The wall of alimentary canal possesses four layers - serosa, muscularis, sub mucosa, and mucosa. Serosa is the outermost layer and is made up of a thin mesothelium and connective tissue. Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer. Sub - mucosa layer is formed of loose connective tissues containing nerves, blood and lymph vessels. Glands are also present in sub - mucosa of duodenum. Mucosa is the innermost layer lining the lumen of the alimentary canal. Mucosa forms irregular folds (rugae) in stomach and villi in small intestine.