It is now known that organic binding of iodine takes place in the endostyle of Ciona (Barrington, 1957) and Amphioxus (Thomas, 1956; Barrington, 1958), and there isevidence that at least di-iodotyrosine and thyroxine are formed as a result of this processin Ciona (Barrington, 1959; Robertson, personal communication. Since this biosynthesis appears to be a specialized property of a restricted group of cells, it has been suggested (Barrington, 1959) that it is not a chance by-product of endostylar secretion but that the thyroidal products are a biochemical necessity for the animals concerned. Attention has been drawn also to the occurrence of iodine binding at the surface of the tunic (or test) of Ciona (Barrington, 1957), and it is the purpose of the present paper to present some analysis of this latter situation and to enquire as to its possible relationship with the endostylar activity.