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 that when brought together two 

 openings, an upper and a lower, result (fig. 349, 0). The lower 



FlG. 349. Ventral views of siphonate and asiphonate acephals. A, Anodonta cygnea ,' 

 B, Isocardia cor ; (7, Lutraria elliptica. a, anal siphon ; />, branchial siphon ;/, foot; 

 A;', outer, A.", inner gill lamella; wi, mantle; s, shell. 



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

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

 the gills, along with the faeces, through the upper or cloacal open- 

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



FIG. 350. Section of shell of Aru 



;. c, cuticula; p, prismatic layer; I, nacreous 

 layer. 



leaving three openings, one for the protrusion of the foot, the 

 others the two just described, which are now called the incurrent 

 (branchial) and excurrent (cloacal) siphons (fig. 349, B). By 

 further development the margins of these openings are drawn out 



