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In page Skin:

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Sweat glands and sebaceous glands are both unique to mammals, but other types of skin gland are found in other vertebrates.[citation needed] Fish typically have a 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. In amphibians, the mucous cells are gathered together to form sac-like glands. Most living amphibians also possess granular glands in the skin, that secrete irritating or toxic compounds.[6]