Before discussing the different kinds of photoreceptors and eyes it will be useful to have an outline of where eyes fit into the basic narrative of animal evolution. Of the earliest animal groups (phyla) the sponges (Porifera) do not have photoreceptors or anything remotely eye-like, so photoreception was not universal among the first animals. However, many animals of another early phylum, the Cnidaria, which contains the sea anemones, corals, and jellyfish, do indeed have photoreceptors. In one class of cnidarians, the cubozoans that include the notorious Australian `stingers', there are even eyes that really look like eyes, so the capacity to produce eyes was present from a very early stage of metazoan evolution. The Cnidaria, which are radially symmetrical animals, split off from the early metazoan line before the formation of the main animal phyla, collectively known as the Bilateria from their left-right symmetry. Most of these bilaterian animals moved forwards, and so had a head end where most receptors, such as eyes, were located. At some time in the late Precambrian the bilaterians split into three major groups, the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia (Figure 3). Excellent eyes evolved in at least one branch of each group. Before the advent of molecular ways of tracing animal relationships only two major bilaterian groupings were recognized: the protostomes and the deuterostomes. The division was based on their different development: the names mean first mouth and second mouth, from the way the mouth and anus develop in the early embryo. The second of these groups--the Deuterostomia--is the group to which we humans belong, and contains the starfish, the sea squirts, and the chordates, from which the vertebrates arose. In 1997, the Protostomia were divided into their two present groups, the Ecdysozoa and the Lophotrochozoa, on the basis of new molecular evidence. Basically the Ecdysozoa are animals that moult repeatedly as they grow. They include the jointed-limbed animals (arthropods), and these comprise the Chelicerata (horseshoe crabs, scorpions, and spiders), the Crustacea (shrimps, lobsters, crabs, and many smaller classes) and the Insecta (beetles, flies, etc.). Most of the members of these groups have compound eyes and good--sometimes excellent--vision. The Ecdysozoa also include millipedes and centipedes, and perhaps surprisingly the nematode worms, which also moult but are mostly eyeless. The Lophotrochozoa (an unconvincing hybrid word meaning `crest/wheel animals') include most of the rest: the flatworms, the molluscs, the annelid worms, and several smaller phyla. Of these, the cephalopod molluscs (octopus, squid, and cuttlefish) stand out as having eyes that are large, resolve well, and are similar in their capabilities to those of fish.