398 THE ANATOMY OF INYERTEBRATED ANIMALS. 



area is narrowed to a mere groove, and is produced on each 

 side of the mouth into a long spirally-coiled arm, fringed with 

 tentacles ; whence the name of J^rac/dojjoda, applied to the 

 group. 



In this case the tentacula disappear from the anterior 

 margin of the oral disk in the region of the mouth, and are re- 

 placed by a lip-like ridge. Each arm contains a canal, wliich 

 ends in a sac at the side of the mouth. 



In Waldheimia (Fig. 116), the two arms are united to- 

 .getherand their distal portions coiled into a horizontal spiral. 

 In many genera, the margins of the oral area or arms are 

 fixed to processes of the dorsal valve of the shell.^ In this 

 case the arms are not protrusible ; but, according to the ob- 

 servations of Morse,^ they can be straiglitened and extended 

 beyond the shell in Rliynchonella^ which has no brachial 

 skeleton. 



The alimentary canal consists of an oesophagus, a stomach, 

 provided with hepatic follicles, and an intestine. In the ma- 

 jority of existing genera the latter is short, and ends in a 

 cfecum in the middle line of the body (e. g., M'^aldhehnla) ; in 

 others it is long, and opens into the pallial chamber on the 

 right side of the mouth (e. g., Lhifjxda^ Disciiia, and Crania). 



The alimentary canal is invested by an outer coat — the so- 

 called peritoneum — by which it is suspended, as by a mesen- 

 tery, in a spacious "perivisceral" cavity. The walls of this 

 cavity are provided with cilia, the working of which keeps up 

 a circulation of the contained fluid. Lateral processes of this 

 coat — the gastro-parietal and iUo-parietal ba7ids — connect 

 the gastric and intestinal divisions of the alimentary canal 

 respectively, with the parietes.^ 



From the perivisceral cavity, sinus-like, branched prolonga- 

 tions extend into each lobe of the mantle, and end ca^cally at 

 its margins. The lobes of the mantle are probably, together 

 with the ciliated tentacula, the seat of the respiratory func- 

 tion. The sinuses of the pallial lobes of Lingula give rise 

 to numerous highly contractile, teat-like processes, or ampid- 

 Im. During life the circulating fluid can be seen rapidly cours- 

 ing into and out of each ampulla in turn (Morse, /. c, p. 33). 



1 See, for excellent fiprures of these arranprements, and for the shells and ex- 

 ternal form of the body in g^eneral, "Woodward's " Manual of the Mollusca." 



2 " On the Systematic Position of the Brachiopoda." (" Proceedmgs of the 

 Boston Society of Natural History," 1873.) 



3 Huxley. " Contributions to the Anatomy of the Brachiopoda " ("Proceed- 

 insrs of the Roval Society," ISai) ; and Hancock, " On the Organization of the 

 Brachiopoda" ("Phil. Trans.," 1858). 



