288 



CCELHELMINTHES 



strongly arched and has an opening at the posterior end for the passage 

 of the stalk (figs. 277, 278). The flatter dorsal valve frequently bears a 

 characteristic feature in the calcareous skeleton of the arms (fig. 278) which, 

 when present, has very different forms. \\ hen closed the valves completely 



rf 



FIG. 276. Anatomy of Rhynchonella psittacea (after Hancock), a', left, a~, 

 right arm; , opening into the cavity of the arm; d, intestine; e, blind end of the intestine; 

 g, stomach with liver; m, adductors and divaricators of shell; o, cesophagus; />', /'-'. 

 dorsal and ventral mantle lobes; st, stalk; i, 2, first and second septum, on the second a 

 nephrostome. 



enclose the body. When they open the gape is anterior, the posterior 

 parts remaining in contact. Here, except in the Ecardines, a hinge is 

 developed just in front of the posterior margin, consisting of teeth in the 

 ventral valve which fit into corresponding grooves in the dorsal. Opening 



D 



h 



FIG. 277. II aldheimia flavescens (from Zittel). Shell with arms and muscles, 

 a, arm with fringed border (h); c, c', divaricators; d, adductors; D, hinge process (the 

 vertical line shows position of hinge). 



and closing the valves are, contrary to what occurs in Lamellibranchs, 

 active processes, accomplished by appropriate divaricator and adductor 

 muscles (fig. 277). These produce scars on the shell, important in the 

 study of fossil forms. 



