Nephridia in the Larvae of Hemichordates and Echinoderms

The pore canal-hydropore complex in the larvae of echinoderms and hemichordates has long been recognized as an important character establishing a close phylogenetic relationship between the two phyla. An experimental and ultrastructural analysis of this complex in a tornaria and a bipinnaria larva indicates that it is a functional nephridium. The ciliated pore canal drives a constant, unidirectional efflux of coelomic fluid out of the hydropore. Two percent and 14% of the body volume are cleared per hour at the hydropore by a tornaria of Schizocardium brasiliense and a bipinnaria of Asterias forbesi, respectively. Fluid recovery by the coelom is from the blastocoel, the presumptive blood vascular space, across basal lamina and podocytes lining the coelomic cavity suggesting that the discharged fluid is formed by ciliary-driven ultrafiltration. Although invertebrate deuterostomes are believed to lack discrete excretory organs, an analysis of the metamorphosis of the larval nephridia suggests that adult echinoderms and hemichordates possess functional metanephridial systems.