Fig. 2.
Magellania flavescens. Australia. Interior of valves.
A. Peduncle valve: f, foramen for peduncle, below which are the two small deltidial plates; t, hinge-teeth; a, b, c, muscle scars. B. Brachial valve, showing the reflected loop for support of the “arms.”
The shell is constructed of very minute prisms of calcareous substance imbedded in an organic matrix. In Lingula the shell is formed of alternating layers of horny and calcareous substance.
The shell-valves are either hingeless, or joined by a hinge in which teeth in the peduncle valve fit into sockets in the brachial valve. The Brachiopoda are primarily divided into two sections, Inarticulata and Articulata, based on the absence or presence of a hinge.
The division into Orders is based on the relation of the peduncle to the valves in its passage between them or through one of them. In the most primitive Brachiopoda (Lingulidæ), the peduncle simply passes out between the valves and not through a foramen or pore in one of them; hence the group is named Atremata (a, not, trema, pore). In the next group, including the families Discinidæ and Craniidæ, the peduncle passes through a fissure in the edge of the peduncle valve, the fissure in recent forms becoming closed round to form a slit-like foramen; this group is named Neotremata (neos, new, trema, pore). In the third group, Protremata (pro, in front of, trema, pore), which includes the Thecidiidæ, the peduncle lies at the apex of a triangular fissure in the peduncle valve, and secretes a calcareous plate to fill in the gap. In the fourth group, Telotremata (telos, final or complete, trema, pore), including the Terebratulidæ, etc., the triangular fissure in the peduncle valve is filled in by two calcareous plates termed deltidia, secreted by the edges of the mantle.
The valves are hingeless in the first two Orders (Inarticulata), and hinged in the last two (Articulata).
The Body.—The body usually occupies only a comparatively small space in the posterior or peduncle end of the shell. From each side of the body there is given off a thin expansion, the mantle which lines the inner surface of the shell. The space between the valves is termed the mantle-cavity. The mouth is situated in the centre of the front wall of the body or floor of the mantle-cavity. The front wall gives rise to a horseshoe-shaped platform surrounding the mouth and bearing on its upper edge ciliated tentacles, or cirri, which set up currents carrying food towards the mouth. In many genera the platform is produced into two coiled “arms” (Figs. 3 and 5), which fill up the mantle-cavity.
The name Brachiopoda (brachion, arm, pous, foot) was given to the group because these “arms” were supposed to be homologous with the Molluscan “foot.”
The mouth leads into a gullet, which opens into a stomach and intestine. In the more primitive forms the intestine terminates in a vent, but in the higher forms the distal end of the intestine has become atrophied, and consequently the gut ends blindly.
The body-cavity contains fluid, and is in communication with a system of sinuses in the lobes of the mantle (Fig. 5). Bands of muscles pass across from valve to valve. The peduncle consists of a horny outer sheath surrounding longitudinal and transverse bands of muscles.