(f) What, now, is the function of these three varieties of cells of the retina? Schewiakoff regards his visual cells (pyramid cells), as the name implies, as having a visual function. That they have such it seems reasonable to suppose, since they have an axial fiber in their pyramids. If the pyramid cells are visual cells, it appears that the prism cells also are such. Indeed, since these are the only ones present in the proximal eye and the more numerous ones in the distal eye, and like the pyramid cells have an axial fiber in their prisms, it seems that they are the visual cells _par excellence_ of the Cubomedusan eye. Also, the analogy between the prisms and pyramids on the one hand, and the rods and cones of the vertebrate eye on the other hand, does not seem to be so far fetched. It may be of interest, here, to briefly consider Patten’s theory of color vision.[5b] The gist of Patten’s theory is this: In the eyes of certain molluscs and arthropods, in the parts of the retinal cells corresponding to my prisms and pyramids, he not only finds an axial fiber (or fibers) but finer fibrils that extend at right angles from these axial fibers to the surface of the rods (I shall here, for convenience, call the prisms, pyramids, etc., rods) where they probably become continuous with other fibrils in the surface of the rods. These fibrils from the axial fibers are arranged in superimposed planes, and if I understand rightly, an axial fiber with its radiating fibrils may be compared to the axial wire with its radiating bristles of a brush used for cleaning bottles, provided the bristles of such a brush be arranged in superimposed planes. The lateral arrangement of the fibrils will, of course, be modified according whether a rod is circular, hexagonal, square, etc., in transverse section. It will also be remembered (p. 49) that Patten describes the retinal cells studied by him as composed of twin cells, and he gives the name _retinophora_ to a pair. The system of fibers and fibrils in the rods he names a _retinidium_. Centrad the axial fibers are continued past the nucleus as a nerve fiber. The fibrils extending laterally in superimposed planes from the axial fiber of a rod, Patten supposes to be the ones stimulated by the incoming rays of light, the retinophora being so arranged that the light rays entering them are parallel to the axial fibers or perpendicular to the lateral fibrils of the retinidium. Again, since the rods are usually the shape of truncated pyramids or cones the lateral fibrils, which are perpendicular to the axial fibers, are of different lengths accordingly as they are situated at the larger or smaller end of a rod. Patten assumes similar fibrils to exist in the rods and cones (particularly the cones) of the vertebrate eye, and he thus makes a general application of his theory. He supports himself in this rather sweeping generalization by the claim to have demonstrated the twin-cell nature of the cones in amphibia and fishes.