(Science: cell biology) a skeleton, or support structure, which supports the organisms body from the outside and is formed from the ectoderm. All arthropods (spiders, insects, crustaceans, horseshoe crabs, etc.) possess one.
Any structure that is formed from the ectoderm in vertebrates, like nails, claws, hair, fur, horns, or teeth. (Note: does not include skin, which is an organ.)