178 



INVERTEBRATE ANIMALS. 



is only one adductor muscle, but ordinarily there are two (Fig. 

 81, 3). These muscles leave distinct scars or " muscular im- 

 pressions " in the dead shell, so that it is easy to determine 

 how many were present in any given shell. The margin of 

 the mantle, too, is muscular, and leaves upon the shell a dis- 

 tinct line where it was attached, this being known as the 

 " pallial line " (Lat. pallium, a mantle), as shown in Fig. 81. 



As regards the shell of the bivalves, the following are the 

 chief points to be noticed. Each valve of the shell (Fig. 81) 

 is to be regarded as essentially a hollow cone, the apex of 

 which is turned more or less to one side. The apex of the 

 valve is known as the " umbo " or " beak," and is turned tow- 

 ard the mouth of the animal. Consequently, the side of the 



Fie. 81.— Shells of Lamellibranchiata. 1. Cyda« amnica, a shell with two addnctor 

 muscles, and an "entire "mantle-margin. 2. Ttapa) pi/iiasfro, a shell with two adduc- 

 tors, and an indented pallial line. 8. P&ma eph/vppvmn^ a shell with one adductor 

 muscle : o Pallial hue ; o Scars left by the adductors ; c Siphonal impression. 



shell toward which the beaks are turned is known as the 

 " anterior " side, and it is usually much shorter than the oppo- 

 site or " posterior " side. The side of the shell at which the 

 beaks are situated is known as the " dorsal " margin ; and here 

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

 distance along a line which is known as the " hinge-line." 

 The union between the valves is usually effected by means of 

 interlocking parts or " teeth," and there is often a band of 

 homy fibres passing between the two valves just behind the 

 beaks. In many cases, there is also a series of homy fibres 

 placed perpendicularly between the beaks, so as to be com- 

 pressed when the shell is shut. By the elasticity of these, 

 and of the external ligament, when present, the valves of the 

 shell are opened, without any effort of the animal, simply by 

 relaxing the adductor muscles. The valves are shut again by 

 the contraction of the adductor muscle or muscles. 



