Further, we may conceive each of the centrad processes of the long pigment cells as receiving a fiber from one of the sensory cells directly as well as indirectly, as just described. While I have been able to demonstrate only a single centrad process for the sensory cells (prism and pyramid cells), yet this does not exclude the possibility of a nerve fibril passing out from such a centrad process to one of the processes of the long pigment cells, and it seems possible that this constitutes the reflex mechanism. That nerve fibrils ramify in ganglion and sensory cells, and may even leave these cells to join those of other cells, has been well demonstrated by Apathy,[6] so that my finding only a single process of the visual cells leading centrad without giving off lateral fibers cannot be a serious objection. Again, fine nerve fibers coming off from the main centrad process of sensory cells in medusæ have been figured by other observers, among whom I mention the Hertwigs. Careful macerations at the seashore would probably demonstrate them for Charybdea. Hesse thinks that the eyes of the Alciopidæ are adjustable. He describes what he supposes to be muscle fibers just exterior (distal) to the lens, and believes that a contraction of these fibers would have the effect of forcing the lens nearer the retina, or _vice versa_. His supposition, like mine, needs experimental verification. Hitherto the only instance known of accommodation in the eyes of invertebrates was that described by Beer[17] for Cephalopods.