Summary:
- The evolution of fish from hagfish to finned fish is a long and involved process.
- One step in this evolution involves the change in function of gills.
- Invertebrate chordates use their gills to filter food out of water, not to absorb oxygen.
- Gills consist of many thin, folded tissues that provide a large surface area for oxygen uptake.
- With more oxygen absorbed by the gills, fish could become much larger and more active.
- More than 1,000 species of fish are at risk of extinction because of human actions.