Respiration in animals

Animals, which belong to amphibia, reptilian, aves and mammalia, use the lungs for breathing. Amphibians breathe through their skin, when they are in water. Smaller animals have different organs for breathing.

Breathing in fish: Fish have a pair of gills. The breathe air; dissolved in water; through gills.

Breathing in  cockroach: Cockroach and other insects breathe through spiracles and tracheae. There is a network of hollow tubes running through the body of an insect. These hollow tubes are called trachea. Each trachea opens on the body surface in the form of very small pore. These pores are called spiracles.

Breathing in earthworm: Earthworms breathe through their skin. The skin of earthworm always remains moist, which helps in breathing.
Breathing in Unicellular Animals: Unicellular animals breathe through the cell membrane.