The work described in this paper is a contribution to the general problems of rearing the larvae, and growing the spat of the European flat oyster, Ostrea edulis L.
The specific aspect of the problem dealt with in detail is the place taken by minute motile organisms (naked dinoflagellates and other unicellular algae) in the food requirements of the developing veliger larvae.
Six such flagellate organisms, ranging in size from 1·5 to 7 μ in diameter, have been isolated from sea water, and are maintained in pure culture, under conditions described. Pending specific identification, these are denoted by letter-labels only. These organisms have been used, over a period of years, in feeding experiments with oyster larvae.