136 INTRODUCTION TO ZOOLOGY. 



2. Notice one line near the edge, much deeper 

 than the others. Tliis is the pallial line. 



3. Examine the hinge teeth, near the umbo on the 

 right. Are there different kinds of teeth? Do you 

 find corresponding teeth on the left valve ? What is 

 the use of these teeth? Draw. 



4. Find a scar near the anterior end of the shell. 

 This is the place where the anterior adductor muscle 

 was attached. You may call it the anterior adductor 

 impression. Find a similar scar, the posterior adduc- 

 tor impression, near the posterior end of the shell. 

 Can you find another muscle scar on the shell? 



5. Draw the inside of the right valve. 



Internal Structure. 



1. Remove the left valve carefully. Find the 

 mantle, or pallium, which covers the entire body. 

 Where is it thickest? 



2. Find the anterior and the posterior muscles. 

 What do these muscles do? 



3. Find the siphons at the posterior end. In the 

 salt-water clam, the siphons are permanent and are 

 an inch or more long. In the fresh-water clam, these 

 siphons are formed by the folding together of the 

 posterior edge of the mantle. They are not perma- 

 nent, and are manifested clearly only in the living 

 clam. Examine tlie edges. 



4. Carefully remove the mantle. Find the gills. 

 How many on the left side? Study their structure. 

 Draw. 



5. In front of the gills, find the labial palpi. How 

 many? What shape? Draw. 



6. Between the palpi, find the mouth. 



7. Notice the foot. Which way does it point? 



8. How are the gills attached to the body? To 

 each other? 



