EVOLUTIONARY ORIGIN OF THE VERTEBRATE NEPHRON

The primitive form of the vertebrate nephron consists of a vascular fi ltration surface overlain with podocytes, a specialized coelomic cavit y to receive the ultrafiltrate, and a tubule for modification to final urine. Although previously thought to be unique to the vertebrates, t his design is now known to be widespread among invertebrates, includin g 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 morphol ogy of adult and larval invertebrates suggests that filtration excreto ry organs, as protonephridia and metanephridial systems, evolved in th e lineage to the bilaterally symmetrical animals and each consisted mi nimally of a filtration cell, a urinary compartment, and tubule joined to the exterior. Invertebrate metanephridial systems and protonephrid ia are discussed as homologous structures composed of homologous cells (podocytes, terminal cells; also nephrocytes). The ontogenetic and ph ylogenetic distribution of nephridia is correlated with body design, e specially body size.