All fish, during the larval stage of their development, feed first upon the contents of the yolk sac which, when they are hatched, is attached to their ventral surface. When the yolk is absorbed, the larvae feed upon the microscopic plankton that abound in the water on every side. The surface water, with its warm temperature, high plankton content and sunlight, forms an ideal nursery for the very young fish of all species. Demersal fish, as they complete the larval stage of their development and descend into deeper water, have to rely for their food either upon the various species of young shellfish and crustacea that drop from the surface water as they develop, or hunt for their food amongst the small fish, mollusca, crustacea, worms and seaweeds of the sea-bottom. Plaice feed chiefly upon cockles and other mollusca, which in their turn feed upon diatoms. The cod is almost omnivorous, greedily devouring small fish, crustacea, worms or mollusca; its favourite food, however, is shrimps and prawns. These, in their turn, feed upon smaller invertebrates, for example small jellyfish and larval molluscs, and these upon microscopic plankton. Pelagic fish, herrings and mackerel, feed almost entirely upon the larger plankton, mainly copepoda (small, shrimp-like crustacea). These may be present in the surface water in enormous quantities at certain times. In many cases, shoals of herring or mackerel probably follow special swarms of copepoda. Mackerel also feed upon young fish, hermit crabs, and prawns. With a few notable exceptions, the various species of demersal fish feed upon smaller fish. Thus— The hake, normally a deep-water fish, ventures inshore in pursuit of herrings, pilchards, mackerel. The ling, turbot, brill, dog fish live entirely upon small fish. The dog fish swarms on certain fishing grounds and is often a serious pest to the drift-net fishermen, destroying their nets as well as the fish that are attached to them. The whiting, like the cod, feeds upon small fish, and upon crustacea and mollusca. The food of the haddock consists of mollusca, crustacea and marine worms, etc. The sole lives on small crustacea, for example shrimps, and marine worms. Skates and rays feed upon mollusca and crustacea. Most shellfish live in shallow water and feed upon the plankton. The methods by which fish obtain their food differ greatly according to the species of the fish. Pelagic fish, e.g. herring and mackerel, sprat and pilchard, obtain their food almost automatically as they swim open-mouthed through the water in which it abounds. These direct plankton-feeders possess comb-like structures—the gill-rakers—attached behind the gill openings, and as the food-bearing water streams through the mouth and gill openings of the fish, these structures strain the food from it. The fish licks the plankton from its gill-rakers with its tongue and swallows it.