Brachiopods are sessile creatures, attached to the substratum usually by a fleshy stalk - the pedicle. They are enclosed in a shell with dorsal (or brachial) and ventral (or pedicle) valves, each of which is bisected by a median plane of symmetry, and which differs in shape from the other. The valves are hinged at the posterior end, near the pedicle, and anteriorly they gape open while the animal feeds. Thus to orientate a brachiopod, the larger valve, from which the pedicle emerges, is placed ventrally, and the pedicle is positioned posteriorly. The composition and morphology of valves vary with species. (The terms dorsal and ventral are used anatomically and may not reflect the orientation of the animal in life.)
The internal space between the two valves is divided into a posterior body cavity and anterior brachial cavity by the thin epithelial body wall (cross-section). The brachial cavity is much the largest and contains a pair of coiled filamentous structures called the lophophore. This serves as a respiratory and food gathering organ having free contact with the surrounding water when the shell is open. The body cavity, containing muscles, alimentary canal, digestive diverticula, the reproductive system is restricted to the postero-median part of the shell and from here flaps of mantle epithelium extend forwards lining both valves internally. This mantle tissue secretes the shell material and contains coelomic cavities within which strands of reproductive tissue develop during the breeding season.
The internal soft part anatomy is known in detail for comparatively few species. However, it seems that the anatomy is relatively stable throughout the phylum, the main difference being that the alimentary canal is open-ended, having an anus, in the Inarticulata, whilst in the Articulata it is blind-ended. The general structure of the lophophore is a long tube with a pair of ridges along its length, between which is a food groove ([l][f][/f]# foodgrooves). The outer side bears long, closely spaced rows of filaments with frontal and lateral ciliated surfaces. The food groove is continuous around the lophophore and leads postero-medianly to the mouth, situated on the transverse section of the lophophore, by the body wall. Inside the lophophore, canals connect to the coelomic or body cavity posteriorly. In some species the lophophore is supported by branching spicules of calcium carbonate, as well as by the turgidity of the coelomic fluid and, in addition, some genera have a skeletal brachial loop built within part of the lophophore. The body wall, separating the brachial and body cavities, is situated between the crura and lies close to the anterior edges of the muscles. In the body wall are paired excretory pores connecting to conical excretory glands, the metanephridia, suspended in the body cavity on either side of the intestine.
The alimentary canal is simple and similar in all articulate species investigated. The mouth, at the mid posterior point of the lophophore, leads to a short oesophagus, which curves towards the dorsal valve, and a roughly U-shaped expanded stomach. Around this are large digestive diverticula. In the Articulata, a short blind intestine extends ventrally, normally finishing near the base of the diductor muscles. The Inarticulata have an anus which, in the case of lingulids, is at the end of a long twisted intestine, and in conical discinaceans and craniaceans terminates a shorter U-shaped gut.
Little is known of the reproductive system of brachiopods; whilst most species appear to be dioecious, some are hermaphroditic. The reproductive glands develop from coelomic epithelium in the body cavity and, as they enlarge, they commonly extend anteriorly into the mantle canals. In sexually mature individuals the ovaries normally show as light brown to orange-red masses adherent to the insides of the valves; the testes are less obvious and lightly coloured. Ova and sperm are discharged through the metanephridia.
The nervous and circulatory systems are poorly known and can only be seen by careful dissection. Circulation is open and the blood free from cells, but contractile “hearts” lie medianly, usually on the stomach. The nervous system consists of enteric ganglia, situated around the oesophagus, from which nerves serve the muscles, lophophore, mantles and alimentary tract. No special sensory organs have been proven, other than the marginal setae, which appear to sense movement in the surrounding water. Some species from shallow water seem to be light sensitive, but no receptors have been isolated.
The two brachiopod classes, the Inarticulata and the Articulata, differ quite markedly, but are distinguished principally on the hinge apparatus of the valves; the Articulata having a pair of teeth in the ventral valve, fitting paired sockets in the dorsal valve (deltidial plates); the Inarticulata rely upon internal muscles to hold the valves together and to move them apart, so for this reason their musculature differs from that of the Articulata.
The articulate species of the North Sea mostly have biconvex shells, but some have somewhat flattened dorsal valves pressed closely against the hard substratum. All species have a pedicle attachment, and the pedicle aperture or foramen, situated in the delthyrium, may be restricted in size by variably developed deltidial plates. The external surfaces of the valves are marked by growth lines, developed during pauses in the secretion of shell, and by various forms of radial ornamentation. Radial ribs form prominent ridges on the valves and extend anteriorly from the umbos. In addition there may be fine radial striations, or tubercles, commonly associated with ribs.
The two mineralised valves are the most obvious features of brachiopods, and are important in the recognition of species. All articulate valves are made of calcium carbonate, whereas inarticulates may be calcareous or have shells made of thin layers of chitinophosphatic material. Calcareous valves tend to be light-coloured and brittle, whilst chitinophosphatic valves are normally dark coloured and somewhat leathery. Microscopy reveals that the calcareous shell of brachiopods is layered. The outer surface is covered by a thin brown cuticle, the periostracum, which may become abraded from parts of the shell during life; below this is a uniformly thin layer of calcite, the primary layer, in the form of minute needles perpendicular to the external surface. Inside this the secondary layer is of variable thickness and may form structures on the inside of the valves. The secondary layer thins peripherally and in articulates, it is typically composed of long fibres, 10 to 20 µm wide, dipping more or less anteriorly towards the inner surface. When exposed on the internal surface of valves these fibres form an overlapping pattern, called the internal shelf mosaic, which is characteristic for major groups of brachiopods. Inarticulate calcareous valves have lamellose secondary layers. Some articulates have a tertiary layer, on the inner surfaces, in which the calcite is in the form of prisms perpendicular to the inside of the valve.
In addition to the above structures, three conditions of brachiopod shell are recognized which characterise broad groups of taxa. These shell types are impunctate, endopunctate and pseudopunctate. An impunctate shell lacks either endopunctae or pseudopunctae and is characteristic of all living rhynchonellids. The endopunctate condition is of regularly spaced narrow canals penetrating from the inside of the valve almost to the outside of the primary layer. In life these canals, the endopunctae, accommodate evaginations of the mantle epithelium, called caeca (caeca), whose purpose remains to be fully understood. Recent work indicates that the caeca probably store food materials, and they may help inhibit boring into the shell by other organisms. All the Terebratulida are endopunctate. The pseudopunctate condition is known from fossil brachiopods and is recognized by inwardly facing conical flexures in the microstructural layering of the shell, which commonly form small tubercules on the insides of the valves. These shell conditions can usually be recognized with the aid of a good hand-lens.
Internal skeletal structures, visible on dead articulate valves from which the soft tissues have been removed, are important in the recognition of species. In the middle of dorsal valves, close to their posterior margins, there are scars of muscle attachment manifesting themselves as a ridged and grooved, elevated or depressed region, known as the cardinal process (internal dorsal valve); in its most clearly developed form it is a prominent bilobed knob-like structure. The cardinal process is flanked by the sockets, into which fit the pair of teeth. The sockets are variable in form but usually are bounded medianly by prominent socket ridges, which may be supported from the floor of the valve by hinge or crural plates. The morphology and disposition of these plates are important taxonomically. From the anterior ends of the socket ridges, skeletal structures project forwards which, in life, help support the lophophore. In rhynchonellid brachiopods these projections are called crura (sing. crus) while in living terebratulids the crura grow forwards and develop into complicated loops which support much of the length of the coiled lophophore. In some taxa this brachial loop (internal) is connected not only posteriorly to the crura but also medianly onto a median septum running down the middle of the valve from near the cardinal process. The morphology of the loop commonly alters considerably during growth of the animal but its form, and that of the crura, play important roles in taxonomy.
Less obvious, but still commonly visible on the insides of the valves, are the sears of muscle attachment. The articulate species have three sets of paired major muscles. The adductor muscles are used to close the valves and attach to the dorsal valve postero-medianly, divided, where present, by the median septum. Their ventral attachment scan are more or less opposite and centrally placed. The diductor muscles open the shell and are fixed dorsally to the cardinal process, posteriorly of the hinge axis which passes through the teeth. These muscles divide ventrally and attach to the ventral valve on either side of the adductor muscles, but commonly extend further forwards. Thirdly, there are several pairs of muscles attached to the proximal end of the pedicle, situated within the ventral umbo. These muscles allow the brachiopod to swivel around the pedicle, so changing its position relative to the substratum; they are called pedicle adjustor muscles and may leave scars in the dorsal valve near the socket ridges. In ventral valves their scars are usually closely associated with the diductor muscle sears.
On the insides of both valves, extending towards the margins, shallow branching grooves may be distinguished which mark the positions of mantle canals in the mantle tissues. These coelomic canals, connecting posteriorly to the body cavity, carry nutrients to the mantle tissues and contain part of the generative strands from which ova and spermatozoa develop. The insides of ventral valves have fewer structures than the dorsal valves. In addition to muscle scars and traces of mantle canals there is a pair of teeth situated antero-laterally of the delthyrial opening. The shape and detailed dispositions of teeth are variable and in some taxa they are supported by dental plates. The pedicle opening may be within the delthyrium, restricted apically or marginally by deltidial plates, or the delthyrium may he almost completely covered by shelly plates so that the pedicle extrudes from a small hole almost at the tip of the ventral umbo. During life the pedicle opening may become enlarged by abrasion against the hard surface of attachment.
Source: Brunton and Curry, 1979.