
26 SALT AND FRESH-WATER CLAMS. 
muscles relax (Pl. 1, fig: 5). While the sea-clam lies buried 
in the mud, head downward, with but little power of loco- 
motion, the fresh-water clam has the faculty of moving 
through the mud or sand in which it lies partially embedded. 
Fig. 6, plate 1, represents the natural attitude of the Unio, 
or fresh-water clam. It will be seen that the tubes are 
above the level of the sand. The foot is very large, and 
with it the Unio is enabled to move along slowly, the shell 
wedging its way through the sand, leaving a groove or fur- 
row along the river bottom, and often the collector takes 
advantage of these tracks in finding them. 
But little is known regarding the development of the sea- 
clam, or Mya, as it is technically termed, but it is similar to ` 
that of the Unio. In these the eggs issue from the ovaries, 
and find their way into the cavities of the outer gills. "There 
they develop until they are furnished with a little triangular 
shell, large enough to be recognized by the unassisted eye. 
At this stage they are discharged by thousands into the 1 
water, and are left to take care of themselves. It has been 
ascertained that they attach themselves by a little thread to 
the river bottom, thus preventing them from being swept 
away, though it is probable that not one in a hundred ever 
reaches maturity, as fishes and other aquatic animals feed 
upon them. Fig. 8, plate 1, represents the shell of the young 
Unio. 
Many of the common fresh-water clams produce pearls, [ 
though the black mussel, with a white pearly interior, often- 
times produces pearls of considerable clearness. These pearls 
getting in between the mantle and shell.. This irritates the 
-animal, and this irritation causes the animal to deposit upon 
the partiele layer after layer of pearl. In China, the natives 
taking advantage of their knowledge of the way in which 
pearls are formed, have shown their ingenuity by making 
flat lead castings of their little idols. These they insert in 
a species of fresh-water clam, by first wedging the shells - 
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