CHAPTER III. 



THE ANATOMY AND PHYSIOLOGY OF A FRESH-WATER MUSSEL. 



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



(A) Examine its outer surface. Note : 



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

 posterior extremity. 



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



3. The lines of growth, running concentric to the umbo. 



4. The ligament, consisting of a band of uncalcified elastic fibres, 

 and uniting the two valves together along their dorsal margins. 

 "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." In 

 the closed shell the ligament is on the stretch. 



(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 ^th 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 behind 

 and continuous with the posterior adductor impression. 



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

 inferior margin of the shell and joining the two adductor impres- 

 sions. In Anodonta this line is even throughout its whole extent, 



