The Larval Anteroposterior Bilateral Asymmetrical Axis of Echinoderms; a Landmark for Establishing Ambulacral Homology

The features of the torsion process in crinoids and blastozoan echinoderms indicate the correspondence of the anteroposterior bilaterally asymmetric larval axis to the E–BC plane of symmetry of adult pentaradiate animals. This correspondence is common to all echinoderms. This confirms the initial role of the ambulacrum E in the appearance of the ambulacra C and A as a result of duplication. The ambulacra D and B arose as a result of branching from the initial ambulacra E and C, respectively. The homology of the ambulacra based on the restated Lovén’s Law is confirmed by the correspondence between the location of the larval plane and the constant place of closure of the ambulacral ring in all extant echinoderms, as well as by hydropore and anus migration. The point of closure of the ambulacral ring at the late stages of ontogeny in many groups of echinoderms determines the position of the new morphogenetically active A–CD plane, adjacent to the initial larval E–BC plane. The hypothesis about the initial bilateral asymmetry of echinoderms, associated with the incomplete coordination of the development of the right and left sides of the body at the initial stage of the formation of Bilateria, is substantiated.