I40 MOLLUSKS 



shell, what would the present margin become? What has 

 each line of growth been? What does this suggest con- 

 cerning the past history of the animal ? On the dorsal side 

 of the shell behind the umbone, look for the edge of tha 

 shell. What color is the shell at the union of the two 

 valves? Is it the same material as the other part of the 

 shell? This is the spring of the hinge. Draw outside of 

 the shell, and name all the parts you have found. 



(3) Study the inside of the shell. Note the differences 

 between it and the outside. The inside is called pearl. 

 Why ? Observe the line parallel with the margin, the pallial 

 line. Note the hinge and the modification of the shell for it. 

 Can you find out now how the hinge works ? Fit the parts of 

 the shell together and fi.nd of what use each part is. Draw 

 inside of shell. Break the shell across, or saw it, and note 

 its structure. Can you tell now how each line of growth 

 was added ? When a line of growth is added is it added on 

 the margin only? Can you account for the position and 

 shape of the umbones? Draw a section of shell. What 

 is the composition of the shell? Drop a little weak hydro- 

 chloric acid on a piece of the shell and note what follows. 

 What does this show? 



(4) The Body, (a) Open the shell by thrusting the scaK 

 pel under the right valve at the anterior and posterior ends, 

 and severing the rigid, tense muscles. The shell now opens. 

 Note the mantle, a membraneous covering of the body. Is it 

 attached to the shell? Where? Is it the same thickness 

 throughout? Examine the margin of the mantfe, and com- 

 pare it with the margin of the shell. Do you think there 

 is any connection between them ? How is the shell formed ? 



(b) Follow the mantle carefully around to the posterior 



