OF THE GULF OF MANAAR. 
105 
to the shell of the upper one, the third one having again 
another adhering to it. 
Migratory habits. 
Thus we trace in the larval form, in the foot, in the 
byssus, and in the shell indications of the pearl oyster being 
migratory, markedly so in its earlier stages, and decreas- 
ingly so with age ; and the further questions are, why it 
should migrate, and whither. The first migration as larv(e, 
as many as twelve millions to a parent, is possibly more or 
less involuntary ; and the countless myriads provide, by the 
law of chances, that some at least shaM fall on favorable 
spots, as well as leave a margin for destruction by the 
enemies of the oyster. The subsequent migrations must be 
voluntary to escape from uncongenial surroundings. What 
these incongenialities are we may in a measure conjecture 
from the similitudes of the edible oyster. 
The Spat. 
Lest we should be again misled, it is necessary to allude 
to a mistake which has obtained with reference to the spat of 
our pearl oyster. The spat of Avicula fucata is, when small, 
almost flat, the valves being exceedingly thin, vsdth scarcely 
a perceptible concavity. They are, also, the exact color of 
their siuroundings. Consequently, they have always escaped 
observation, and the native divers frequently finding the 
oyster beds covered with innumerable minute shells that 
bore an approximate resemblance to the pearl oyster, con- 
fidently concluded them to be the spat of the pearl oyster, 
and when these minute shells were followed in the course 
of time by growing oysters it was not unnaturally concluded 
that they had grown from spat to oyster, the more so as the 
true spat, from being thin and the color of its surroundings 
was not seen. And this mistake of the divers was, 
unfortunately, accepted and stereotyped by a plate in 
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