Animals of the World

Water is home to a vast share of animal life, and fish are the most numerous group of vertebrates. They breathe through gills that extract oxygen from water, propel themselves with fins, and are usually covered in protective scales. Fish range from tiny species in streams and ponds to large predators of the open ocean, and they fill many ecological roles as grazers, hunters, and prey.

Amphibians such as frogs, toads, and salamanders bridge water and land. Most begin life as aquatic larvae that breathe with gills and later develop lungs as they mature, allowing the adults to live partly on land while returning to water to breed. Their moist skin plays a part in breathing, which makes them sensitive to changes in their environment and useful indicators of habitat health. The transformation from a gilled larva into an air-breathing adult, known as metamorphosis, is among the most striking life cycles in the animal world.

Beyond fish and amphibians, the animal kingdom includes reptiles, birds, mammals, and a huge variety of invertebrates such as insects and arachnids. Each group has adaptations suited to its way of life, from the camouflage of forest animals to the speed of open-country hunters. Many species today face pressure from habitat loss, and conservation programs work to protect those that are rare or endangered.

Frequently asked questions

How do fish breathe underwater?

Fish use gills, which extract dissolved oxygen from water as it flows over them, allowing the fish to breathe without coming to the surface.

Why do amphibians need both water and land?

Many amphibians begin life as aquatic larvae and breed in water, while the adults can live on land. Their moist skin also depends on damp conditions.

Why are amphibians sensitive to their environment?

Because they breathe partly through their skin and pass through an aquatic stage, amphibians are easily affected by pollution and habitat change, making them useful environmental indicators.