EVOLUTIONARY ORIGIN OF THE VERTEBRATE NEPHRON

The primitive form of the vertebrate nephron consists of a vascular filtration surface overlain with podocytes, a specialized coelomic cavity to receive the ultrafiltrate, and a tubule for modification to final urine. Although previously thought to be unique to the vertebrates, this design is now known to be widespread among invertebrates, including most of the protochordates, and especially their larvae. Goodrich's rejection of the homology of invertebrate nephridia and the vertebrate nephron, based on a lack of germ-layer correspondence, is shown to be either unsupported by facts or logically dubious. Comparative morphology of adult and larval invertebrates suggests that filtration excretory organs, as protonephridia and metanephridial systems, evolved in the lineage to the bilaterally symmetrical animals and each consisted minimally of a filtration cell, a urinary compartment, and tubule joined to the exterior. Invertebrate metanephridial systems and protonephridia are discussed as homologous structures composed of homologous cells (podocytes, terminal cells; also nephrocytes). The ontogenetic and phylogenetic distribution of nephridia is correlated with body design, especially body size.