METAMORPHOSIS OF STICHOPUS CALIFORNICUS (ECHINODERMATA: HOLOTHUROIDEA) AND ITS PHYLOGENETIC IMPLICATIONS

Descriptions of holothurian metamorphosis are based on data from the relatively specialized order Apoda. Metamorphosis for a relatively unspecialized aspidochirote, Stichopus californicus, is described here. Metamorphosis in Stichopus is characterized by the following features: the madreporic vesicle is a calcite secreting syncytium, not a coelom. Stichopus has no separate axocoel and no transient axial complex forms during metamorphosis. The buccal podia form from the water vascular ring, not the radii. The axes of bilateral larval symmetry and the pentaradial adult symmetry are congruent; therefore, the secondarily derived symmetry in holothurians is the pentaradial symmetry of the adult, as in the other extant echinoderms. No axial or visceral torsion occurs during metamorphosis. The enclosed ambulacra form in a manner distinct from that of the ophiuroids and echinoids. Perivisceral coelomic pores develop near the end of metamorphosis, and before the definitive anus forms. Assignment of holothurians with echinoids to the subphylum Echinozoa is discussed.