The wall of the alimentary canal is made of four layers, viz. serosa muscularis, sub-mucosa and mucosa. Serosa is the outermost layer and is made up of a thin mesothelium with some connective tissues. The epithelium of visceral organs is called mesothelium. Muscularis is formed by smooth muscles which are usually arranged into an inner circular and outer longitudinal layer. An oblique muscle layer may be present in some regions. The sub-mucosal layer is formed of loose connective tissues.