Gromia oviformis has an organic shell or wall modified at one point to form a specialized oral region through which the protoplasm extrudes as pseudopodia. Whereas the shell appears to be predominantly proteinaceous with additional acid mucopolysaccharide and lipid components, the gel-like oral capsule, the main feature of the oral region, appears to be composed mainly of acid mucopolysaccharide. The changes in form undergone by the oral capsule during the opening and closing of the oral region are described. The shell, apart from the oral capsule, normally contains a large amount of ferric iron. When sections, from which the iron had been removed, were immersed in seawater reinforced with iron or in aqueous iron solutions, it was found that iron was only taken up by the shell when the concentration was greater than 12.5 mg Fe per litre. Only when the concentration was in the region of 200 mg Fe per litre did the shell take up an amount comparable with that which is normally present. It appears unlikely that during life a reaction between the iron in normal sea-water and the shell can be wholly responsible for the iron content of the organic wall. Alternative sources of iron are discussed. A great reduction in the amount of iron taken up from iron solutions under conditions of low pH, and after the sections have been methylated, points to the importance of carboxyl groups in the binding of iron by the shell. The nature of the union of the iron with the wall is discussed and shown to be similar in some ways to that in leather tanning processes where inorganic tanning agents are employed.