This is a lancelet.
Skin is commonly referred to as any external tissue of an animal, including the coverings of microscopic invertebrates. However, in a narrow sense, this term is applied only to the outer coverings of chordates. The thinnest skin is found in the lancelet, a small marine creature (in the photo). It consists of a single layer of epithelial cells that are attached to a dense extracellular matrix. It is nearly impossible to separate such skin from the body.
Among mammals, the thinnest skin is observed on the wing membranes of bats: in the case of the noctule bat, for example, its thickness is only 20 microns in some areas.
