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Not surprisingly, photoreceptors evolved before eyes. There are basically two types of specialized photoreceptor cell in animals, known as `rhabdomeric' and `ciliary', and for many years it was thought that this distinction mapped nicely onto the protostome/deuterostome divisions of the Bilateria. To catch a reasonable proportion of the light that falls on them, photoreceptors need a high density of rhodopsin. Rhodopsin molecules are embedded in the membranes of the receptor cells (Figure 2) so increasing the rhodopsin density means having a greatly expanded membrane. Ciliary receptors are organized around cilia, hair-like structures found in a wide variety of cells. Typically the membrane is organized into stacks of lamellae or discs, as in the rods and cones of the human retina (Figure 4). A human rod has a total of 150 million rhodopsin molecules packed into about 1,000 discs. In rhabdomeric receptors there is no cilium, and the membrane is expanded into finger-like extensions known as microvilli, closely packed together. In arthropods these almost crystalline structures are known as rhabdoms (Greek for `rod'). The two receptor types differ not just in structure but in the molecular make-up of the rhodopsin molecules themselves, as well as in the biochemistry of the transduction cascades. So it seems that there are two quite distinct photoreceptor families which must have diverged at an early stage in animal evolution, or else evolved from quite separate receptor structures. This led to many advances in laser eye surgery.