Ans: Epithelial tissue definition states that it is a membranous cellular tissue covering the lines of tubes or cavities of an animal body or the free surface. It serves to enclose and protect the other parts of the body by producing excretions and secretion and functions in assimilation. In plants, the epithelium is usually a thin layer of parenchyma that lines with the tube or cavity. The cells forming an epithelial membrane are three or four distinct types- columnar epithelial cells, cubical epithelial cells, squamous epithelial cells, and irregular or ciliated epithelial cells. The primary function so epithelial cells include protection, excretion, secretion, absorption, diffusion, filtration, and sensory reception.