ABSTRACT
Fishes are the first vertebrates, descendents of the early chordates* that evolved from marine, deuterostome invertebrates. The extant lancelets and tunicates both marine and stenohaline, are modern invertebrate chordates, with the lancelets generally accepted as closest to the early fish lineage, based on molecular data. Despite probable specialization during at least 400 million years of evolution, 70 species of modern hagfishes represent the best model available for any osmoregulatory mechanisms that may have been present in the basal vertebrate clade. The hagfish, opisthonephric kidney has 30 to 40 segmental glomeruli, connected to short, nonciliated neck segments that drain into paired archinephric ducts that are structurally similar to proximal tubules in other vertebrates. The hagfish glomerulus has the same basic structure as other vertebrates, with a capillary endothelium, mesengial cells, basement membrane, and podocyte forming the putative filtration surface. The hagfish gill morphology is very different from that found in lampreys and jawed piscine vertebrates.
