152 The Life Story of the Fish 



so they are in the same position as a bony fish in fresh water. 

 They absorb it directly through their membranes, and since 

 it is more dilute than their blood, they can use it without 

 alteration to form their urine. Sharks do not drink. 



It is only recently that all of these facts were learned. 

 They tell us at last what the difference between a salt-water 

 fish and a fresh-water fish really is. They bring us a new 

 understanding of the obstacles which have to be overcome by 

 such fish as the salmon and the eel when they pass from river 

 to marine life or the reverse. And they force us to modify 

 our old expression about drinking like a fish. For, since 

 sharks and fresh-water fish do not drink at all, if we want to 

 have the phrase mean anything, we must qualify it, and say 

 that a man drinks "like a marine bony fish." 



Like the human being, the fish has a urinary bladder. This 

 is an example of the incorrigible neatness of nature, for there 

 seems to be no real reason for such an organ in an animal 

 which is wet all the time anyway. Its existence, however, 

 offers no material for the teleologist. It was not given the 

 fish in preparation for evolution into a mammal, for the 

 mammal's urinary bladder is formed from an organ which 

 does not exist at all in the fish. In the marine fishes it seems 

 to be particularly useless, for the amount of urine passed is 

 very small. The fresh-water fishes, on the other hand, have a 

 very large discharge — as high as 100 to 150 cubic centimeters 

 per day per kilogram of fish. An idea of what this means can 

 be had by realizing that if human beings operated at the same 

 rate, a 150-pound man would have to excrete from 10 to 15 

 pints of urine a day. As man's bladder has a capacity of only 

 about three-quarters of a pint, this would be highly in- 

 convenient. 



And one further comparison is of interest. In the fish, the 

 anal opening is in front of the uro-genital, which is some- 

 times divided into two, whereas in humans the position of 



