76 SHELL GALLERY. 
to the valves in its passage between them or through one of them. 
In the most primitive Brachiopoda (Lingididai), 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 Discinidce, 
and Craniida, 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 Thecidiidm, 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, Telo- 
tremata (telos, final or complete, trema, pore), including the Tere- 
Iratulidoz, 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 (orachion, 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. 
