Internal Workings 149 



tinct zones are formed, between which there is no inter- 

 change of water, with the result that the lower levels, cut 

 off from the oxygen supply, become depleted of that ele- 

 ment, and uninhabitable to fish. Such lakes are said to have 

 thermodinesy the name given to the dividing layer between 

 the upper, warm, light, oxygen-rich zone and the lower, cold, 

 heavy, oxygen-poor zone. 



The sharks and rays breathe just as other fish do. They 

 differ, however, in this way: that the clefts between the gills, 

 instead of opening into a common gill chamber with a single 

 external aperture for the escape of the water, open separately 

 through the skin to the exterior (see Figure 21). There 

 may be from five to seven clefts, with the corresponding five 

 to seven slitlike openings in the skin so characteristic of the 

 appearance of sharks. In addition, there is the small round 

 opening in the top of the head, the spiracle, considered to be 

 a degenerate gill slit, which we discussed in Chapter V. 



EXCRETION 



Excretion means the removal from the body of the un- 

 usable or injurious products which have been formed by the 

 operation of the bodily machine. It does not refer to the 

 removal of that fraction of the food which, having no nutri- 

 tive value, has been allowed to pass through the digestive 

 tract without being absorbed. 



In both fish and human a certain amount of excretion takes 

 place through the respiratory organs and through the skin, 

 but the principal method in both is by solution of the mat- 

 ter to be excreted in water and its passage out of the body 

 as urine. In both, it is the kidneys which prepare the urine, 

 but the way in which these organs function is so unlike in 

 the different forms that it brings about a startling dissimilar- 

 ity in the way in which they go about getting their supply of 

 water. The human does it by drinking fresh water. The 



