CH. XIl] THE CIRCULATION OF NITROGEN 285 



with the result that precisely the same nitrogenous waste substances 

 are again formed. 



Another fraction of the nitrogen compounds entering the sea 

 begins an upward series of transformations. A small proportion of 

 the larger sea-weeds which utilise these substances is, as we have 

 seen, eaten by marine animals ; and another, and much larger, 

 proportion of the protophyta consuming these same things becomes 

 the food of innumerable creatures. I have already stated that 

 there is evidence that many marine invertebrates feed upon the 

 dissolved carbon compounds present in sea water, and it is 

 probably the case that these animals also utilise, as sources of food, 

 some of the dissolved nitrogen compounds (though perhaps much 

 less of these than of the carbon food-stuffs). But it is also the case 

 that many molluscs, other invertebrates, and even fishes habitually 

 feed upon diatoms. At once then some of this nitrogen utilised by 

 the protophyta returns to the land, for the fishermen catch the 

 cockles, mussels and oysters, and the mullets and sardines which 

 have eaten the diatoms. Yet again we find that the ubiquitous 

 copepods feed upon the peridinians and other holophytic and myxo- 

 trophic protozoa, and a great host of marine animals feed upon the 

 copepods. Thus the herrings, mackerel, shads, pilchards, sprats, 

 and other fishes habitually eat the copepods and smaller Crustacea, 

 and these fishes are caught for the public markets, the herrings, 

 mackerel, shads and pilchards to be used directly as human food, 

 and the sprats to be made into sardines and anchovy paste. Again 

 many fishes eat the diatom-feeding molluscs, thus the plaice and 

 flounder do so to a very great extent, and other fishes also make 

 use of molluscs as occasional food animals. Plaice and flounders 

 are then caught and taken ashore as human food. Many other 

 fishes in their juvenile phases eat the diatoms and other protophyta, 

 and though the fishermen do not intentionally catch these little 

 fishes, the latter are still necessary if we are to have big ones. 



Further many large fishes like the whiting, turbot, skate, ray 

 and cod eat smaller fishes, or eat invertebrata, and if we examine 

 the food of these latter animals we will find that it is either 

 smaller fishes which eat the protophyta directly, or it consists of 

 molluscs, or other invertebrata, which directly or indirectly also 

 utilise the diatoms, peridinians, and their congeners as sources of 



