BRACHIOPODA. 81 



the Brachiopoda, which have hitherto been regarded as Molluscs, are 

 closely related to the Bryozoa. 



The Brachiopoda possess a large body, enclosed in a bivalve shell, 

 of which one valve is anterior (dorsal valve), the other posterior 

 (ventral valve) (fig. 554). Both valves lie upon corresponding folds 

 of the integument (mantle lobes), and are often connected on the 

 back by a kind of hinge, above which the usually more arched 

 ventral valve projects like a beak. This ventral valve is either 

 directly fused with foreign bodies, or the animal is attached by a 

 peduncle projecting through the opening of the beak (fig. 554 St). 

 The peduncle may, however, pass out between the two valves (Lin- 

 gula). The valves of the shell are cuticular structures secreted by 

 the skin and impregnated with cal- 

 careous salts; they are not opened 

 by a ligament, but by special groups 

 of muscles (fig. 554 Met) ; they are 

 closed also by muscles which are 

 placed near the hinge, and pass 

 transversely from the dorsal to the 

 ventral surface through the body 

 cavity (fig. 554 Ma). 



The body is bilateral and enclosed 

 by the shell\ it possesses two large 

 reduplications of the integument, 

 the two mantle lobes, which are 

 applied to the inner surface of the FIG. 555. Dorsal valve of shell of 



Shell. The edffeS of the mantle ^imiaaus^alis with the bracliial skeleton 

 6 (after Hancock). 



lobes are thickened, and carry very 



regularly-arranged seise. The mantle may also produce within its 



own substance calcareous spicules or a continuous calcareous network. 



The mouth is placed between the bases of the two spiral arms and 

 leads into the oesophagus ; the latter passes into the intestine, which 

 is attached by ligaments and surrounded by large hepatic lobes. The 

 intestine either describes a single bend, or is of considerable length 

 and coiled (Discina, Lingula). In the latter case it opens into the 

 mantle cavity by an anus placed on one side of the middle; 

 while in the hinged Brachiopoda (Terebraiula, Waldkeimia) there is 

 no anus, and the intestine ends blindly in the body cavity (fig. 554). 

 Sometimes the end of the intestine is continued into a string-like 

 organ (Thecidium). 



The two buccal arms are supported by a hard framework cor - 

 VOL. IT. 6 



