198 



IXVEIITEBRATE ANIMALS. 



being known as tliu "pallial line" ( Lat. piilliinn',;i mantle), as shown 

 in tig. 138. 



As regards the shell of the Bi\:ilve.s the following are the chief 

 points to he noticed. Each vahe of the .shell (tig. 138) is to be 

 regii.rded as essentially a liullow <-iaie, the ajiex oi which is turned 

 more (jr less to one side. The a|ie.\: of the valve is known as the 

 ''und)o" or " l>eak," :ind is tunii'd towai'ds the mouth of the aiuniah 

 C'onsei|Uently, the side of the .shell towards which the beaks a.re 

 turned is known as the "aid-erior" side, and it is u.sually much 

 shorter than the opiwisite or "posterior" side. Tlie side of the shell 

 at which the beaks a.re situated is known as the "dorsal" niargin ; 

 and here the valves are united to one another for a longer or shorter 



Fi-, l:ls._Lpft v.ihT or Cjilhen;,. ,-I,i>j,ir i:,t\vr W Iwiinll A, AntiTinr ni.irpn ; 



li, I'.isliiiur i,i;iiL'iii : C, Vrntriil m.'ir-iii i.r li.isr; /. Uliil"i; h Li-.-aiiR-iit ; c Ciil- 

 .liiiiil Idolli ; // I,;Hrr:il Ovlli; ,/ AiiliTidr aild iii'lnr ; „' ro.strrinv ad.llictov ; 

 J. J'allial lilir ; .J I':i||i:il .villus riillsi-a liy llir IvtriirO ir Ijinsi-lrs ,if tli,' sililloilS. 



dist.'ince along ,'i line which is known as the "hinge-line." The 

 union liet«ecii (lie x.alves is nsu;dly ett'ccted by means of inter- 

 lorkiic;' p.-iiis or " Icc4li," and llici'e is often a band of horny hbres 

 passini; belwi'cn tlir two v;d\-cs just behind the l.ieaks. In many 

 cases theiv is ;dso a .staios of lioiny tibres placed perpendicularly 

 between tln' boaks. ,so ;is to be couipi'essed wlien tlie sliell is .shut. 

 By tlie elasti<'it,y of these, ;uid af the e.xtei'u.al ligament when ])re.s- 

 eid, \]]r. valx'es (.f the sliell .-ne opened, without any effort of the 

 auiui;\l, simply by relaxin;,;- llio ,'iddMetor niusidc.s. The valves are 

 ;shiit .I'jviin liy the con tract ion of llic adductor muscle or muscles. 



,\.^ .dii',id\- said, tile m.aicin of llic m.aollc lc,i\a's on the shell a 

 diclinci iiniaessioh-llic "j.allial line"-,aial by inspection of this, 



