Facts At Your Fingertips: The living world – Types of animal
There are millions of animal species living on Earth.
They are divided into two main groups: vertebrates, such as mammals and fish, which have a backbone; and invertebrates, such as insects and mollusks, which do not have a backbone.
VERTEBRATES
There are five main groups of vertebrates—amphibians, reptiles, fish, birds, and mammals. They all have internal skeletons but come in all shapes and sizes and live in very varied places.
Amphibians, such as frogs, lay their eggs in water and live both in the water and on land.
Reptiles, including alligators, have scaly skin and lay their eggs on land.
Fish, such as goldfish, spend all their lives in water, where they can breathe using gills.
Birds, including eagles, have feathers and many of them can fly.
Mammals are warm-blooded, have hairy skin, and feed their young on milk. They include tigers and humans.
INVERTEBRATES
About 97 percent of all known animal species are invertebrates. Insects and crustaceans have hard external skeletons and bodies made from segments.
Starfish have simple bodies with bony plates just under the skin, while soft-bodied worms and squid do not have a skeleton at all.
Anthozoa, such as coral, fix themselves in one place and feed on algae or plankton.
Asterozoa, including starfish, are star-shaped, with arms growing out of a central disk.
Arachnids have eight legs, which have joints. They include spiders and scorpions.
Gastropods include snails and slugs. They move around using one muscular foot.
Malacostraca, such as crabs, have shells and heads made of five segments.
Insects have six legs, and many have two pairs of wings. They include butterflies and ants.