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Brachiopods are benthic marine invertebrates with a pair of shells or 'valves' and a tentacular filter-feeding and respiratory organ called a lophophore held within the mantle cavity (a layer of soft tissue that secretes the shell). The two valves sometimes articulate by a simple hinge (but never a ligamentary system as has evolved independently in the bivalve molluscs and ostracod arthropods), but they are different in shape and size from each other, although symmetrical through their median axis. The 'pedicle' valve typically has teeth at the hinge line and an opening (foramen) through which a tough, chitinous pedicle emerges, fixing the animal to the substrate. The 'brachial' valve typically has sockets that articulate with hinge teeth, a projecting cardinal process for muscle attachment and in many cases calcareous 'arms' called brachidia. The lophophore is a ciliated structure. It is often formed of two main branches that are variably spiralled or lobed and supported by its own hydrostatic skeleton, although calcareous support in the form of looped brachidia or short struts are found in many groups. The shell is secreted at the edge of the mantle epithelium and is composed of three layers, an outer periostracum (of chitin or protein), middle calcareous layer (calcite or apatite) and inner layer of apatite or calcite with protein (e.g. collagen) or chitinophosphate. Other key points of the anatomy include a gut, diductor muscles for opening the shell, adductors for closing it and adjustors for moving its position on the pedicle. Nephridia remove nitrogenous waste and release gametes (produced in the gonads) that are fertilised in the sea and develop into free-swimming planktonic larvae, later settling to and adopts their benthic, adult form. Of note, although superficially resembling bivalves, brachiopods are a completely independent group: most obviously, bivalves are symmetric between their valves and construct their shells from aragonite or calcite.
Brachiopod classification is a topic of debate, but current taxonomy defines two 'inarticulate' subphyla (with a toothless hinge), the Linguliformea and Craniiformea and one large 'articulate' subphylum (with a hinge held together by various kinds of teeth), the Rhynchonelliformea. Representatives of all three major groups appear in the Early Cambrian, and may have evolved from stem taxa such as the tommotiid Paterimitra, which shares similarities with paterinid lingulids. On a broader scale, brachiopods are members of the Lophotrochozoa, related to groups such as phoronids, annelids, molluscs and bryozoans. Brachiopods were diverse and ecologically dominant filter-feeders in the Palaeozoic, but were decimated by the Permo-Triassic mass extinction, declining afterwards to only five surviving lineages today (Lingula, Crania, Rhynchonellida, Thecideida and Terebratulida).
Brachiopods provide a window into evolutionary convergence in terms of specific body forms ('homeomorphy'), anatomical structures such as caecae, statocysts, lophophores, brood chambers and larval eye spots, as well as convergent ecological assemblages in deep sea ecosystems such as mid-ocean ridges and cold seeps.
Homeomorphy is very well document at all levels, with various forms appearing repeatedly (diverging from the typical biconvex shell form) within and between brachiopod groups and even very distant relatives among the bivalves. Homeomorphs evolve through adaptation to similar environments, and may be isochronous, appearing at the same time in multiple taxa, or heterochronous, re-evolving several times independently over time. A shining heterochronous example involves some unusual brachiopods of the Order Productida and bivalves known as rudists. Productides were large articulate brachiopods whose peak diversity was in the later Palaeozoic, becoming extinct after the Permian. Adults lived almost buried in sediment (quasi-infaunal) and featured a flat lid-like brachial valve atop a deep, convex pedicle valve, often with root-like supporting spines. Highly modified productides of the sub-order Strophalosiidina were dominant reef-forming brachiopods of the Carboniferous, and included genera such as Cyclacantharia that may have been capable of generating rhythmic flow across its feeding organs (instead of the typical ciliary pumping). This form appeared again in Cretaceous reef-building rudist bivalves (Hippuritoida) which look superficially like productids, with a flat lid-like upper valve and lower cone-shaped valve, although they did now evolve active pumping. Within the brachiopods, isochronous homeomorphy is clearly seen between the Rhynchonellida (e.g. Rhynchonella) and Spiriferida (e.g. Cyrtospirifer). These articulate brachiopods independently evolved a zig-zag commissure (shell margin) and differential growth leading to an outward fold of the pedicle valve and a dip or sulcus in the brachial valve. The zig-zag form created a longer commissure for efficient food intake when the shell was only open a very small amount, and prevented entry of large indigestible particles.
The lingulid Discinisca (Devonian to Recent) is an inarticulate brachiopod that converges on limpets (gastropod molluscs), attaching its limpet-like form to boulders in protected habitats. At least four times in brachiopod history, shells evolved internal punctuations (punctae) that were filled by tubular outgrowths of the mantle termed caecae. Among the rhynchonelliformes, functional caecae evolved independently in the Enteletacea (Orthida), Spiriferinacea (Spiriferida), Rhynchoporacea and Terebratulida, where they function in storage, respiration, toxic inhibition of boring predators or periostracum repair. In a more distant example of convergence, remarkably similar caecae are also found in fissurellid limpets.
A number of female brachiopods retain their fertilised eggs in brood chambers, demonstrating a form of parental care also known in many other animals, from fish to snails and also Antarctic echinoderms.
The brachiopod feeding organ, the lophophore, shows a high degree of structural convergence. Spirolophe lophophores composed of a simple twin spiral evolved independently in lingulids and three rhynchonellatan Orders, and a rare multi-lobed (ptycholophe) lophophore is inferred in certain productids (e.g. Falafer), strophomenids, megathyrids and has been observed in the living thecidean Lacazella.
Sensory eye-spots are found in larvae of articulates such as Lacazella (Thecideida), Terebratella (Terebratulida) and Tegulorhynchia (Rhynchonellida). These eye-spots provide light sensitivity during the free-swimming stage of the life cycle, and are reminiscent of those in several free-living protistans (e.g. Chlamydamonas) and even certain bacteria. Larval stages of inarticulate brachiopods (linguliformes, craniiformes) have statocysts, organs for balancing that are found in many other animals, for example cephalopods, cnidarians and some crustaceans.