THE MUSSEL OR CLAM AND OTHER BIVALVES 147 



soft, but it is well protected by the shell. This shell consists of 

 two parts called valves (Fig. 83), and hence mussel-like animals 

 have received the name of bivalves. Each valve is built up of 

 concentric layers of calcium carbonate (Fig. 83, 10) extracted 



ds 



aam 



m 



vs 



ma 



FIG. 84. Internal organs of a mussel. 



a, anus; aa, anterior aorta; aam, anterior adductor muscle; b, brain; 

 ds, dorsal siphon ; ec, excretory canal ; ep, excretory pore ; f , foot ; g, gill ; 

 gg, genital gland ; i, intestine; k, kidney ; 1, liver; Ip, labial palp, ; m, mouth ; 

 ma, mantle ; pa, posterior aorta ; pam, posterior adductor muscle ; pc, peri- 

 cardium; pg, pedal ganglion; pw, pericardia! wall; r, rectum; ra, right 

 auricle ; rpo, reno-pericardial opening ; s, stomach ; v, ventricle ; vg, visceral 

 ganglion ; vs, ventral siphon. (After Jammes.) 



from the water by the animal and added to the shell by a mem- 

 brane just under the shell, known as the mantle (Fig. 84, ma). 

 The oldest part of the valve is that near the hinge where the 

 lines of growth are shortest; this part is the umbo (Fig. 83, 6). 

 One cannot tell the age of a mussel by counting the lines of 



