36 FIRST BOOK OF ZOOLOGY. 



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. 



The pupil may mark with a pen the names of all the 

 parts upon the inside of a fresh-water mussel. 



35. When the mussel is opened by separating the ad- 

 ductor 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 Avill 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 continu- 

 ally exert itself by contracting the addxictor 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 open- 

 ing 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 closed, the rubber is drawn out, or stretched 

 across the back, and, if allowed to regain its elasticity, the 

 lids are pulled apart. This experiment illustrates the way in 

 which the ligament acts in those shells which have the liga- 

 ment external. 



