THE AGE OF FISHES 181 



economy of the creatures within it. Between the consumer and 

 the producer is an elaborate chain of distributors, wholesalers, 

 and retailers; and this has its undersea counterpart, which, as 

 we shall see later, is greatly influenced by the circulating 

 waters of the River itself. Besides the whales there are such 

 fishes as herring and menhaden which also feed directly on 

 the plankton. In turn they become the food of cod and other 

 carnivores. They strain their food from the water through a 

 sieve made of slender filaments, the gill rakers, which project 

 from the front side of each gill arch. Some bottom-feeding 

 fish, like haddock, soles, and plaice, feed on small shellfish, 

 starfish, and worms, but these in turn trace their food back to 

 the drifting life, and their own young are part of the plankton. 

 Even here, however, we have not reached the end of the food 

 chain; for we still have the further complexities of plankton 

 feeding on plankton. At the end of this long line of animals 

 eating still smaller animals we come to the microscopic plank- 

 ton plants, the pasturage of the sea, the real producers. 



We have already mentioned diatoms, which Sir Joseph 

 Hooker rightly took to be important among the food plants 

 in the plankton, but these are not alone, though in the cooler 

 waters they bulk large in the drifting plant life. In warmer 

 waters, as the diatoms become less plentiful their place is 

 taken by dinoflagellates, which have the same plantlike way of 

 feeding, but which are in other respects very different; for 

 whereas the diatoms are encased in an armor of silica the dino- 

 flagellates either have a thin cellulose shell or are completely 

 naked. Though so small that their size is reckoned in thou- 

 sandths of an inch the naked dinoflagellates may sometimes 

 grow in such vast numbers that the sea is discolored for miles. 

 This was the cause of a lethal ''Red Tide" that spread inter- 

 mittently along the Gulf of Mexico shores of western Florida 

 during 1947. Though harmless in ordinary concentrations, the 

 myriads in the ''Red Tide," as many as 60 million to a pint of 



