaving only a few families during f Mesozoic era, most of which are living even today. From the fossil recon it is evident that the animals belonging to the phylum Brachiopod are successful group with a geological age ranging from early Paleozoic Recent.