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. [Illustration: Fig. 3. _Magellania flavescens._ (After Davidson.) A. Interior of dorsal valve to show the “arms”; some of cirri removed on right side; _v_, mouth. B. Longitudinal section, with a portion of the animal. ]