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In Clavelina, Ciona, Phallusia and Styela, the branchial sac, the stomach and the intestine are involved in the absorption of dissolved amino aids and glucose. Autoradiography and double countings on 14C and 3H labelled samples show that, in the first hour, the branchial sac can concentrate several hundred times the dissolved nutritive molecules from dilute solutions, whereas the digestive tract is not involved significantly. In Phallusia the hepato-pancreas has a specific activity higher than in the intestine and may be an organ of storage. In all species, the branchial sac absorbs the largest proportion of labelled material.