222 THE CONDITIONS OF LIFE IN THE SEA [PART III 



includes those which feed upon the plankton, and which possess 

 special organs of alimentation for the capture of these microscopic 

 organisms. The w^halebone whales feed upon the pelagic ptero- 

 pods, and the baleen strainers in the mouth form a filtering 

 arrangement wherewith the animal separates the food organisms 

 from the water taken into the mouth. Fishes, such as the herring, 

 sprat, mackerel, shads, and many others, feed upon the pelagic 

 copepoda and schizopods ; and these animals possess gill-rakers, 

 which are fine comb-like structures placed on the internal margins 

 of the gill-bars, and which form a straining apparatus which 

 separates the micro-crustacea from the water taken into the 

 mouth. Many molluscs eat the micro-plankton, w^hich they obtain 

 from the sea water by causing the latter to pass through the fine 

 interstices of the gills, thereby filtering out the particles of food 

 wdiich are then taken into the mouth by means of ciliary action. 

 It is probable that very many other animals are plankton eaters, 

 thus the micro-crustacea doubtless feed in this manner. The 

 molluscs, like the mussels and cockles, which feed on the plankton, 

 are herbivores, since they appear to subsist chiefly on the diatoms. 

 Some fishes, of which the grey mullet is the best known example, 

 eat the finer algae, and the diatoms with which the latter are 

 usually associated. These fishes " browse " upon the sea-weeds 

 grooving on stones, &c., in much the same way as a cow eats 

 herbage. Other fishes, like the sardine, also feed upon diatoms, 

 but they obtain these in the same way as the herring obtains the 

 copepods and schizopods on which it subsists. 



Thus there are marine animals which eat the organisms of the 

 plankton. But if we push our enquiry into the food organisms of 

 sea animals far enough we find that the plankton creatures are 

 the last links of chains of food organisms. Thus the cod feeds on 

 plaice, which feed on molluscs, which feed on diatoms, and other 

 protophyta. The plaice is a food organism of the first degree, the 

 mollusc one of the second degree, and the diatom one of the third 

 degree. Generally speaking we find that the food organisms of 

 the nth. degree of a carnivorous animal are such as belong to the 

 protophyta of the plankton. And this is why we speak of the 

 plankton as constituting the ultimate organised food-stuff of 

 the sea. 



In all these cases the metabolic processes concerned in nutri- 



