104 LABORATORY EXERCISES 



a heavy rain? In the morning look for the tracks of earth- 

 worms; describe them. Find also the holes leading to the 

 earthworm burrows; account for the heaps of earth about them. 

 How does the earthworm go through the ground? What 

 effect does it have upon the soil? 



d. If the laboratory has a leech, examine its movements. 

 Compare the leech with the earthworm. 



EXERCISE 96 

 MOLLUSKS 



Materials. Shell (both halves) of the freshwater clam (mussel), of 

 the saltwater clam, of the oyster; a snail shell; if possible, a living clam 

 and a living snail. An oyster "on the half shell" will also be useful. 



a. Study 337 of the text. Examine the clam shell and the 

 oyster shell; compare the two. 



Describe the appearance of the outside of the clam shell. 

 Which part of the shell was the top (dorsal part) and which the 

 bottom (ventral part) in the living animal? Are the two 

 halves of the clam shell exactly alike? 



If you have a living clam, find which is the anterior (head) 

 end; then find which is the right valve and which the left valve 

 of the shell. Draw an outside view of the left valve, showing 

 the markings and the hinge. Trace the outside markings all 

 the way around the shell. Label the parts of the drawing. 



6. Describe the inside of the clam shell; then draw the inside 

 view of the left half, showing the markings as accurately as pos- 

 sible. Why does the presence of these marks indicate that the 

 shell increases in size? Which part of the shell is tfye oldest? 



c. Examine the insides of both halves of the shell for the spots 

 at which the two muscles (anterior adductor and posterior ad- 

 ductor) were attached (see Fig. 261, text). Can you see any 

 evidence that a spot may not always have been at the same place? 



