七鳃鳗 / Lamprey

No jaws, still swimming! I help scientists discover which parts of your body plan came before biting.
没有上下颌，照样游泳！我帮科学家寻找你身体构造中比咬东西更古老的部分。

Sources
Lamprey Hox genes and the evolution of jaws
https://www.nature.com/articles/nature02616
Heterotopic shift of epithelial-mesenchymal interactions in vertebrate jaw evolution.
https://www.science.org/doi/10.1126/science.1068310
Diversity and function of adaptive immune receptors in a jawless vertebrate.
https://www.science.org/doi/10.1126/science.1119420
Species reference
https://en.wikipedia.org/w/index.php?title=Lamprey

Research context
Lampreys provide a living comparison with jawed vertebrates. Their development and nervous systems help identify which traits predate jaws and which evolved later in the lineage that includes humans.