Millions of different kinds of animals live on Earth. Animals are found throughout the world, from the freezing polar zones to the hottest deserts. They live on land and in the water. Animals come in a huge variety of shapes and sizes, ranging from tiny insects and worms to giant elephants and whales. Like plants, animals are living things. They need food and water to live. Unlike plants, which make their own food, animals feed themselves by eating plants or other animals. In addition, animals differ from plants because they can sense what goes on around them. Their bodies allow them to move around in reaction to their surroundings. They use their senses and movement to find food, mates, and safety. Animals can be divided into many groups. One of the easiest ways to group animals depends on whether they have a backbone. Animals that have a backbone are called vertebrates. Animals that do not have a backbone are called invertebrates. About 95 percent of all animals are invertebrates. InvertebratesSome invertebrates have soft, unprotected bodies—for example, sponges, jellyfish, and worms. Most soft-bodied invertebrates have no legs. Many live in water, but some worms live on land. Other invertebrates have a tough skin to protect their bodies. The tough skin of echinoderms is covered in spines, or stiff points. Echinoderms include starfish, sand dollars, and sea urchins. These animals all live in oceans. Mollusks are invertebrates that often have a hard shell around their soft body—for example, oysters and snails. Squid and octopuses are also mollusks even though they lack a shell. Many mollusks live in water, but some live on land. Invertebrates with jointed legs are called arthropods. They have a hard covering called an exoskeleton. Their bodies are divided into segments, or sections. Examples of arthropods are insects, spiders, crustaceans, and centipedes. Arthropods live on land and in water all over the world. They make up more than three fourths of all types of animals on Earth. VertebratesFish are the oldest known vertebrates. They live in water and are often covered in scales. Most fish have a bony skeleton. However, sharks and rays have a skeleton made of cartilage, a strong and flexible tissue. Animals that live part of their life on land and part of their life in water are called amphibians. Amphibians include frogs and salamanders. Most young amphibians live in water and breathe with gills. As they grow, they form lungs and legs and move onto land. The first animals to live completely on land were reptiles—for example, lizards, snakes, and crocodiles. Dinosaurs were reptiles, too. Reptiles have dry, scaly skin. Some reptiles, such as turtles, spend a lot of time in water, but they breathe air. Animals that have wings and feathers are called birds. Examples include hummingbirds, pigeons, and hawks. Most, but not all, birds can fly. Many live on land and in trees. Some, such as ducks and penguins, spend a lot of time in and around water. Animals that feed their babies with milk from the mother are called mammals. Mammals include mice, deer, seals, monkeys, and humans. Mammals live on land in all parts of the world. A few types, such as whales, live in the ocean but still must breathe air. Mammals develop inside their mother's body and grow hair at some point. They have highly developed brains that help them to learn from their behavior. |