Definition: "A modified non-motile cilium which functions as a light-detecting organelle in the retina. OS cilia have an extended membrane system which consists of arrayed membranous disks resembling a stack of coins. In rods/rod cells, these disks are discontinuous, being divided into lobules or \"incisures\", and are surrounded by a plasma membrane. In cones/cone cells, the disks and plasma membrane remain joined as one continuous membrane system and incisures are not present."