38 STUDIES OF ANIMAL TYPES 



Note the hinge teeth along the dorsal edges. How many 

 are there on each valve ? Note how they fit together and 

 interlock with one another. Of what use are they ? 



Note and number the muscle scars, two at the anterior 

 end of the valve and two at the posterior end. At the 

 anterior end near the dorsal margin is the large anterior 

 adductor muscle scar and above and just posterior to this 

 is the small, obscure anterior retractor muscle scar. At the 

 posterior end of the valve and near the dorsal margin is 

 the large posterior adductor muscle scar. Above and just in 

 front of this is the small posterior retractor muscle scar. 



Also note the pallial line parallel with the ventral edge 

 of the valve. 



Make a drawing to show these points. 



E. STRUCTURE OF THE SHELL. Break the shell and ex- 

 amine the broken edge with a hand lens. Note that it 

 consists of three layers. The inner layer is composed of 

 mother-of-pearl and is secreted by the mantle. The middle 

 layer, which is dark in color, consists of many-sided prisms 

 placed at right angles to the surface of the shell. The 

 outer layer forms the greenish or brownish epidermis. 



Place a shell in dilute hydrochloric acid until the mineral 

 matter is all dissolved out. What is the characteristic of 

 the remaining portion? This portion is composed of the 

 animal matter and is called conchiolin. 



Weigh a shell and then burn out the animal matter in the 

 fire. Weigh again and determine the loss if any. 



F. TOPICS FOR FURTHER STUDY. Outline the processes of 

 digestion, circulation, and respiration in this animal. Write 

 out the chief characteristics of the mussel. Fix firmly in 

 mind its position in the animal kingdom. Make a study 

 of its near relatives, the marine clams, oysters, etc. 



