Most articulate lampshells are less than 1 in (25 mm) across; in some instances, however, they can grow as much as 2 in (50 mm) in width. They contain two very conspicuous hinged valves (or shells) that are composed of scleroproteins and calcite (a form of calcium carbonate). The shells are segmented in a radial pattern; for the most part, they are usually brown in color, or sometimes greenish when surrounded by algae. Only a few lampshells are naturally white. The upper (dorsal) valve is smaller than the lower (ventral) valve. The valves are connected by a joint that can articulate or pivot when teeth located on the ventral valve are inserted into sockets on the dorsal valve along a hinge line. In this manner, the shell can close in front of the hinge and open from behind the hinge, thus locking itself at the rear. The hinge restricts the organism's movements to simple opening and closing. Only a slight anterior gap of about 10° is apparent when the valves are locked in place. This gap at the apex of the two shells resembles the shape of the spout of ancient Roman and Greek oil lamps—whence the English name "lampshell."
The space inside the shell is divided approximately into two halves, the mantle cavity and the body or coelomic cavity. The body of an articulate lies between the shell valves, sheathed in mantle tissue that secretes the shell valves. The coelom lies between the body wall and the gut, and surrounds the lophophore and its tentacles. The lophophore is a crown-shaped structure of hollow tentacles with a pair of arms or brachia (from which the name brachiopod is derived), one brachium on each side of the mouth. The lateral arms of the lophophore are partially supported by a cartilaginous axis on the dorsal valve and a fluid-filled canal within each brachium. The arms climb in a spiral pattern into each half of the mantle cavity.
Articulate lampshells have three sets of muscles that control the movements of their valves. The largest are the adductor muscles, used to close the shell. These muscles usually have two, occasionally four, closely spaced foundations on the ventral valve and four distinctly separated foundations on the dorsal valve. The posterior adductors close the shell rapidly, while the anterior adductors hold the shell tightly closed over longer periods of time. The diductor muscles are used to open the shell for feeding; they have two large foundations on the ventral valve and an attachment area on the rear tip of the dorsal valve. The adjustor muscles, positioned for moving the shell relative to the pedicle, have two attachments near the posterior end of each valve. The pedicle itself is a cylindrical stalk of horny material used to anchor the organism on such substrates as pebbles, stone particles, or coral. The pedicle in articulates ranges from absent in a few species to short and nonmuscular to very long, flexible, muscular, and branched. The pedicle does not expand where it leaves the shell but emerges through a slit or notch on the ventral valve. The pedicle lacks internal muscles and a coelomic lumen; is supported by connective tissue; and is composed of short papillae on its distal end that may branch, wrap around a substrate, or penetrate a soft substrate with acid secretions.
The short U-shaped intestine of articulates begins at the lower end of the stomach and ends in a blind pouch with no anus. The excretory system consists of segmental organs known as metanephridia, which are short tubes ending in large nephrostomes. Articulate brachiopods have a simple circulatory system containing colorless blood with only a few cells. The system includes a dorsal vessel that functions as a primitive pumping heart. The simple nervous system consists of a small dorsal ganglion, or group of nerve cells, behind the mouth and a large ventral ganglion in front of it. A nerve ring around the esophagus connects the various parts of the nervous system, including a nerve running into each tentacle. Articulates do not possess special sense organs, but the mantle margin is most likely an important area of sensory reception.
The setae (bristles) on the mantle are thought to transmit stimuli to receptors in the mantle epidermis.
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