Histological comparisons of intestinal internal structures were made for the monophyletic lamprey group comprising parasitic Lethenteron japonicum, and nonparasitic L. kessleri and the northern form of L. reissneri, in order to verify the speciation hypothesis that the nonparasitic species have been derived from a congeneric parasitic species. In the larval stage of each species, the mucosal epithelial cells were regularly arranged around an inner layer of intestine, including the typhlosole. At the metamorphosed stage, L. japonicum possessed functional mucosal folds, reflecting an adaptive change for parasitic feeding after metamorphosis. The two nonparasitic species, in which feedings are absent after metamorphosis, also exhibited mucosal folds albeit in a degenerative condition, indicating the likely presence of functional or at least rudimentary mucosal folds in an ancestral parasitic species. This finding supports a previously advocated direction of speciation in lamprey satellite species, namely nonparasitic L. kessleri and the northern form of L. reissneri speciated from ancestral stocks of parasitic L. japonicum.