110 LIFE IN THE SEA [CH. 



we try to show that such an animal as we have been 

 discussing nourishes itself in the same way as a 

 mammal does. Again a sponge causes a current of 

 water to flow through the system of cavities per- 

 meating its body, and it nourishes itself by removing 

 the food matter contained in this water of circulation. 

 If we estimate the quantity of carbonic acid excreted 

 by the sponge in the course of an hour, say, we can 

 find what was the quantity of carbon contained 

 in the parts of its body which were oxidised during 

 that time that is to say we find what was the 

 quantity of carbon contained in the food which it 

 must have taken in order to keep itself from losing 

 weight. Now we know what quantity of carbon is 

 contained in the water in the form of plankton and 

 it is therefore easy to calculate what quantity of sea- 

 water must be filtered by the sponge in order to get 

 enough food, on the assumption that all its food is 

 contained in the plankton. The volume of water so 

 found is ridiculously large far too large to make it 

 possible for the sponge to have filtered it in the time 

 required. 



We can show also, by actual experiment, that 

 many marine animals, even fishes, can be kept for 

 many months in water which is filtered so that it 

 does not contain any plankton. The fish or other 

 animal may live in good health for months in such 

 circumstances provided the water is kept cool, and is 



