Cuticle

Cuticle, a sheath formed outside a layer of cells, either by their secretory activity or by a modification of their external portions. In the strict sense, a cuticle is not in itself cellular, but consists of the products or of the modified portions of underlying cells. The thin envelope which may be readily stripped off a leech or earthworm when killed in spirit supplies a convenient example. A cuticle is usually formed outside relatively passive cells, but even ciliated Epithelium (q.v.) may have its cuticular outer layer through the pores of which the cilia emerge. By continuous modifications of the cells considerable thickness of cuticle may be developed—as e.g. the hard lining of the gizzard in many birds. By chemical modification of a well-developed cuticular formation very varied protective and offensive skin-structures often result. Thus the peculiar gelatinous, cellulose-containing tunic of Ascidians is for the most part a cuticle; the shells of molluscs are cuticular formations plus lime; the rasps of snails are formed from cuticle; the hard armour of Arthropods is a cuticle associated with the formation of Chitin (q.v.); the bristles, jaws, and firm sheaths of many worms are also cuticular, and so on. For the use of the term in special connections—e.g. the cuticle of the hair or of the teeth, see special articles. The term must not be confused with cutis, one of the names for the under skin or dermis; nor should it ever be used as equivalent to skin. See CELL, EPITHELIUM, SKIN.