Internal Workings 151 



through the gills into the sea. The only way, then, for the 

 marine fish to obtain water is to drink. It actually takes large 

 quantities of salt water into its stomach. This is absorbed into 

 the blood, but since the kidneys of the fish, unlike those of 

 the human, are capable of excreting only urine more dilute 

 than its blood, it must separate the water from the salt in 

 order to have dilute water to form its urine. This task is 

 assigned to its gill membranes, which do what is called os- 

 motic work by excreting the salt back into the sea. In other 

 words, they force the salt to go in the direction of the 

 stronger solution, in spite of its natural tendency to go the 

 other way. This leaves the water free to be formed into 

 dilute urine by the kidneys, and at the same time prevents 

 the accumulation of too much salt inside the fish's body. 



And so the fresh- and salt-water fishes have to have quite 

 different excretory mechanisms. In the former, water con- 

 stantly seeps in through the gills, and the kidneys must be 

 able to remove it in large quantities from the blood stream. 

 In the latter, water is taken copiously into the stomach to 

 make up for what escapes through the gills j the kidneys 

 must use as little of it as possible for the excretory solutions j 

 and the gills must be able to get rid of the excess salts which 

 the water brings in. Neither set of equipment is suitable for 

 use in the other habitat, and so the salmon and the eel, which 

 spend parts of their life in each, have to have both sets. How- 

 ever, switching from one to the other is not always a simple 

 matter. Some species of fish can support a swift change, but 

 a young salmon cannot be taken from a stream and dumped 

 into salt water. It is only when the proper physiological 

 moment arrives, signalized by the appearance of the silvery 

 coating of guanin in the skin, that he can make the change 

 safely. 



The sharks and rays as usual do things differently. In 

 them, the concentration of the blood is higher than sea water. 



