Epidermis (zoology)
The Epidermis is an epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 (sheet of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

) that covers the body of an eumetazoa
Eumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage...

n (animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

 more complex than a sponge). Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis
The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....

, which forms a boundary with the epidermis at the mouth.

Sponges have no epithelium, and therefore no epidermis or gastrodermis. The epidermis of a more complex invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

 is just one-layer deep, and may be protected by a non-cellular cuticle
A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...

. The epidermis of a higher vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 has many layers, and the outer layers are reinforced with keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

and then die.
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