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THE POPULAR SCIENCE MONTHLY. 



valves, and cut tlirough the muscles, before the valves will open. 

 These muscles are called the adductor muscles^ and the scars or im- 

 pressions on the valves are called the adductor muscular impressions. 

 Very close to the adductor muscular impressions are seen smaller im- 

 pressions, and these indicate where the muscles are attached whicli 

 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 Eight Vaive of a Fresh- Water Mussel. 

 f, Cardinal Teeth ; I, Lateral Tooth ; II, Lifcament ; aa. Anterior Adductor Impression ; pa, Poste 

 rior 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 pallia! line, and indicates where the mantle is at- 

 tached 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 liga- 

 ment 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 tiglitly 



