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Brachiopoda

Brachiopoda (brākēŏp´ədə), phylum of shelled sessile or sedentary marine animals, commonly known as lamp shells, and characterized by a peculiar feeding organ, the lophophore. The shell consists of two parts, called valves, that completely enclose the body; the external appearance of the animal is much like that of a bivalve mollusk, or pelecypod, such as a clam. However, the valves of a lamp shell cover the top and bottom of the animal, while those of a clam cover the right and left sides. Furthermore, the internal anatomy of brachiopods does not resemble that of pelecypods; the two groups are not related. There are two classes in the phylum: the Inarticulata, members of which have the valves held together by muscles alone, and the Articulata, members of which have interlocking processes that form a hinge. A complex set of muscles opens the shell for feeding and closes it for protection. In most brachiopods a short stalk called a pedicel, or peduncle, emerges between the valves or through an opening in the lower valve. Most sessile brachiopods attach to objects by means of the pedicel, but a few lack pedicels and attach directly by the ventral valve. Burrowing lamp shells have long pedicels, which they contract to retreat into the burrow. The lophophore consists of two tentacle-bearing arms, often spirally coiled, one on either side of the mouth. The tentacles have cilia that create currents, drawing water-bearing food particles and oxygen into the shell. Food particles are trapped in mucus on the tentacles and moved by the cilia to the mouth. Oxygen is absorbed through the body wall. Brachiopods have a simple digestive and nervous system, and are equipped with excretory organs called nephridia. The open circulatory system includes a contractile vessel, or heart, and sinuses for the flow of the colorless circulatory fluid to various parts of the body. Reproduction is sexual and the sexes are usually separate. In most species the eggs and sperm are shed into the sea, where fertilization results in the development of free-living, ciliated larvae. The larvae settle to the bottom after developing rudiments of the adult structures. A few species brood their young. Brachiopods are believed to be related to the shell-less bryozoans, or moss animals (phylum Ectoprocta), which also have a lophophore. Abundant at the start of the Cambrian period, brachiopods were widespread and numerous in ancient seas. About 12,000 extinct species are known, and members of the largest species were almost 1 ft (30 cm) in diameter. Fewer than 325 species are extant today, and these are relatively small, usually 1 to 2 in. (2.5–5 cm) across. All are marine and most prefer shallow water; they are sporadically distributed, although some are very abundant locally. Among the better known lamp shells are the burrowing Lingula (class Inarticulata) and the stalkless, sessile Crania (class Articulata).

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Brachiopoda

Brachiopoda (lampshells) A phylum of solitary, benthic, marine, bivalved, coelomate, invertebrate animals that have existed from the Lower Cambrian to the present day. Brachiopods are commonly attached posteriorly to the sea bed by a stalk (pedicle), but may be secondarily cemented, or free-living (e.g. the fossil form Productus which, like many productids, was spinose, thick-shelled, and lived partly buried in the mud of the sea bed). Usually they consist of two unequal valves: a larger pedicle (ventral) valve and a brachial (dorsal) valve, lined by reduplications (mantle lobes) of the body wall which enclose the large mantle cavity. They are bilaterally symmetrical about the posterior-anterior mid-line of the valves (i.e. through the valves, compare BIVALVIA). The characteristic feeding and respiratory organ, the lophophore, surrounds the mouth and is covered by ciliated tentacles. It may be a simple horseshoe but more often forms two ciliated arms or brachia that project through the gape (thus giving the phylum its name). The alimentary canal is divided into oesophagus, stomach, and intestine, with or without an anus. The nervous system consists of a circum-oesophogeal ring with a small aggregation of nerve cells on the ventral side. The excretory organs are one or two pairs of nephridia (excretory tubules) also acting as gonoducts (for the release of eggs and sperm). The circulatory system is open, with the contractile vesicle (heart) near the stomach. Considerably more than 3000 fossil species are known and about 100 are alive today; these are widely distributed and occur at all depths. Brachiopods are divided into three classes: Lingulata, Inarticulata, and Articulata.

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"Brachiopoda." A Dictionary of Earth Sciences. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Brachiopoda

Brachiopoda (lamp shells) A phylum of solitary, marine, bivalved, coelomate invertebrates that live attached to the seabed by a muscular stalk (pedicle), or are secondarily cemented, or are free-living. Usually they consist of a larger, pedicle (ventral) valve and a brachial (dorsal) valve, lined by reduplications (mantle lobes) of the body wall which enclose the large mantle cavity. They are bilaterally symmetrical about the posterior-anterior mid-line of the valves. The lophophore surrounds the mouth and is covered by ciliated tentacles. Brachiopods first appeared in the Lower Cambrian and since then have undergone numerous adaptive radiations, the first involving the inarticulate brachiopods. The articulate groups became more important after the Cambrian. Many of the larger articulate groups are now extinct.

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"Brachiopoda." A Dictionary of Zoology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Brachiopoda

Brachiopoda (lampshells) Phylum of c.260 species of small, bottom-dwelling, marine invertebrates. They are similar in outward appearance to bivalve molluscs, having a shell composed of two valves; however, unlike bivalves, there is a line of symmetry running through the valves. They live attached to rocks by a pedicle (stalk), or buried in mud or sand. There are 75 genera including Lingula, the oldest known animal genus. Most modern brachiopods are less than 5cm (2in) across. More than 30,000 fossil species have been found and described.

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"Brachiopoda." World Encyclopedia. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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Brachiopoda

Brachiopoda A phylum of marine invertebrates – the lamp shells. They live in shallow waters, attached to a firm substratum by means of a flexible stalk (peduncle), and are protected by a bivalved shell consisting of dorsal and ventral valves. A food-gathering lophophore protrudes from the shell. Brachiopods thrived in Palaeozoic times but are now much less numerous; living brachiopods include Terebratella, with an articulated shell; and Lingula, in which the shell valves are held together by muscles only.

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"Brachiopoda." A Dictionary of Biology. . Encyclopedia.com. 16 Aug. 2017 <http://www.encyclopedia.com>.

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