Morphological and ecological characteristics of two montacutid bivalve species endo- and ecto-symbiotic with holothurians were studied in the Ryukyu Islands, Japan. The endosymbiotic species Entovalva lessonothuriae sp.nov. was found inside the oesophagus of the holothurian Holothuria (Lessonothuria) pardalis. The bivalve had an internal shell wrapped by a mantle and a well-developed bilaterally flattened foot, with a byssal gland on the right-hand side that attached to the wall of the oesophagus. The foot, usually bent to the left, enabled the bivalve to dodge through the sand flow in the host's oesophagus. The presence of well-developed gills suggests that the bivalve is still a filter-feeder. The bivalves were generally found in pairs, i.e., a larger female, sometimes brooding veliger larvae, and a smaller male. The ectosymbiotic species Anisodevonia (gen.nov., separated from Devonia) ohshimai was attached to the skin of a synaptid holothurian, Patinapta ooplax. The bivalve could creep actively on the host using its foot, which was bent to the right. The left- and right-handedness of the foot flexion corresponded to an endo- and an ectosymbiotic life-style, respectively. It is suggested that the endosymbiotic species evolved from an ectosymbiotic ancestor without the loss of locomotory, respiratory, and feeding functions.