Fish and Marine Life Illustrations

About mussel anatomy:

• beak - the oldest part of the shell, this is the hump-like structure near where the two valves (shells) are hinged together
  
• anterior and posterior adductors - the adductors draw the two valves (shells) together
  
• mantle - the following description is from Zoo Lab (http://bioweb.uwlax.edu/zoolab/Table_of_Contents/Lab-5b/Mussel_Dissection_1/mussel_dissection_1.htm):

"...the mantle is a sheath of skin that hangs down in two folds around the soft body and encloses a mantle cavity... The outer side of the mantle secretes the shell (if present), while the inner side is ciliated,  and along with gills or lungs that develop from it,  participates in gas exchange."

• gills - used for both feeding and breathing. "As the mussel sucks water into its shell to breathe, it also draws in the microscopic plankton which it filters and sends to its mouth." (Source: The Open Door Web Site: Breathing Underwater)
  
• foot - used for digging and anchoring
  
• teeth - help the valves (shells) to interlock
  
• ligament - causes the valves (shells) to open and close
  
• mouth
  
• digestive tract
  
• incurrent and excurrent siphons - circulates water into and out of the mussel

More about freshwater mussels:
University of Massachusetts: Freshwater Mussel
Animal Diversity Web: Family Unionidae

Also see:

Mussel anatomy (Unionidae) is a 2006 illustration by Gina for Minnesota Department of Natural Resources. An opened shell exposes the animal's internal structures in pale colors and careful outlines, and labels tie every named part to its place. The beak, the oldest part of the shell, is the hump-like structure near the hinge.