A FRESH-WATER MUSSEL. 79 



near each end of the gape, the large muscle which holds the two valves 

 together. 



Note that the shell now stands widely open. Push it shut. Will 

 it stay in this position ? Understand the mechanism of opening and 

 closing the shell. Break the hinge ligament and place the half of the 

 shell containing the body of the animal in water. 



Exercise i. Draw the outside of the left valve, natural size. 



INNER SURFACE OF THE SHELL. Examine now the inside of the 

 left valve. Note the following: 



a) Parallel with the edge and a short distance from it is the pallial 



line. Look on the right valve for the cause of this delicate 

 streak. 



b) Beneath and a short distance ahead of the umbo the pallial line 



ends in the anterior adductor impression. A similar posterior 

 adductor impression lies near the opposite end of the shell. 

 What caused these impressions ? 



c) Just above each of the large impressions is a smaller scar marking 



the insertion of the anterior and posterior retractor muscles 

 of the foot, respectively. The large and small anterior impres- 

 sions are slightly separated; the two corresponding posterior 

 scars are contiguous. 



d) Immediately behind the lower half of the anterior adductor impres- 



sion is another small scar made by the protractor muscle of the 

 foot. 



e) If the specimen is a species of Unio, you will find several hinge 



teeth just beneath the hinge ligament. \Vhat purpose do they 

 serve ? Anodonta has no such teeth. 



f) Observe an outer layer of the shell periostracum composed 



of conchiolin, a tough, horn-like substance analogous to chitin. 



g) An inner, smooth layer, the nacre, with a pearly luster. 



h) Between them is the prismatic layer, formed of minute prisms 

 of calcium carbonate separated by thin leaves of conchiolin. 

 Examine the edge of a piece of freshly broken shell. 



