ABSTRACT In view of the interest attaching to the nature and function of the madreporic vesicle, I made a careful examination of a number of young living Ophiuroids from tow-nettings at the Millport Marine Station in June, 1918. In particular specimens (probably of Ophioglypha albida), at the stage when the young star is a flattened disc with five blunt arms, each with five tentacles, the madreporic interradius is slightly wider than the rest, and is marked by the projection in its neighbourhood of a still persisting larval arm-rudiment. Careful focussing through the tissues of this interradius, in recently taken specimens examined in saltwater, never failed to reveal the presence of a rhythmically pulsating thin-walled cavity entirely comparable with the pulsating madreporic vesicle of an Asterias larva. The pulsations are extremely regular, occurring once in every eleven or twelve seconds, and they continued for over an hour in several of the specimens examined. They could be made out both from the oral and from the aboral side, and nothing similar was revealed by search in the other interradii. One gets the impression that the essential part of the pulsation is the emptying and filling of spongy tissue to one side of the vesicle, but this appearance is not nearly so definite as in Asterias (2, p. 248) and Porania (3, p. 40) larvæ. Bury (9, p. 76, 1896) noted in Echinus microtuberculatus that during contraction the floor of the vesicle projects far up into its cavity, and that the pulsation is certainly continued in the earliest post-larval stages, though whether it occurs in the adult he was unable to say. Recently MacBride has figured and called attention to a similar projection in sections of late larvæ of Echinocardium cordatum (5, p. 263, Pl. 19, fig. 10A).