Systemic circulatory pressure provides the work for the single effect in the metanephros

ABSTRACT: The evolution from the reproductory organ mesonephros to the excretory organ metanephros was simple. As gills no longer excreted excess matter, it basically had to leak out somewhere else, and the mesonephros that already had vents to the “outside” for reproductive purposes was where that leak could be facilitated. Replication of the quantity of mesonephros could meet the demands of excretion, and, by simply draining them all through a funnel, the systemic circulatory pressure that was already there could also generate a “single effect” that produced an osmotic gradient.