An element which seems to have been too little regarded in the physiology of sponges is the permanent tension of their walls. The contours of the surfaces, particularly where they rise over projecting spicules, are alone sufficient to demonstrate that there is surface-tension between the protoplasm of the sponge and the sea-water. Both the outer and the inner surfaces of a cylindrical or of a spherical sponge unite, therefore, in exerting a force which tends to contract its diameter.