The Influence of Environment 313 



action following an alteration in the constitution of the 

 sea water is in some of the cases due to an increase in 

 the permeability of the membranes of the cell, whereby 

 substances can diffuse into the cell which when the 

 proper balance prevails cannot diffuse. For this 

 balance the ratio of the concentration of the salts with 

 univalent cation Na and K over those with bivalent 



, , , salts 



cation Ca and Mg -^ - is of the greatest 

 CCa+Mg salts 



importance. 



6. The importance of this quotient appears in 

 the so-called "behaviour" of marine animals. We 

 have mentioned the newly hatched larvae of the 

 barnacle in connection with heliotropism. These larvae 

 swim in a trough of normal sea water at the surface, 

 being either strongly positively or negatively helio- 

 tropic. They collect as a rule in two dense clusters, 

 one at the window and one at the room side of the 

 dish. If such animals are put into a solution of NaCl+ 

 KC1 (in the proportion in which these salts exist in 

 the sea water), they will fall to the bottom unable to 

 rise to the surface. They will, however, rise to the 

 surface and swim energetically to or from the window 

 if a certain quantity of any of the chlorides of a biva- 

 lent metal, Mg, Ca, or Sr, is added, but these movements 

 will last only a few minutes when only one of these 

 three salts is added; and then the animals will fall to 

 the bottom again. If, however, two salts, e. g., MgCl 2 



