150 The Life Story of the Fish 



fresh-water fish does not drink at all, but absorbs water. 

 And the salt-water fish drinks salt water. 



An explanation of all this in terms of osmotic pressure, 

 hypotonicity, and the like, would turn our hair gray, but we 

 can set up a few conceptions which, while they will probably 

 turn the chemists' and physicists' hair gray, will give us an 

 easy understanding of how the system works. 



The first point is simple. A solution is a combination of 

 water and some other substance. If the proportion of the 

 other substance to the water is high, the solution is strong, 

 or concentrated 5 if it is low, the solution is weak, or dilute. 

 Both fresh water and sea water have salt in them, but the 

 sea water is a much stronger solution. Now, if two solutions 

 of different strengths are separated by what is called a semi- 

 permeable membrane, they will tend to equalize. Water will 

 pass through the membrane from the weak solution into the 

 strong solution until they are of equal strength. 



The second point is simple. Blood, both fish and human, 

 is a stronger solution than fresh water. Therefore, fresh 

 water will pass through semi-permeable membranes into 

 blood. In man, such membranes are found only inside the 

 body, and he must therefore drink In order to get water into 

 the stomach where it can be absorbed. In the fish the semi- 

 permeable skin is, as we have seen, normally rendered 

 inactive by the slime, but the functioning semi-permeable 

 gills are in constant contact with water. Through these gills 

 fresh water constantly goes into the more concentrated blood 

 and must be taken out of the blood by the kidneys and 

 turned into dilute urine for excretion. The fresh-water fish 

 does not have to drink. 



The third point is also simple. The fish's blood is a weaker 

 solution than sea water j therefore, sea water does not pass 

 into it through the gill membranes. The tendency is for the 

 reverse to happen — for water to pass out of the fish's bJood 



