The larval apical organ in the Holothuroid Chiridota gigas (Apodida): Inferences on evolution of the ambulacrarian larval nervous system

Within the deuterostomes, the similarity of the dipleurula-type larvae of echinoderms (auricularia, bipinnaria) and hemichordates (tornaria) is striking. Here we describe the serotonergic system of the auricularia larvae of the apodid sea cucumber Chiridota gigas to broaden the comparison of the dipleurula-type larval nervous system in the Holothuroidea. This larva has a simple serotonergic nervous system largely composed of the apical organ. The apical organ is a concentration of cells and fibers along, and spanning between, the portions of the ciliary band that traverse the larval apex. Bipolar and multipolar cells give rise to fibers that connect the bands. In contrast to other echinoderm larvae, this larva does not develop serotonergic cells around the mouth. The similar topology and structure of the apical organ of the auricularia and the tornaria suggests that the serotonergic apical organ of the Ambulacraria (Echinodermata + Hemichordata) may have originated as a group of nerve cell bodies and fibers that developed in association with ciliary band sectors at the larval apex. © 2006 Marine Biological Laboratory.Maria Byrne, Mary A. Sewell, Paulina Selvakumaraswamy, and Thomas A. A. Prows