Freshwater mussels are sometimes called "two shelled " or bivalves (bi-valve). Bi - meaning two and valve - meaning shell. The freshwater mussel lives inside of the shells. Together the shells act like a house and protect the freshwater mussel. As the freshwater mussel grows it makes the shell bigger, so it never has to look for a new home. Unlike the hermit crab that has to find a new shell when it out grows the one it has.
Make a bivalve with your hands. The first thing you need to do is make cups with both of your hands. Now bring your cupped hands together so that both your thumbs are touching. Keep your hands cupped. Now keep your thumbs together, and open your pinky finger side of the cup. This is what a bivalve looks like.
Scientists use the different parts of the shell to help identify what kind of freshwater mussel it was. Here is a picture of the inside of a freshwater mussel shell.
The adductor muscle scar is where the adductor muscle attaches to the live mussel shell. The adductor muscle is used to hold the two shells together allowing the mussel to close its shell.
The lateral teeth and pseudocardinal (pseudo = false) teeth are not used for chewing up food, but instead they are used to keep the two shells or bivalve together. A freshwater mussel has a tooth on the inside of each shell or valve. When the valves are together the teeth lock together like a puzzle piece. This keeps the two shells from slipping around and hurting the freshwater mussel inside.
The umbo is where the two valves (or shells) are held together.
The retractor muscle scar is where the retractor muscle attaches to the live mussel shell. The retractor muscle is used to open the mussel shell.