five groups: The ciliary opsins (cilopsins, c-opsins), the rhabdomeric opsins (r-opsins, rhabopsins), the xenopsins, the nessopsins, and the tetraopsins. Four of these subclades occur in Bilateria (all but the nessopsins). However, the bilaterian clades constitute a paraphyletic taxon without the opsins from the cnidarians. The nessopsins are also known as anthozoan opsins II or simply as the cnidarian opsins. The tetraopsins are also known as RGR/Go or Group 4 opsins and contain three subgroups: the neuropsins, the Go-opsins, and the chromopsins. The chromopsins have seven subgroups: the RGR-opsins, the retinochromes, the peropsins, the varropsins, the astropsins, the nemopsins, and the gluopsins. Animal visual opsins are traditionally classified as either ciliary or rhabdomeric. Ciliary opsins, found in vertebrates and cnidarians, attach to ciliary structures such as rods and cones. Rhabdomeric opsins are attached to light-gathering organelles called rhabdomeres. This classification cuts across phylogenetic categories (clades) so that both the terms "ciliary" and "rhabdomeric" can be ambiguous. Here, "C-opsins (ciliary)" refers to a clade found exclusively in Bilateria and excludes cnidarian ciliary opsins such as those found in the box jellyfish. Similarly, "R-opsin (rhabdomeric)" includes melanopsin even though it does not occur on rhabdomeres in vertebrates. Ciliary opsins Ciliary opsins (cilopsins, c-opsins) are expressed in ciliary photoreceptor cells, and include the vertebrate visual opsins and encephalopsins. They convert light signals to nerve impulses via cyclic nucleotide gated ion channels, which work by increasing the charge differential across the cell membrane (i.e. hyperpolarization.) Vertebrate visual opsins Vertebrate visual opsins are a subclass of ciliary opsins that express in the vertebrate retina and mediate vision. They are further subdivided into: Photopsins - those responsible for photopic vision (daylight), which are expressed in cone cells; hence also cone opsins. Photopsins are further subdivided according to their spectral sensitivity, namely the wavelength at which the highest light absorption is observed (λmax). Vertebrates generally have four (SWS1, SWS2, RH2, LWS) classes of photopsins. Mammals lost Rh2 and SWS2 classes during the nocturnal bottleneck, so are generally dichromatic. Primate ancestors later developed two distinct LWS opsins (LWS and MWS), leaving humans with 3 photopsins in 2 classes: SWS1 (OPN1SW) and two forms of LWS (OPN1LW, OPN1MW). Scotopsins - those responsible for scotopic vision (dim light), which are expressed in rod cells; hence also rod opsins. The most common form of scotopsin is rhodopsin, thus usually denoted Rh1. Extraretinal (or extra-ocular) Rhodopsin-Like Opsins (Exo-Rh) These pineal opsins, found in