Phylum Brachiopoda
Brachiopods are marine invertebrates in which the soft parts are enclosed by two deposits of shell termed valves. Thus superficially they resemble the bivalves (pelecypods). However; they are distinguished from them by the inequality of the two valves in size and shape, and by equilateral symmetry of each valve.
Q.
Compare in a table between Brachiopods and pelecypods.
The name brachiopoda (brachio = arm, pod = foot), refers to internal paired appendages (brachia or lophophore) which at first were assumed to function for locomotion, like the foot of mollusks.
The brachiopoda are chiefly inhabitants of the shallow sea bottom and they live in a fixed position (sessile). They may be anchored by a stem-like attachment termed pedicle or fastened to some foreign objects such as another shell. Some brachiopods are fixed by cementation of the shell or by projecting spine.
A few species of brachiopods live in deep water along the continental slops. No brachiopoda are known to have lived in fresh water. Lingula is an example of brackish water brachiopods.
Two main type of brachiopod shell are recognized: mostly diminutive forms having valves held together without hingement (inarticulates) and small to large forms having valves which bear teeth and sockets for articulation along edges hinged together (articulates).
Feeding:
The food of brachiopods consists of microscopic material, which is drawn to the interior of the slightly open valves by water currents induced by movement of cilia on arms or brachia. Food particles are filtered by means of a food- gathering organ called lophophore.
SOFT PARTS:
The brachiopod animal consists of soft parts which carry on essential body functions and hard parts which are mainly external and serve for protection.
The soft parts consist of the body covering (mantle), a digestive tract, visceral organs, various muscles and tentacle -bearing appendages, termed brachia. The mantle lines the inside of both valves and contains cells which deposit the mineral substance of the shell (calcium carbonate). In certain brachiopods the mantle projects outward into the shell which consequently produces a perforate or punctuate structure. Shells which don’t have these perforation are called impunctate.
Pedicle:
is a muscular stalk which serves as the attachment organ of brachiopods. It is attached to the floor of one of the valves called pedicle valve.
Musculature:
In Brachiopods there are three kinds of muscles, two are responsible for closing and opening the two valves when contracted, they are known as adductor and diductor muscles respectively. The third muscle is called adjustor muscle and is responsible for adjusting the position of individuals.
Q. Why do we find most brachiopod shells closed, on the other hand most pelecypod shells are not so?
HARD PARTS:
Brachiopods secrete a hard skeleton called shell, which consists of two valves enclosing the soft part. The valves are held together by muscles in case of inarticulate brachiopods and by teeth and sockets in case of articulate brachiopods. Teeth are found in one valve (pedicle) whereas sockets in the corresponding valve (brachial).
SHELL COMPOSITION AND STRUCTARE:
Brachiopod shells are classified into two groups on the basis of their chemical composition. Most inarticulate brachiopoda have hard parts composed predominantly of calcium phosphate and chitinous organic matter, such shells are termed chitinophosphatic. In most articulate brachiopods the shell is composed of calcium carbonate (calcite), such shells and termed calcareous.
Structurally
, the chitinophosphatic shells are of two types: one in which thin phosphatic laminae alternate with chitinous ones and the other in which calcium phosphate and chitinous material are uniformly admixed.Calcareous shells are of three types called impunctate, punctate and pseudo-punctate. In each there are two shell layers: an outer thin finely laminated layer and an inner thick layer composed of inclined calcareous fibers. In impunctate shells, these layers are solid. In punctate shells, the inner fibrous layer is perforated by fine tubes or pores extending from the interior almost to the outer surface. Pseudo-punctate shells lack pores, but the fibrous layer contains rod-like bodies of calcite, the ends of which commonly form projections on the shell interior. Because the rod-like bodies tend to be dissolved more readily in weathering than the surrounding fibrous materials, rounded cavities resembling puncta may be produced. Thus the shell is falsely punctate.
MORPHOLOGICALLL FEATURES:
Brachiopod shells are distinguished from bivalve shells by the inequality of the two valves.
Shells of articulates are characterized by the possession of a hinge line which coincides with the posterior margin of each valve. The hinge line is absent in inarticulate brachiopoda. The pointed end of the pedicle valve is called beak and the arched part of the valve near the beak is called umbo. Each valve has a beak although that of brachial valve may project little or not at all above the hinge line. The beaks mark the beginning of shell growth as shown by concentric lines around them (growth lines) or by radially disposed ribs (costae) or shell corrugations (plication, folds, sulcus) which generally diverge from them.
The posterior part of the shell is that part near the hinge line whereas the anterior is that part away from it.
Length is the distance from the beak to the anterior extremity of the shell. Width is the maximum distance between opposite points on the lateral margins of a valve or shell. Thickness is the maximum distance between opposite points on the surface of the two valves.
At the posterior part of the shell there is an area between the beak and the hinge line, this area is called interarea. Beneath the beak the interarea of either valve may be interrupted by a triangular open space termed delthyrium in the pedicle valve and notothyrium in the brachial valve. The delthyrium provide space for passage of pedicle. It is sometimes covered by shell substance called deltidium if it consists of a single plate or deltidial plates if it consists of two plates. Those are termed chilidium and chilidial plates on the brachial valve.
The rounded opening for passage of pedicle is termed foramen or pedicle opening.
The hinge line of articulate brachiopods carries teeth and sockets where teeth are found on the pedicle valve and sockets on the brachial valve.
SHELL FORM:
Brachiopod shells have diverse shapes. Some shells tend to become transversely widened and so having the greatest width along the hinge line (e.g. Spirifer). Other shells become very narrow transversely and longitudinally extended (e.g. Terebratula).
The shape of one valve with respect to the other is important in the identification of brachiopods. The following shapes are recognized:
HOMEOMORPHY:
Homeomorphy is an external resemblance among shells belonging to different genera. This means that shells belonging to different groups may have similar shapes, also their pattern of ornamentation may be nearly identical. In such cases, one should examine the internal structure
of the shell which shows that these shells differ radically, or the shell substance of one is impunctate whereas the other is punctate (perforate).There are two types of homeomorphy: one refers to closely similar but unrelated shells having the same age (isochronouse homeomorphy), the other type includes two or more shells of different geologic age in which one simulates the other (heterochronous homeomorphy).
CLASSIFICATION OF BRACHIOPODS:
Brachiopods are divided into two classes termed articulata and inarticulata.
CLASS ARTICULATA:
Valves hinged, calcareous, generally bearing well-defined teeth and sockets. Early Cambrian- Recent.
CLASS INARTICULATE:
Valves unhinged, lteeth and sockets, shell generally chitinophosphatic. Early Cambrian –Recent.
GEOLOGIC HISTORY
They appeared in Cambrian and were abundant in the shallow-water deposits during the Paleozoic.
They were decimated in Permian but some genera continued during the Mesozoic and were abundant in the Jurassic.
They are still living today especially in deep and cold waters. This may represent a shift in environment through time.
