• 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.
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