398 THE ANATOMY OF IXYERTEBRATED 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 Brachiopoda, 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, which 

 ends in a sac at the side of the mouth. 



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

 gether and 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. 1 In this 

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

 servations of Morse, 2 they can be straightened and extended 

 beyond the shell in Rhynchonetta, 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 

 caecum in the middle line of the body (e. g., Waldheimia) ; in 

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

 right side of the mouth (e. g., IAnguld, Discina, 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 ileo-parietal bands — connect 

 the gastric and intestinal divisions of the alimentary canal 

 respectively, with the parietes. 3 



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

 tions extend into each lobe of the mnntle, and end crecally 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 Lingida give rise 

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

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

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



1 See, for excellent figures of these arrangements, and for the shells and ex- 

 ternal form of the body in general, Woodward's " Manual of the Mollusca." 



2 " On the S\ Btematic Position of the Brachiopoda." (" Proceedings of the 

 Boston Society of Natural History," 1873.) 



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

 ings of the Roval Societv," 1854); and Hancock, " On the Organization of the 

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



