Brachiopoda, Or Brachiopods(Gr. arm, and foot), till within a few years universally regarded as one of the classes of mollusca, named by Cuvier from two long, ciliated arms, which project from the side of the mouth, and with which they create currents that bring them food. By De Blainville and Owen they were called palliobranchiata, from pallium, a mantle, and Iranchia, gills, the delicate mantle covering the body constituting the respiratory apparatus of the animals. They are bivalve, differing from the conchifera in the valves being always unequal; yet they are symmetrical and equal-sided. The valves are dorsal and ventral, instead of right and left, the smaller and lower being generally considered the dorsal valve. By the old naturalists they were commonly called lampades, or "lamp shells," from the resemblance of their form to that of the antique lamps; the hole for the wick in these being represented in the shell by the curved beak of the ventral valve, through which the organ passes by which the animal attaches itself to any substance. The brachio-poda all belong to salt water. They are found attached to corals, to other shells, and to the under sides of shelving rocks.
Though a low animal type, no other class exhib-itsvsuch a great range in time, geographical distribution, and depth of water; they are found from the Silurian to the present epoch, from the poles to the tropics, and from near high-water mark to the greatest depths reached by the dredge. Among the earliest forms of animal life were the lingulae of the lowest fossiliferous rocks. This genus has continued through all the series of formations, during which multitudes of other forms have been introduced and spread through an immense number of species, which have long since disappeared, leaving no type of their family in existence; but the ancient genus lingula is still met with in the Pacific and on our Atlantic coast; and the terebratula and discina, which were introduced in periods nearly as remote, have representatives living in many parts of the world. More than 1,000 extinct species have been described. They constitute a large proportion of the shells found so abundantly throughout the New York system, as the spirifer, productus, atrypa, strophomena, etc.
They were most numerous in the Silurian and Devonian epochs, since which they have been declining; there were about 700 in the palaeozoic age, not more than 200 in the cretaceous period, and there are fewer than 100 at the present time, of which the best known genera aoe lingula, terebratula, discina, rhyndbnella, and crania, all of which are very old forms. - Naturalists have for some years been of the opinion that the brachiopods and polyzoa form a natural anatomical class, defined by Prof. Hyatt as a sac closed at one end by a disk, surrounded by free tentacles, and perforated by a toothless mouth from which hangs the alimentary canal. Some recently have been inclined to add the ascidians, and to separate the three from the mollusca, under the name of molluscoida; the ascidians seem to form the connecting link of the mol-luscoids with.the bivalve mollusks; the first two agree in having but one aperture to the atrial chamber, and a complicated muscular system intersecting the visceral cavities.
Prof. E. S. Morse, in " Memoirs of the Boston Society of Natural History," 1871, from the study since 1869 of terebratulina and discina, in all stages of growth, finds the following articulate characters, which induce him to remove the brachiopods from mollusks: the shell is like that of Crustacea in its tubular structure, scale-like appearance, and chemical composition; in lingula there is 42 per cent, of phosphate of lime, and only 6 per cent of carbonate of lime; the horny bristles or setae fringing the mouth are remarkably worm-like; they are secreted by follicles, surrounded by muscular fibres, and freely movable. Gratiolet has compared the circulatory system with that of Crustacea, and Burmeister has shown the resemblance between the respiratory apparatus of lingula and that of certain cirripeds. The oviducts resemble the similar organs in worms in their trumpet-shaped openings; the part bearing the cirri, and the mantle covering the arms, are comparable to similar parts in tubicolous worms. From French and German authors we have many proofs of their affinity with the worms in embryological characters.
These views were confirmed by Prof. Morse's study of the living lingula on the coast of North Carolina, near Fort Macon. Here he ascertained that they make a tube in the sand, like annelids; the peduncle is hollow, distinctly ringed, with longitudinal and circular fibres, very contractile, and remarkably worm-like; they have also red blood, and the sexes are distinct. His conclusions are that they are "true articulates, having certain affinities with the Crustacea, but properly belonging to the worms, coming nearest the tubicolous annelids: they may be better regarded as forming a comprehensive type, with general articulated features." Possibly they have affinities with the mollusks, as homologies have been pointed out between the polyzoa and tunicates or ascidians.