76 SHELL GALLERY. 



to the valves in its passage between them or through one of them. 

 In the most primitive Brachiopoda (LinguUdce), 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 GraniidcB, 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, Peotremata {pro, in front 

 of, trema, pore), which includes the Tlucidiidm, 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- 

 hratididcB, 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 {hracMon, arm, pons, 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. 



