THE ORIGIN OF LOVÉN'S LAW IN GLYPTOCYSTITOID RHOMBIFERANS AND ITS BEARING ON THE HEMICOSMITID PERISTOMAL BORDER

Few echinoderms show true five-fold symmetry. Most have feeding ambulacra with three proximal rays (A, shared BC, and shared DE) extending from the peristome. Five rays form when the shared ambulacra branch distally into the B, C, D, and E ambulacra. Lovén's Law, found in several echinoderm clades, states that the A, C, and E ambulacra are somehow similar, but different from the B and D ambulacra which themselves are similar. It is manifested in glyptocystitoid rhombiferans by a single left proximal brachiole in the A, C, and E ambulacra, and double left proximal brachioles in the B and D ambulacra. Double left brachioles result from an ontogenetic shift of the shared BC and DE brachioles onto the B and D ambulacra following the developmental insertion of the lateral oral plates (O2 and O5). These plates insert to the right of the shared brachioles forcing them onto the B and D ambulacra before the development of the distal ambulacra.

In hemicosmitids (a clade closely allied with blastoids and glyptocystitoids including Caryocrinites and Hemicosmites) there are only three ambulacra primitively. Confusion in the homology of the peristomal bordering elements occurs because 1) many hemicosmitids have a tegmen of modified cover plates obscuring this region, 2) some elements are lost through paedomorphosis and, 3) some proximal ambulacral elements are incorporated into the theca. The large interradial peristomal bordering plates are homologous to the early-developing O1, O3, and O4 plates of glyptocystitoids, and form the triradial portion of the ambulacral system. The primitively late-developing O2 and O5 plates are lost via paedomorphosis. Consequently, hemicosmitid ambulacra are A, shared BC and shared DE, and development of the distal B, C, D, and E ambulacra is truncated from the ontogeny. Two small plates bordering the hydropore and gonopore in the CD interray are homologous to O6 and O7. Six large, paired, radially positioned plates that form the arm facets are proximal ambulacral floor plates incorporated into the summit. Three small plates positioned between the orals and the proximal ambulacral floor plates are incorporated brachiole mounting plates and mark the position of juvenile, summit-mounted brachioles, homologous to the brachioles of the shared ambulacra of glyptocystitoids.