Brachiopoda

Century Dictionary and Cyclopedia

1. A class of mollusk-like animals distinguished by the development of two labial (generally called brachial) appendages, diverging from either side of the mouth. The animal is invested in a mantle which extends laterally and forward, is highly vascular, and secretes a shell composed of dorsal and ventral valves opening aborally; it is without foot or branchiæ, respiration being effected by the branchial mantle. By the older naturalists the species were regarded as bivalve shells, or at least as true mollusks; but by later writers they have been separated as representing (alone or with Polyzoa) a peculiar branch or subkingdom Molluscoidea, and approximated to or associated with the worms, Vermes. The class is generally divided into two subclasses or orders, Arthropomata or Clistenterata, and Lyopomata or Tretenterata. The families of the inarticulate or lyopomatous brachiopods are the Lingulidæ, Craniidæ, and Discinidæ, all of which have living representatives. The families of the articulate or arthropomatous brachiopods are the Terebratulidæ, Rhynchonellidæ, Theciidæ, Spiriferidæ, Koninckinidæ, Pentameridæ, Strophomenidæ, Orthidæ, and Productidæ. The species are very numerous, nearly 4,000 having been described; they are mostly extinct, and all marine. They flourished especially during the Silurian period, and some Silurian genera, as Lingula, are still extant. See cut under Lingulinœ. Many of the species, especially of the family Terebratulidœ, are known as lampshells.
2. In recent years extensive studies of the Brachiopoda have been made: by Joubin on the anatomy of recent species, by Hall and Clarke on the anatomy and taxonomy of fossil forms, and by Beecher on ontogeny and phylogeny. It is now generally conceded that a classification based on the absence or presence of hinge-teeth, which is the foundation of that introduced by Deshayes, Owen, Bronn, and Huxley, or on the perforation or closing of the anus, as in King's scheme, has a value restricted to fully accomplished phylogenetic phases or ontogenetic conditions. In Beecher's classification, variation in the nature of the pedicle-opening is assumed as the basis of ordinal divisions, and the stages of shell-growth and its accessory developments of those of minor value.
3. This classification establishes the following orders: Alremata, in which the valves are unhinged and the pedicle emerges freely between them. These are subcircular or elongate shells in which growth takes place freely in all directions except posteriorly. Obolus and Lingula are typical genera.
4. Neotremata, in which the valves are also hingeless, cone-shaped, with the pedicle restricted to the ventral valve and the pedicle-aperture modified by a deltidium or listrium. These are circular, often largely calcareous shells (Acrotreta, Orbiculoidea, Discina). Crania and its allies have lost the pedicle by atrophy and are either attached by cementation of their valves or are wholly free.
5. Protremata, in which an articulate hinge is developed in progressed forms, the pedicle is restricted to the ventral valve, the deltidium originates on the dorsal side of the larval body but becomes ankylosed to the ventral shell and modifies the pedicle-passage, and the brachia are unsupported by calcareous processes. (Strophomena, Thecidium, Productus, Orthis, Pentamerus.)
6. Telotremata, in which the pedicle-passage is shared by both valves in early growth, but is confined to the ventral valve at maturity and modified by deltidial plates which are morphologically distinct from the deltidium and have a later origin in ontogeny. The valves are hinged, the brachia supported by calcareous crura, loops, and spirals. (Rhynchonella, Terebratula, Spirifer, Atrypa.) With the increased knowledge of the structure, distribution, and vertical range of the extinct brachiopods, the number of genera has been augmented to about 400, living and extinct, represented by some thousands of species, 2000 species being recognized in the Paleozoic rocks of North America alone, while about 150 species are now living.

Wiktionary

1. n. A taxonomic phylum within the superphylum Protostomia — the brachiopods.

GNU Webster's 1913

1. n. (Zoöl.) A class of Molluscoidea having a symmetrical bivalve shell, often attached by a fleshy peduncle.

WordNet 3.0

1. n. marine invertebrates that resemble mollusks