. Animal life as affected by the natural conditions of existence. Animal ecology. FORCE OF WATER. 203 exposed to them; some they will annihilate, others they will transport against their will to other spots, and others again they will affect by hindering or promoting their growth. The resistance of animals to currents.—The effects of water in motion may be displayed in two different ways : in the first place, as sudden and irregular blows, as in the beating of waves or surf; secondly, equally and uninterruptedly, as in currents. Excepting in some few instances, we need not en- quire more close
. Animal life as affected by the natural conditions of existence. Animal ecology. FORCE OF WATER. 203 exposed to them; some they will annihilate, others they will transport against their will to other spots, and others again they will affect by hindering or promoting their growth. The resistance of animals to currents.—The effects of water in motion may be displayed in two different ways : in the first place, as sudden and irregular blows, as in the beating of waves or surf; secondly, equally and uninterruptedly, as in currents. Excepting in some few instances, we need not en- quire more closely into the effects of sudden shocks, for either they at once destroy the creatures exposed to them, or these are able to withstand them ; this, however, can but rarely be the case under heavy blows. Currents are far more important. In general we estimate the pressure of a current, in seas, rivers, or torrents, by its velocity, assuming that the current exerts its force perpendicularly to the body resisting it. Such. Fig. 58.—Mollusca that cling tightly to rocks by the foot, a, Patella, the shell of which entirely covers the soft parts, which are pressed down on the rock ; 6, Navicella, fully extended, only the tentacles projecting beyond the front of the shell. cases, however, but rarely occur in nature, and in places where a current or high waves break perpendicularly to the cliff a very small number of animals can live—such, for instance, as are sufficiently protected by the strength and foim of their shells against destruction, or by the sucking power of their foot against being torn or washed away, like Patella in the sea (fig. 58) or Navicella95 in strong mountain torrents; or forms which not only are covered by a hard external shell, but whose shell is grown to the rock, such as the Sea-acorns (Balanidee). The pressure to which these animals are subjected, either perpendicular or lateral, must sometimes be enormous; it would be interesting to ascertain by experimen
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