Brachiopoda (brākēŏpˈədə) [key], 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).