The characteristic forms of the different species of flat fish are gradually assumed by the young fish during the period of their larval development. The appearance of a newly-hatched young plaice exhibits little change during the first week or so, other than that due to the gradual disappearance of the yolk sac. The young fish grows very slowly, and, twenty-one days after hatching, is only 3/8 of an inch in length. For thirty days the development of the young fish is entirely symmetrical. During the succeeding fifteen days, the shape and appearance of the fish become profoundly modified. The left eye gradually moves upwards and forwards, until it attains its final position above and in front of the right eye. At the same time, the fish gradually acquires a new swimming position, finally swimming on what is really its left side. This left side becomes colourless. With these changes in form and habit, there proceeds a transformation in the diet of the fish. At twenty-one days it feeds upon the young stages of various crustacea. Gradually it acquires a taste for copepoda and the larvae of mollusca and crustacea. After its metamorphosis is complete, it feeds upon various worms, small shrimps and small, bottom-living crustacea. The adult plaice feeds upon mollusca of the cockle and mussel families.
=The Migration of Fishes.= Fishes, like birds, migrate over great distances at certain seasons of the year. In most cases, this migration occurs just before spawning, and is evidently connected directly with the spawning instinct. True marine fishes, such as the herring, haddock, plaice, cod, associate in vast numbers at spawning time, choosing a locality in which the temperature and food supply will be favourable to the development of the young larvae. Generally, the spawning ground is in deep water. The eggs are buoyant, and drift up to the warm surface water and hatch out amongst the plankton. The herring differs from most other pelagic fish in laying its eggs in relatively shallow water, over a rocky bottom covered with seaweed. The eggs are denser than sea-water and are covered with an adhesive substance, so that they sink to the bottom and become attached to the stones and seaweed.