BRACHIOPODS, or BRACHIOPODA, the class of shelled worms, formerly placed among mollusks. The class is named Brachi opoda from the feet-like arms, fringed with tentacles, coiled up within the shell, and which correspond to the lophophore of the Polyzoa and the crown of tentacles of the Sabella-like worms. The shell, which lives attached to rocks, is in shape somewhat like an ancient Roman lamp, the ventral and larger valve, be ing perforated at the base for the passage through it of a peduncle by which the animal is attached to rocks. The shell is secreted by the skin (ectoderm), and is composed of car bonate (Terebratulina) or (Lingula) largely of phosphate of lime.
The body of Brachiopods is divided into two parts, the anterior or thoracic, comprising the main body-cavity in which the arms and viscera are contained, and the caudal portion, that is, the peduncle. The part of the body in which the viscera lodge is rather small in pro portion to the entire animal, the interior of the shell being lined with two broad lobes, the free edges of which are thickened and bear seta, as seen distinctly in Lingula. The body-cavity is closed anteriorly by a membrane which separates it from the space in which the arms are coiled up. The pallial chamber is situated between the two lobes of the mantle (pallium) and in front of the membrane forming the anterior wall of the body-cavity. In the middle of this pallial chamber the mouth opens, bounded on each side by the base of the arms. The latter arise from a cartilaginous base, and bear ciliated tentacles, much as in the worm Sabella. In Lingula, Diseina and Rhyncho nella, they are developed, in a closely wound spiral, as in the genuine worms (Amphitrite). In Lingula the arms can be partially unwound, while in Rhynchonella they cannot only be un wound but protruded from the pallial chamber. In many recent and fossil forms the arms are supported by loop-like solid processes of the dorsal valve of the shell, but when these proc esses are present the arms cannot be pro truded beyond the shell. The tentacles or cirri on the arms are used to convey to the mouth particles of food, and they also are respiratory in function, there being a rapid circulation of blood in each tentacle, which is hollow, com municating with the blood-sinus or hollow in each arm, the sinus ending,in a sac on each side of the mouth.
The digestive system consists of a mouth, oesophagus, stomach, with a liver-mass on each side, and an intestine. The mouth is bordered by two membranous, highly sensitive and mov able lips. The stomach is a simple dilatation of the alimentary canal, into which empty the short ducts of the liver, which is composed of masses of caeca. The liver originally arises as
two diverticula or offshoots of the stomach. The short intestine ends in a blind sac or in a vent, and is, with the stomach, freely suspended in the perivisceral cavity by delicate membranes springing from the walls of the body.
The nervous system consists of two small ganglia above, and an infracesophageal pair of larger ganglia., and there are two elongated ganglia behind the arms, from which nerves are given off to the dorsal or anterior lobe of the mantle.
The larva is top-shaped (trochosphere) and is quite active, swimming rapidly about in'every direction.
While in their development the Brachiopoda recall the larva` of the true worms, they resemble the adult worms in the general ar rangement of the arms and viscera, though they lack the highly developed nervous system of the Annelids, as well as a vascular system, while the body is not jointed. On the other hand they are closely related to the Polyzoa, and it seems probable that the Brachiopods and Polyzoa were derived from common low ver mian ancestors, while the true Annelids prob ably sprang independently from a higher an cestry. They are also a generalized type, hav ing some molluscan features, such as a solid shell, though having nothing homologous with the foot, the shell gland or odontophore of mollusks.
In accordance with the fact that the Brachiopods are a generalized type of worms, the species have a high antiquity, and the type is remarkably persistent. The Lingula of our shores (Glottidia pyramidata) lives buried in the sand, where it forms tubes of sand around the peduncle, just below low-water mark from Chesapeake Bay to Florida. It has remarkable vitality, not only withstanding the changes of temperature and exposure to death from various other causes, but will bear transporta tion to other countries in sea-water that has been unchanged. Living lingulm have been carried from Japan to Boston, Mass., the water in the small glass jar containing the specimens having been changed but twice in four months. The living species of this cosmopolitan genus differ but slightly from those occurring in the lowest fossiliferous strata. Between 80 and 90 living species are known, most of them living, except Lingula, which is tropical, in the tem perate or Arctic seas, while nearly 2,000 fossil species are known. The type attained its maximum in the Silurian Age, and in Palaeozoic times a few species, as Atrypa reticularis, ex tended through an entire system of rocks and inhabited the seas of both hemispheres. Con sult Zittel-Eastman, 'Textbook of Palae ontology' (New York 1900).