Jawless fish still swim beside the vertebrates
Agnatha means without jaws: a paraphyletic grab-bag of vertebrates that never grew biting mouths. Living forms—hagfish and lampreys—total about one hundred twenty species and form Cyclostomi, closer to each other than to sharks or bony fish. Fossils reach back to the Cambrian, long before jaws remade vertebrate life.
Living agnathans lack paired fins, keep a notochord into adulthood, and breathe through seven or more paired gill pouches. Lampreys sport a light-sensitive pineal eye; hagfish secondarily lost vertebrae yet still count as vertebrates once molecular and developmental data overturned the older Craniata-only framing. Skin replaces scales: hagfish pour slime that can clog a predator's gills, while many extinct armoured forms wore heavy bony or dentine shields. Almost all lack paired appendages, though fossil osteostracans and pituriaspids did grow paired fins later inherited by jawed descendants.
Physiology stays simple. They are ectotherms with a long, fairly uniform gut and no distinct stomach. Lampreys may scavenge or parasitize fish and marine mammals; anticoagulants in their secretions keep blood flowing. Fertilisation is external in lampreys; hagfish mating remains poorly known, and lifetime egg counts are guessed near thirty with scant parental care. About five hundred million years ago vertebrates invented two styles of adaptive immunity; jawless lineages use leucine-rich-repeat receptors rather than the V(D)J immunoglobulin system of jawed vertebrates.
Calling the whole assemblage Agnatha is convenient history more than neat cladistics. Extinct conodonts and cephalaspidomorphs fill early Palaeozoic seas in the same jawless grade. Modern work keeps hagfish and lampreys together as cyclostomes while treating extinct jawless groups as successive branches on the road to gnathostomes. The enduring lesson is that jaws were an upgrade, not a requirement for vertebrate success—cyclostomes still patrol cold oceans with rasps, slime, and ancient immune tricks.
Source: Agnatha
