360 



MOLLUSC A. 



palliata, the line at the hinder end makes a large bay (pallial 

 sinus) (fig. .348, s). Since the mantle folds are membranes with 

 free margins, it follows that when the shell is closed these edges are 

 pressed together, which would prevent the free entrance and exit 

 of water. To accommodate this each mantle has its margin exca- 

 vated at the posterior end, so tliat when brought together two 

 openings, an upjier and a lower, result (fig. 340, C). The lower 



Fig. 349. — Ventral views of siphonate and asiphonate acephals. -4, Anodnnia cygnfa ; 

 f/, J.wc'irdin €<')• : C, Lntrt(ria elliptiai. a, anal siphon ; /', branchial siphon ;/, foot; 

 k\ outer, t/', inner gill lamella; in, mantle; «, shell. 



of these is the branchial opening by wliicli fresh water passes into 

 the mantle (branchial) chamber; it flows out after j)assing over 

 the gills, along witli the fa?ces, tlirough the upper or cloacal open- 

 ing. In many bivalves the free edges of the mantle grow together, 



Fig. 350.— Section of shell of .iiimlmita. c, cnticula; j\ prismatic layer; /, nacreous 



la>"er. 



leaving three openings, one for tlic jirotrusiou of the foot, the 

 others the two just descriljed, wliicli are now called the incurrent 

 (brancliial) and excurrcut (cloacal) siphons (tig. 340, 7?). By 

 furtlier development the mai'gins of tliese (.ipenings are drawn out 



