THE LIFE OF THE UNIO 



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Fig. 5. Quadrula heros, or "Washboard" mussel exposed. 



MA., mantle folded back to show the other proportions; M., 



mouth; P., palpi; R, foot; I. G., inner gill; O. G., 



outergill ; B., branchial opening. 



tance so that the teeth always lock, and the mantle 

 cushion swells when they are open and keeps them from 

 slipping. Inside of the mantle and hung along the hinge 

 line is a sort of bag which contains the vital organs. It 

 takes up a large space in the shell and is called the ab- 

 dominal sac. This extends below and in front into a 

 tough muscular organ called the foot. The foot is 

 shaped something like a tongue and can be projected 

 from the shell and worked into the ground. A good idea 

 of its use can be obtained by referring to Fig. 6, which 

 shows a Lampsilis rectus, or black sand-shell mussel plow- 



