190 THE BIOLOGY OF THE ANIMAL 



ness? Of how many parts or valves does the 

 shell consist ? How are the valves held together ? 

 Can you distinguish a right and a left side to the 

 shell ? Dorsal and ventral sides ? Anterior and 

 posterior ends? Do you find that some of the 

 shells of the same kind are more convex than 

 others? Of what use is the shell? Put a small 

 piece into weak hydrochloric acid. "What is the 

 result ? Compare with the " shell " of the lob- 

 ster and the integument of the insect. How is 

 the mussel shell probably formed ? 



JSTotice that a portion of the shell is usually 

 covered with a deposit of mud. Which portion 

 is it ? How thick is the deposit ? What is the 

 direction of the line between the clean and the 

 mud-covered portion ? Does the latter portion 

 show more or fewer signs of wear, such as ab- 

 sence of "epidermis," fading of colors, smooth- 

 ness of surface, etc., than the other? 



h. The single valve. 



On the outside study 



1. The shape. — What is the shape of its outline? 



Are the two ends shaped alike? The margins? 

 Where is the valve thickest ? Where thinnest? 



2. The epiostracum or "epidermis." — Does it 



cover the entire valve ? What is its color ? 

 Are there any variations in color? Where is 

 this covering thickest? Can the epiostracum 

 be removed from the shell ? What is its use ? 



3. The hinge- ligament. ^ — What is its position? 



What reason can you give for this ? What is 

 its shape ? Of what is it made ? Soak it in 

 water, if the shell be dry, until the ligament 



