The mollusca, e.g. mussels, periwinkles, oysters, deposit their eggs in the sea-water. The eggs float to the surface, hatch out, and drift about with the other constituents of the plankton. The fully developed larvae fall to the sea bottom and become attached to seaweed and stones. The period of incubation of fish eggs varies according to the species of fish, and for the same species is prolonged by a low temperature. Plaice eggs, fertilized in January, hatched in eighteen days; others, fertilized in April, were hatched in nine days. All fish, on emerging from the egg, enter upon a larval stage in which they resemble each other very closely (_see_ Fig. 1). (Thus, the larvae of plaice are quite symmetrical, like those of the cod or other round fish.) The newly hatched larvae drift helpless in the water for two or three weeks, during which time they subsist upon the contents of the yolk sac, which they carry attached to their ventral surface. When this is exhausted, they feed upon the microscopic plankton which abound in the surrounding water. 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.