Big Picture

Amphibians are vertebrates that spend their lives in both water and land. They’re ectothermic and have complex nervous and circulatory systems. Amphibians reproduce sexually, and they lay their eggs in the water. Eggs hatch into larvae with long tails that allow them to swim in the water. Amphibians are divided into three orders: frogs and toads, salamanders and newts, and caecilians. All amphibians are descendants of the lobe-finned fish.

Key Terms

Amphibian: Vertebrate that lives in both land and water.

Keratin: A tough protein in the skin, feathers, nails, and other parts of tetrapod vertebrates.

Tetrapod: Vertebrate with 4 limbs.

Ectothermy: Body temperature is controlled from the outside through behavioral changes such as basking in the sun.

Cloaca: Body cavity that stores waste/gametes and has an opening that allows them to leave the body.

Structure & Function

  • Amphibians have moist skin without scales
  • The keratin in the skin of amphibians is not very tough, so substances can pass easily through their skin
  • Amphibians are ectothermic, so they’re active when it’s warm and sluggish when it’s cold

Organ systems:

  • Complex circulatory and nervous systems with 3-chambered heart, lots of sense organs
  • Amphibians have digestive, excretory, and reproductive systems
  • All systems are connected to the cloaca

Reproduction & Development

  • All amphibians reproduce sexually, and fertilization happens internally or externally
  • Species have different ways of attracting mates (such as mating calls)
  • Eggs need to be laid in water and are covered in a jelly-like substance
  • When eggs hatch, larvae start off as tadpoles that swim in water, then grow into adults that can live on land
  • Tadpoles have tails, gills, and no legs
  • Adults are tetrapods that have lost the tails and developed lungs
Image Credit: Illustrations by Meyers Konversations-Lexikon, colored and arranged by TomCatX, Public Domain

Classification, Evolution, Diet


3 orders of amphibians:

  • Frogs and toads: Long back legs, adults don’t have
  • Salamanders and newts: long body, short legs, adults have tails
  • Caecilians: Worm-like body, no tails


  • Amphibians evolved from lobe-finned lungfish; they were the earliest land vertebrates
  • Amphibians live all over the world


  • Larvae eat small insects and algae
  • Adults are carnivores - eat insects, snails, worms, etc.