ABSTRACT
Pteraster tesselatus (order Velatida, family Pterasteridae) has direct development because it lacks a larval stage. Coeloms arise from seven separate enterocoels that evaginate from novel regions of the archenteron. The water-vascular coelomic system develops from the first five enterocoels (homologous to hydrocoel lobes) that extend radially, in a transverse orientation, from the central region of the archenteron. All other coelomic compartments derive from two enterocoels that evaginate later in development from posterior regions of the archenteron. This forms the adult coelomic organization directly from the gastrula without a metamorphosis. The direct pattern of coelom formation in P. tesselatus led to not only morphological simplification and the loss of larval features, but also provided for precocious development of the water-vascular system. Although most pterasterids brood their young, P. tesselatus broadcast spawns and has pelagic development. Podia form early and are used for attachment to the benthos in Heu of larval (brachiolar) structures. The novel pattern of morphogenesis of the circumoral canal in P. tesselatus may have provided a pathway for the evolution of the unique water-vascular geometry (dual circumoral canals) of the newly discovered deep-sea echinoderms, the concentricycloids. We agree with recent interpretations of the concentricycloids as progenetic velatid asteroids, rather than a new class.
