THE CLAM. 137 



9. Observe two chambers inside the mantle cavity. 

 The one containing the gills, body, foot, etc. , is the 

 branchial chamber. The other one, which is above 

 the body, is the cloacal chamber. 



10. Before taking the clam out of the shell, look 

 for the nervous system. On the lower surface of the 

 posterior adductor muscle, find a double yellowish 

 body, the visceral ganglia. Find nerves leading from 

 the pair. Draw. 



11. Just behind the upper portion of the anterior 

 adductor muscle is a small ganglion, the left cerebral 

 ganglion. Find the short nerve connecting it with 

 the right cerebral ganglion. Draw. 



12. Just below the body in front of the foot, find 

 two pedal ganglia. Draw. 



13. In the cloacal chamber, just above the 

 body, find the heart. The intestine passes 

 through the heart, but has no communication with 

 it. In the live clam, the beat of the heart is 

 easily seen. Can you find one artery passing 

 forward and two passing backward from the 

 heart? 



14. Trace the intestine until it opens into the 

 cloacal siphon. Does it go above or below the adduc- 

 tor muscle? 



15. Beginning at the mouth, trace the intestine as 

 far as you can. Some will trace its entire course. 



Suggestive Questions. 



1. How does the clam get food? 



2. How does the clam move? 



3. How does the water get into the cloacal 

 chamber? 



4. Of what use are the siphons? 



5. Does a fresh-water clam or a salt-water clam 

 close its shell the more closely? Why? 



