The author presents the results of an analysis of the test in Pygomalus ovalis , P. analis , Pygorhytis ringens, Collyrites sp. and Disaster n. sp. where she has observed the presence of the 5th genital plate. Its displacement is discussed and is shown to be similar to that occurring in the ontogenetic development of Echinocardium cordatum. Moreover, in order to homologize the plates of the apical system in echinoids, the writer has also used the crystallographic method, on the ground that the optic axis of the particular plates displays a fairly constant inclination angle. It is supposed that in some genital plates the inclination angle of the optic axis is determined by its position in the remains of the larval skeleton around which these plates arise.