In fact, there should be sixteen a arahats, or sixteen ayasmas (Arahat is transliterated into Chinese as Aluohan, or simply Luohan). According to the Buddhist canons, under the Buddha’s instructions, sixyteen of his disciples would not enter into Nibbana. In Nandimittavadana written by Ayasma Nandimitra of Siharatta (now Sri Lanka) in the 2nd century AD, the names of the sixteen arahats and localities of their residences were recorded. After the book was translated into Chinese by Dhammacariya Xuan Zang, the sixteen Arahats were held in universal esteem by the Chinese Buddhists. By the time of the Five Dynasties, with the growing popularity of paintings and sculptures, representation of sixteen arahats was gradually superseded by the representations of eighteen arahats. Presumably the painters intended to add Nandimitra and Xuan Zang, the writer and the translator of Nandimittavadana into the list. However, when later generations put the names of Arahats on the paintings, Nandimitra was mistakenly listed as the seventeenth resident Luohan, and the name of the first Arahat was repeated as the eighteenth. Though this mistake was already found out in the Song Dynasty, the formulation of the Eighteen Luohans became widespread in China, probably because of the many inscriptions and paintings made by famous calligraphies and painters as well as men of letters such as Guan Xiu, Su Dongpo, and Zhao Songxue. (From Essentials of Buddhism: Questions and Answers)