us 



MOLLUSCA 



arc o]icneil hv an clastic hinge ligaDioit usually placed dorsal to and be- 

 hind ihc l"LiuL,'c. The shell is closed Ijy addiiitor iiiiisclfs which extend 

 through the IkhIv from shell to shell, lea\-inL^ their impressions on the 

 inner surface (I'ig. 310). Usually there occur an anterior and a posterior 

 adductor ciiually well developed (Ditnyaria) ; less frei|ueinly the anterior 

 is rudimentary (Heteromyaria) or entirel)' disappears (Monomyaria). 

 ^^'hen the muscles are relaxed the elastic ligament opens the \'al\-es. 



The Inirroiloiil hinge is the typical form (fig. 310"); eacli vahe bears a group 

 of teeth near the umbo, those of the left aUernaling witli those of the riglU. 

 Besides these canliihil U\ili there are lalcral laili in front and liehind. often pro- 

 duced into ridges. The Hganicnt lies behind the hinge ami is usually \isible 

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

 interior (internal ligament, fig. 31S). The so-called siliizoiloiit anti dcsinodoiit 

 hinges are modifications of the hctcrodoiU. Then there are .Vcephahi of a])par- 

 ently primitive character wliich either lack the hinge (dysodoiil), or haw one 

 composed of numerous teeth in a series symmetrical to the unilio (Id.yndoiil), 

 or of two strong teeth likewise svnnuelrical to the umbo (isodiiiil). In these 

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

 be either external or internal. 



Fir.. ;^2o. — \'cntra1 yiews of siphonate and asiiihonatc acenhals. .1, Aiimlivila 

 cy,[,'/;o;; l\ I soiarilia cor; (_', Liilrana clliplii\i. a, anal siphon ;/>, braiuhial siiilion ; 

 /, fuot; 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 arc 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 pallia! line parallel to the margiii 

 (fig. 318). In many sjiecies (the Sirtupalliata) the line at the hinder cml 

 makes a large bay {J>allial sinus) (fig. 319, i-). Since the mtmtle folds 

 are membranes with free margins, it follows that \yhen the shell is closed 

 these edges are pressed together, wliich would prevent the free entrance 

 and exit of water. To accommodate tliis each mantle has its margin 



