52 MOLLUSC A. 



of dead shells several scars are seen, some oval and others 

 mere lines ; the former are the marks of the adductor mus- 

 cles, <?, e, that move the valves, opening and shutting them. 

 Near by are smaller scars, showing the position of the 

 pedal muscles that moved the foot. The line running par- 

 allel with the margin of the shell is called the pallial line, 

 /, and shows where the mantle was attached to the shell. 



FIG. 54. Bivalve shell, a, beak ; k, 

 base ; , l>, hinges ; c, d, d, principal 

 teeth ; //, ligament ; e, e, adductor 

 muscles ; /, lines of growth ; /, pal- 

 lial line. 



6 



FIG. 55. Mollusk, without siphon. 

 6", shell ; m, lower half of man- 

 tle; m', a piece of the upper half; 

 g, breathing-gills ; 7i, heart ; /v, 

 liver ; Ip, lips ; 0, opening of 

 mouth ; , anus, where refuse is 

 thrown out ; ms, muscle hold- 

 ing shells together ; c, elastic 

 cushion forcing them apart. 



Infernal Organs. Opening the valves, we note the two 

 leaves of the mantle, or body-wall, whose function is to 

 secrete and repair the shell. Removing these, we see the 

 gills (Fig. 55, g) or branchiae that are open in front and 

 joined at the back. They appear made up of minute 

 rods covered by a maze of veins, and are provided with 

 cilia. The siphon (Fig. 56, s), or so-called blackhead, 

 when present, projects through the mantle, and in the 

 clam is capable of great distention. It is divided into 



