BRACHIOPODA, (Gk., arm footed, from gpaxtwv, brarhion, arm + 7rolis pens. foot). A class of peculiar animals, having bivalve shells, but differing in important points from the vast majority of recent mollusks with bivalve shells. The mantle or pallium (see MOL LUSK ) in the brachiopods consists of two broad expansions or lobes, covered by the two valves of the shell, which are dorsal and ventral, and in closing all the other soft parts of the animal; while the chief function of these folds is unques tionably the formation of the two-shell valves, it is possible that they are supplied with blood lacunae or vessels, and may servo as the principal organs of respiration. The circulatory system is not satisfactorily known, but there is probably a vascular system present, with a heart placed above the intestine. The nervous system is weak ly developed, but includes a small brain and a larger infra-(esophageal ganglion, with two prin cipal trunks running backward from it. There are no special sensory organs of any kind. The organs by which food is procured are also re markable—two long arms arising from the sides of the mouth, and disposed wholly or partly in spiral curves, when not extended to seek or seize prey, These arms are usually furnished with numerous vibratory filaments, which are sup posed not only to aid in the capture of prey. but in the maintenance of the current necessary for respiration. The arms are often supported by a special calcareous skeleton, attached to the dorsal valve.
Excretion takes place by means of nephridia, of which one or two pairs are pre.sent; these also serve as duets fur the transmission of the sexual products. They open into the mantle
cavity on each side of the mouth. The Braehio poda are attached to solid hodies either by a footstalk or by one of the valves of the shell. When the stalk is present it emerges either be tween the valves or through a hole in the pos terior upward bulging of the larger ventral valve. The Brachiopod.: are most notable because of the fact that in the earliest geological times they were a dominant type; but since the close of the Paleozoic Age they have been steadily decreasing in number. More than 4000 species are known, but more than 95 per cent. are extinct and exist only as fossils. Most of the living brachiopods are small, less than an inch across, but some of the extinct forms were six inches across. All living species are marine, and there is every reason to believe the same is true of the extinct species. Existing species are found in water of moderate depth, sometimes down to 250 fath oms. They are generally grouped in two orders: the .1 rtienlata, or Testicardines, in which the shell-valves are hinged, and the Inarticulate, or Eeardines, without hinge. Examples of Brachio pods are shown on the plate: 1, Crania Bordeni; 2, Spirifer eameratus; 3, Bilobites hiloba; 4, Tropidoleptus oceidens; 5. Pentamerus oblongus; Incites gryphus; 7, Derbya Bennetti; 8, Orthis trieenaria; 9, Athyris spiriferoides; 10. Orthis peetinella (interior) ; I 1,1thipidomella Michelini; 12, Stropheodonta demissa; 13, Camarotcechia congregate.