FOECE or WATER. 203 



exposed to them ; some they will annihilate, others they will 

 transport against their will to other sjDots, 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 

 thoy 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 cun-ent 

 exerts its force pei'pendicuLirly to the body resisting it. Such 



h 



Fig. 58.— Mollusca that cling tightly to rocks by tie foot, a, Patella, the shell of which 

 entirely covei-s the soft parts, which are pressed down on the rock ; b, Naxictlla, 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 form of their shells 

 against destruction, or by the sucking power of their foot 

 against being torn or washed away, like Palella in the sea 

 (fig. 58) or Navicella'^^ 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- 

 (Balanidce). The pressure to which these animals are subjected, 

 either perpendicular or lateral, must sometimes be enormous ; 

 it would be interesting to ascertain by experiment how 

 great it actually is in individual cases. A knowledge, how- 

 ever, of the maximum of pressure which can be genei-ally borne 

 by the animals above mentioned and others of analogous 



