Animals (or Metazoa) are living creatures with many cells. They are eukaryotes. Animals get energy from other living things. Usually they eat them or are parasites. Some have photosynthetic protists as symbionts.
Most animals are mobile, meaning they can move around. Animals take in oxygen, and give out carbon dioxide. This cellular respiration is part of their metabolism (chemical working). In both these ways they are different from plants. Also, the cells of animals have different cell membranes to other eukaryotes like plants and fungi.
There are many different types of animals. The common animals most people know are only about 3% of the animal kingdom. When biologists look at animals, they find things that certain animals have in common. They use this to group the animals in a biological classification. Several million species may exist, but biologists have only identified about one million.
Animals can mainly be divided into two main groups: the invertebrates and the vertebrates. Vertebrates have a backbone, or spine; invertebrates do not. Vertebrates are the only group to have an adaptive immune system, which may be partly responsible for their size and success.
Some invertebrates are:
The animal mode of nutrition is called heterotrophic because they get their food from other living organisms. Some animals eat only plants; they are called herbivores. Other animals eat only meat and are called carnivores. Animals that eat both plants and meat are called omnivores.
The environments animals live in vary greatly. By the process of evolution, animals adapt to the habitats they live in. A fish is adapted to its life in water and a spider is adapted to a life catching and eating insects. A mammal living on the savannahs of East Africa lives quite a different life from a dolphin or porpoise catching fish in the sea.
The fossil record of animals goes back about 600 million years to the Ediacaran period, or somewhat earlier. During the whole of this long time, animals have been constantly evolving, so that the animals alive on Earth today are very different from those on the edges of the sea-floor in the Ediacaran.
In scientific usage, humans are animals. But in everyday use, humans are often not regarded as animals.
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