CHAPTER XXII 

 MOLLUSCA: THE MOLLUSCS 



THE FRESH-WATER MUSSEL 



Structure (fig. 103). TECHNICAL NOTE. The fresh-water 01 

 river mussel lives commonly in the streams and lakes or ponds in 

 the United States. It frequents muddy or sandy bottoms. Speci- 

 mens can often be secured with a long-handled rake from the shore 

 or picked up in shallow streams with the hand. If possible to keep 

 the animals alive until ready for use, some of their habits may be 

 observed. Place them in a tub or trough with water and mud ; 

 when they have settled themselves put some powdered carmine, 

 starch, or similar substance in the water near them, and note the 

 water-currents. 



Living mussels which have been placed in a dish with 

 mud several inches deep and covered with water will be 

 seen to travel in a definite direction. The end which is 

 in front is the head end. Note the process of thrusting 

 out and retracting the fleshy/^/ which extends between 

 the two valves of the shell. Note that the two valves are 

 held together along the upper, or dorsal, surface by a 

 horny structure, the hinge -ligament. Note near the hinge- 

 line a prominence (umbo) in each valve from which ex- 

 tends a series of concentric lines of growth. The umbo 

 is the oldest part of the valve. Note at the lower edge 

 of the valves a soft membrane with a fringe along its free 

 border. This is the edge of the mantle-lobes, flaps of the 

 body-wall which cover the body and which aid in the 

 functions of respiration and nutrition. 



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