Fig. 16 in New insights into the origin and relationships of blastoid echinoderms

Fig. 16. Ontogenetic development of ambulacra in the callocystitid Lepadocystis moorei (Meek, 1871), Late Ordovician, Ohio, USA, showing addition of oral and ambulacral plates and development of the "BD different" pattern of primary brachioles. A. Triradiate stage with the first pair of ambulacral plates (A1a, A1b, and so on) interpreted as A, shared BC and shared DE. Arrows indicate the positions where the radial water vessels of the shared ambulacra branch to produce a pentaradiate rhombiferan, assuming subterminal branching. B. Pentaradiate stage with a single brachiole per ambulacrum. The two new brachioles are interpreted as the first brachioles in separate ambulacra C and E. Arrows indicate positions where the radial water vessels of all five ambulacra branch to produce second brachioles. C. Pentaradiate stage with two brachioles per ambulacrum. The second brachioles in the shared ambulacra are interpreted as B1 and D1. Note that the subterminal branching has produced the "BD different" pattern of primary brachioles in which the first two brachioles in ambulacra B and D branch to the left as viewed in the growth direction, whereas in ambulacra A, C and E only the first brachiole is on the left. In addition, the second brachioles exhibit a pattern which is A right, B left, C right, D left, and E right, which equals Lovén's Law as restated by Paul and Hotchkiss (2020). M, mouth; O6 and O7, orals 6 and 7 in the conventional interpretation of the oral circlet. Solid lines, plate sutures; dashed lines, outline of ambulacra and brachiole facets. Redrawn from Paul and Hotchkiss 2020: 1096, fig. 8.