The genus of the «symbiotic" alga; its mode of reproduction. V. Green and colourless freshwater sponges; under what circumstances they occur; the nature of their «symbiotic" algae VI. The structure of the colourless))symbiotic" algae; how they arise from the green ones VII. The intrinsic amount of the various green and colourless stages of the «symbiotic" algae in sponges. The factors ruling this amount. How green and colourless sponges keep up their «colour", and how they arise from each other. VIII. The nature of the «symbiotic" association of sponge and green alga; its use to the alga and to the sponge. ... IX. Some other algae occurring in tissues of Ephydatia. .. B. The current of water in the canal-system of the freshwater sponges C. The ingestion of food in the freshwater sponges D. The digestion in the freshwater sponges E. The defecation and excretion in the freshwater sponges .