Skin (comparative anatomy).

Animal survival requires that the internal body components be separated and protected from the external environment. Most single-cell organisms are separated from the environment only by the plasma membrane (cellular membrane). In multicellular organisms, the body surface is covered by a tissue consisting of epithelial cells and connective tissue. The covering is commonly referred to as skin, but the skins of invertebrates and vertebrates have distinct differences. Invertebrates often have a single layer of surface epithelial cells, which is generally referred to as an integument. However, some invertebrates, specifically flukes and tapeworms, have a unique, living surface covering called a tegument. In this situation, the epithelial cells have fused and formed a single bag of cellular components called a syncytial epidermis. Thus, the word “skin” is often reserved specifically to describe the surface covering in vertebrates, but the word “integument” is also used.