The epidermis is the external surface of an animal and performs several functions. It is usually a monolayer of cells, but can also be composed of several layers of cells. Epidermal cells are connected by cellular junctions (anchoring, occluding, and gap junctions), which are of particular phylogenetic importance. Epidermal cells can secrete extracellular material to their basal and apical side, which is called the ECM or glycocalyx/cuticle. The evolution of such structures is discussed in this chapter, including the presence of chitin and the moulting of the entire cuticle. Particular substructures of epidermal cells are the cilia, which form a variety of different arrangements and shapes. Several characters concerning the epidermis can be recognized as evolutionary novelties for particular metazoan taxa.
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