CHAPTER III. 



THE ANATOMY AND PHYSIOLOGY OF A FRESH-WATER MUSSEL. 



TAKE a shell of the Swan Mussel (Anodonta cygnea). 



(A) Examine its outer surface. Note : 



1. Its oval form, blunt at its anterior, but produced at its 

 posterior extremity. 



2. The itmbo, a small blunt prominence near the dorsal border. 



3. The line's of growth, running concentric to the umbo. 



4. The ligament, uniting the two valves together along their 

 dorsal margins. u The relations of the ligament to the shell 

 valves show that strictly speaking the valves ought to be regarded 

 as parts of a continuous structure. The dorsal region of this 

 structure does not undergo calcification, or only to a very slight 

 extent, inasmuch as the economy of the animal requires that it 

 should remain flexible. It is an adaptation of an originally 

 univalve shell." 



(B) Examine its inner surface. Note : 



1. The anterior adductor impression, near the anterior end of 

 the shell. 



2. The anterior retractor impression, a small area continuous 

 with the anterior adductor impression. 



3. The protractor impression, about gth of an inch posterior to 

 the inferior portion of the anterior adductor impression. 



4. The posterior adductor impression, an oval depressed area 

 near the posterior end of the shell. 



5. The posterior retractor impression, a small depression con- 

 tinuous with the posterior adductor impression. 



6. The pallial line, an even curved line running parallel to the 

 inferior margin of the shell. In Anodonta this line is even 

 throughout its whole extent, and the shell is said to be integri- 



