318 



MOLLUSCA 



are opened by an elastic hinge ligament usually placed dorsal to and be- 

 hind the hinge. The shell is closed by adductor muscles which extend 

 through the body from shell to shell, leaving their impressions on the 

 inner surface (fig. 319). Usually there occur an anterior and a posterior 

 adductor equally well developed (Dimyaria) ; less frequently the anterior 

 is rudimentary (Heteromyaria) or entirely disappears (Monomyaria). 

 When the muscles are relaxed the elastic ligament opens the valves. 



The Jieterodont hinge is the typical form (fig. 319); each valve bears a group 

 of teeth near the umbo, those of the left alternating with those of the right. 

 Besides these cardinal teeth there are lateral teeth in front and behind, often pro- 

 duced into ridges. The ligament lies behind the hinge and is usually visible 

 from the outside (external ligament), but is occasionally transferred to the 

 interior (internal ligament, fig. 318). The so-called schizodont and desmodont 

 hinges are modifications of the heterodont. Then there are Acephala of appar- 

 ently primitive character which either lack the hinge (dysodont), or have one 

 composed of numerous teeth in a series symmetrical to the umbo (taxodont), 

 or of two strong teeth likewise symmetrical to the umbo (isodont}. In these 

 cases the ligament is developed in'front of as well as behind the umbo, and may 

 be either external or internal. 



C 



FIG. 320. Ventral views of siphonate and asiphonate acephals. .4, An^donta 

 nea; B, Isocardia cor; C, Lutrana elliptica. a, anal siphon; b, branchial siphon; 

 /, foot; k', outer, k", inner gill lamella; m, mantle; s, shell. 



Since the secretion of shell takes place most rapidly at the edge of the 

 mantle, both are closely united, the union being strengthened by small 

 muscles. So the edge of the shell has a different appearance from the 

 rest, this part being marked off by a pallial line parallel to the margin 

 (fig. 318). In many species (the Sinupalliata) the line at the hinder end 

 makes a large bay (pallial sinus} (fig. 319, 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 



