ii6 ^hell Life 



The only remaining fresh - water bivalve to be 

 described represents a distinct family — the 

 Driessensiidae. It is the Zebra Mussel (Dinessensia 

 ^^olymorpha), which is very similar in appearance to 

 the Common Mussel (Mytihis) of the sea-coasts, but 

 differs from it in having a partition extending across 

 the tip of each valve and thus dividing this portion 

 of the interior into two. The shell is sharply keeled 

 along each valve, and is dull brown in colour. Tlie 

 hinge is without teeth, and the pearly interior is but 

 faintly marked by the muscular impressions. The 

 animal is dark hued, with a paler, clearer, slender 

 and cylindrical foot which spins a strong byssus. 

 From all this it will be seen that 

 Driessensia approaches much more 

 nearly to the marine species than to 

 other fresh-water bivalves. All the 

 latter retain their e2:<j:s until they 



Zebra Mussel • ri 4.U • 



natch; JJriessens'ia, like the marnie 

 bivalves, discharges her eggs into the waters, and 

 the young larvae that come from them are free 

 swimmers. They appear in the middle of the summer 

 on the surface of the water in great numbers. After 

 apparently feeding on the microscopic plant-life of the 

 surface for about eight days, they sink to the bottom. 

 By this time the foot has formed and become very 

 long and narrow, and the shell round, much like that 

 of Splicmum. The swimming organ — the veluvi — 

 has disappeared, and in future the creature must use 

 its foot and its byssus for locomotive purposes. 

 Another point connecting this species with those of 

 the sea is the fact that the edges of the mantle are 

 united almost throuMiout their lencrth. 



