52 MOLLUSCA. 
of dead shells several scars are seen, some oval and others 
mere lines; the former are the marks of the adductor mus- 
cles, ¢,¢, 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, 
J, and shows where the mantle was attached to the shell. 
Fic. 55.—Mollusk, without siphon. 
S, shell; 7, lower half of man- 
tle; m’, a piece of the upper half; 
al &, breathing-gills; 4, heart; Zv, 
Fic. 54.—Bivalve shell. a@, beak; 2, liver; //, lips; 0, opening of 
base; 4, 4, hinges; c, @, d, principal mouth ; 2; anus, where refuse is 
teeth; 4, ligament; e¢, ¢, adductor thrown out; ms, muscle hold- 
muscles ; /, lines of growth; _/, pal- ing shells together; c, elastic 
lial line. cushion forcing them apart. 
Internal 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, ¢) or branchiz 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 
