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LIFE UNDER DIFFICULTIES. capability to resist drought, as well as heat and cold. They are essentially aquatic animals, and can only remain active so long as they are surrounded by water. Yet many of them live in situations which are liable to become dry; streams and ponds go dry in summer, and moss, among which most of the kinds live, only receives occasional moisture from rain and dew and snow. If the rotifers could not cope with this difficulty they would perish in great numbers in dry weather, as rotifers of other orders do. If dried too quickly they are actually destroyed.

If dried more slowly, as when mixed up with grains of mud or sand, or when sheltered in the axils of moss leaves, they appear to have warning of the approaching crisis. They contract into little balls and the skin exudes a kind of varnish which dries and seems then to be quite impervious to air. In this condition they may remain for an indefinite time, and may be blown about as dust by the wind, and thus distributed to all regions of the earth.

Thus the sand of the desert, and the polar snows may receive these living dust particles, which may last have pursued an active existence in the woods or moors of temperate regions; and in either case, if they happen on moist places they may in a few hours