are white." Here the coating is of a rich brown instead of a black
colour, and seems to be composed of ferruginous matter alone. Hand
specimens fail to give a just idea of these brown burnished stones
which glitter in the sun's rays. They occur only within the limits of
the tidal waves; and as the rivulet slowly trickles down, the surf must
supply the polishing power of the cataracts in the great rivers. In
like manner, the rise and fall of the tide probably answer to the
periodical inundations; and thus the same effects are produced under
apparently different but really similar circumstances. The origin,
however, of these coatings of metallic oxides, which seem as if
cemented to the rocks, is not understood; and no reason, I believe, can
be assigned for their thickness remaining the same.
One day I was amused by watching the habits of the Diodon antennatus,
which was caught swimming near the shore. This fish, with its flabby
skin, is well known to possess the singular power of distending itself
into a nearly spherical form. After having been taken out of water for
a short time, and then again immersed in it, a considerable quantity
both of water and air is absorbed by the mouth, and perhaps likewise by
the branchial orifices. This process is effected by two methods: the
air is swallowed, and is then forced into the cavity of the body, its
return being prevented by a muscular contraction which is externally
visible: but the water enters in a gentle stream through the mouth,
which is kept wide open and motionless; this latter action must,
therefore, depend on suction. The skin about the abdomen is much
looser than that on the back; hence, during the inflation, the lower
surface becomes far more distended than the upper; and the fish, in
consequence, floats with its back downwards. Cuvier doubts whether the
Diodon in this position is able to swim; but not only can it thus move
forward in a straight line, but it can turn round to either side. This
latter movement is effected solely by the aid of the pectoral fins; the
tail being collapsed, and not used. From the body being buoyed up with
so much air, the branchial openings are out of water, but a stream
drawn in by the mouth constantly flows through them.
The fish, having remained in this distended state for a short time,
generally expelled the air and water with considerable force from the
branchial apertures and mouth. It could emit, at will, a certain
portion of the water, and it appears, therefore, probable that this
fluid is taken in partly for the sake of regulating its specific
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