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Some fishes dig the sand to make a sleeping space.  Others look for fissures in rocks.  Others even hide away in aquatic algae or under a rock.  But nobody is as creative as the parrot-fish in its search of a safe place to sleep.

Some time before going to sleep, the body of the fish secretes a kind of mucus which forms all around its body a transparent sac having an opening at the mouth.  The latter thus ensures the continuous motion of water though the gills of the fish.  The sac is made in about half-an-hour.  The parrot-fish takes the same amount of time to find its way out of the case.

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