SUMMARY The buccal glands of lampreys develop during metamorphosis and occur in the adults of both the parasitic and non‐parasitic species. Their form varies little in sexually mature lampreys. In the young feeding stages of the sea‐lampreys the glands are relatively much larger and appear to be more active than in the sexually mature adults of the same species. The secretion of the buccal glands is shown to prevent the clotting of blood even when the secretion is obtained from the glands of non‐parasitic lampreys. The significance of this is discussed and it is suggested that the nonparasitic lampreys are neotenous forms living in a restricted environment, but which have not lost the anatomical features possessed by their parasitic relatives.