Animal tissues can be grouped into four basic types: epithelial, connective, muscular and nervous tissue. Epithelial tissue forms the outer covering of the body and internal organs and has several subtypes including squamous, cuboidal and columnar epithelium. Simple epithelium is a single layer of cells while compound epithelium has multiple layers. Epithelial tissues function to protect the body, aid absorption and elimination, and perform secretion in some areas.