86 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



Stagnation is prevented even when no new water is admitted from 

 the outside, for the water in the car is constantly being turned over 

 and the lower strata brought to the top and aerated. When, there- 

 fore, the water of a car of considerable size is kept cool by being sunk 

 into the ocean and shaded from the sun and is continuously forced to 

 the surface so as to be relieved of all waste gases as well as recuperated 

 with oxygen, there is comjDaratively little need of continuous or fre- 

 quent renewal. It is at least reasonable to suppose that, in what we 

 may call (after Birge) the "respiration" of a small inclosed body of 

 water containing a considerable quantity of animal life, the elimination 

 of the waste or toxic gases is necessary, and that aeration which is 

 accomplished by forcing more air into the water only partially fulfills 

 the requirements of respiration. The analogy with the physiological 

 process of respiration would seem to be real. In case of small, very 

 thin, flat animals, where the ratio of surface to the bulk is large, 

 respiration may be continuous and direct without special internal 

 apparatus, and, likewise, shallow water with a large expanse of surface 

 has been found by experiment to need no aeration in order to main- 

 tain animals alive for a long time. On the other hand, in bulky 

 animals, the respiratory apparatus provides always for the elimina- 

 tion of gaseous products of metabolism as inevitably as it provides 

 for the acquisition of oxygen. Likewise the bringing of the lower 

 strata of water continuously to the surface fulfills these two necessary 

 requirements. 



For keeping larval forms which are not exceedingly minute, win- 

 dows covered with screens about 16 meshes to the inch in the bottom 

 of the cars allowing for intake, and similar ones in the sides for the 

 exit of water, are satisfactory. A much finer mesh can be used in 

 this case than would ordinarily be practicable, because the water is 

 drawn in through the bottom screens with considerable force by the 

 upward tendency of the current. It is possible by means of a filter 

 device, which will be described hereafter, to hold fry which would 

 pass through even very fine screens. 



The rotary upward current keeps the particles of food suspended 



