iv] MODES OF NUTRITION 109 



gain in dry organic substance of 713 milligrams, say 

 356 milligrams of proteid, since analyses of mussel 

 flesh give about half the dry weight as consisting 

 of this constituent. Now let us suppose that all 

 this proteid came from the plankton captured by 

 the shell-fish. A good estimate of the amount of 

 plankton contained in northern waters is that 1000 

 litres of water contain, on the average, 168 milligrams 

 of dry organic substance, and since we know the 

 composition of various forms of plankton we can 

 easily calculate the amount of proteid contained in 

 the water : it is 25*2 milligrams. If the solid matter 

 filtered from the water by the mussel were as rich 

 as this and it certainly is not, for the greater part 

 of it consists of fine mud, and if the animal captured 

 all the solid matter contained in the water sucked 

 into its shell and again it certainly does not then 

 it would have to filter 14,200 litres of sea- water in 

 order to obtain this amount of proteid. But even 

 then it must have digested and assimilated all the 

 food matter taken into its intestine and the digestive 

 machinery of no animal is as efficient as this. Above 

 all it must have built up all the assimilated proteid 

 into the form of tissue, and it certainly could only 

 have so disposed of a small fraction of it for the 

 largest part by far must have been oxidised in the 

 production of energy. This example gives us an 

 idea of the difficulties we saddle ourselves with when 



