valves, and cut through the muscles, before the valves will open. These muscles are called the adductor muscles and the scars or impressions on the valves are called the adductor muscular impressions. Very close to the adductor muscular impressions are seen smaller impressions, and these indicate where the muscles are attached which move the foot. These muscles are called the pedal muscles, and the impressions are called the pedal muscular impressions. One occurs just behind the anterior adductor impression; the other will be found just above, and in front of the posterior adductor impression.
Fig. 11. The Right Valve of a Fresh-Water Mussel.
c, Cardinal Teeth; l, Lateral Tooth; li, Ligament; aa. Anterior Adductor Impression; pa, Posterior Adductor Impression; ap, Anterior Pedal Muscular Impression; pp, Posterior Pedal Muscular Impression; p, Pallial Line; u, Umbone.
Besides these marks, we see a delicate and slightly irregular line running from the anterior to the posterior muscular impression, just inside, and nearly parallel with the lower margin of the shell. This line is called the pallial line, and indicates where the mantle is attached to the shell. It will be observed that, when the soft parts are removed from the shell, the mantle adheres along this line.
When the mussel is opened by separating the adductor muscles with a knife, the valves slowly open, and after the animal is removed the valves still remain partly open, and, to preserve them closed, a string has to be tied around them, and in this condition, if the ligament is allowed to dry, the valves will then remain closed. From this it is evident that the ligament acts upon the valves to draw them apart. To keep them closed, then, the animal must continually exert itself by contracting the adductor muscles; and it will be found that, when these creatures are left in the water, undisturbed for a while, the muscles relax, and the valves partly open. The ligament is elastic, and is stretched as it were from one valve to the other, over the back. A possible imitation of the action might be represented by partly opening the lids of a book, and then gluing across the back, from one lid to the other, a sheet of elastic rubber. If, now, the lids are tightly
