The Horny Sponges of the genus Ircinia contain, as well as spongin fibers and collagen fibrils, peculiar filaments which are typical of this genus. The filaments are slender threads, several millimeters in length, about ten microns wide, terminated at each end by a terminal knob. These filaments consist of three layers : a cuticle-like outermost layer, a soft cylinder (forming the bulk of the filament) within this tubular cuticle, and an axial thread in the center. With electron microscope the filaments appear as bundles of microfibrils (about 50 to 70 A in diameter) twisted in an helical fashion, nearly straight in the axis of the filament and becoming increasingly coiled toward the periphery. X-ray diffraction and infrared' spectral studies in conjonction with biochemical analysis have shown that the filaments are collagenous, that they are close to spongin fibers and thus belong to the sponge skeleton. Isolated observations on the synthesis of the filaments suggest that they are at least partially elaborated by nucleolate cells of the Sponge. These cells release a material which seems to undergo a fibrillar organization and a self assemblying process outside the cells.