<P> The gills, located under the operculum, are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide . They are not usually visible, but can be seen in some species, such as the frilled shark . The labyrinth organ of Anabantoidei and Clariidae is used to allow the fish to extract oxygen from the air . Gill rakers are bony or cartilaginous, finger - like projections off the gill arch which function in filter - feeders to retain filtered prey . </P> <P> The epidermis of fish consists entirely of live cells, with only minimal quantities of keratin in the cells of the superficial layer . It is generally permeable . The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods . Instead, in most species, it is largely replaced by solid, protective bony scales . Apart from some particularly large dermal bones that form parts of the skull, these scales are lost in tetrapods, although many reptiles do have scales of a different kind, as do pangolins . Cartilaginous fish have numerous tooth - like denticles embedded in their skin, in place of true scales . </P> <P> Sweat glands and sebaceous glands are both unique to mammals, but other types of skin glands are found in fish . Fish typically have numerous individual mucus - secreting skin cells that aid in insulation and protection, but may also have poison glands, photophores, or cells that produce a more watery, serous fluid . Melanin colours the skin of many species, but in fish the epidermis is often relatively colourless . Instead, the colour of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments . Many species, such as flounders, change the colour of their skin by adjusting the relative size of their chromatophores . </P> <P> The outer body of many fish is covered with scales, which are part of the fish's integumentary system . The scales originate from the mesoderm (skin), and may be similar in structure to teeth . Some species are covered instead by scutes . Others have no outer covering on the skin . Most fish are covered in a protective layer of slime (mucus). </P>

The skeletons of both hagfish and sharks are made of the same cartilaginous material