The shells of organisms in this group may grow as large as 2.75 in (7 cm) in the lingulides and 0.78 in (2 cm) in the discinids and craniids. These shells have a ventral and a dorsal valve; the muscles that close the shell include single or paired posterior adductor muscles as well as paired anterior adductor muscles. The oblique musculature, generally three or four pairs of muscles, controls the rotations and sliding movements of the valves. The lateral muscles of the trunk wall control the hydraulic mechanism that opens the shell. This mechanism involves changes in the pressure inside the organism's coelom, or body cavity. The lophophore or feeding organ is spirolophous (has elongated and coiled lateral lobes), except in Pelagodiscus, which has a schizolophous (lobes with relatively few filaments along its edges) lophophore. The lophophore has no supportive skeleton. The digestive tract ends in an anus. The ring of nerves around the animal's esophagus has either a single or paired dorsal and ventral ganglion (group or cluster of nerve cells).
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