CIVIC PROBLEMS RELATING TO MOLLUSKS 281 



It is quite possible that these humble creatures are doing a work of 

 value many times greater than that represented by their shells and 

 pearls, in the constant purification of our lakes and streams. Experi- 

 ment has shown that a good-sized mussel filters about four gallons 

 of water through its gills per hour, and since it breathes and feeds 

 continuously, this means nearly 100 gallons daily year in and year out. 



FIG. 128. Fresh-water mussels, female, male, and side view, showing 

 growth lines 



Set up two perfectly clean glass aquaria exactly alike, put a mus- 

 sel in one of them, and note the difference in clearness of the water. 

 What may this mean in keeping reservoirs and park waters clear and 

 wholesome ? 



Glochidia (glochis, "arrow point"). Life littory. The eggs develop in 

 the gills of the parent mussel into minute bivalves so unlike the adult 

 that they were long considered parasites. These are the glochidia and 

 when ripe they are extruded into the water. Here further development 

 depends upon their attachment to gills, fins, or other parts of fishes. 

 A long byssus thread is present and probably helps in catching the fish, 

 and some species have the valves tipped with sharp-barbed hooks, which 

 catch into the skin of the fish when they are snapped together. The 



