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Animals can adapt to life in extreme habitats. They belong to the consumers, most of them can move freely and have a nervous system. In contrast to plant cells, animal cells have only a thin cell membrane.

Characteristics of Animals

Animals are unable to manufacture their own food from simple chemical compounds and, therefore, have to consume other organisms, living or dead, as their main carbon source (= heterotrophic), have cells each containing two sets of chromosomes (= diploid), are made up of many cells and develop usually (except sponges) from a blastula. The blastula, a multicellular embryo that develops from the diploid zygote produced by fertilization of a large haploid egg by a smaller haploid sperm, is unique to animals. Their cells have only a thin cell membrane. Multicellularity is not unique to animals, but it is most divers in animals with many cells having highly specialised functions being grouped into tissues and these tissues into organs. Nervous systems and brains are unique to animals. Cells can be linked, with animals showing two unique types of cell-to-cell connections (desmosomes and gap junctions). Most animals take up food through an oral opening. However, some have acquired symbionts. Most animals are freely movable.

Importance of Animals

Animals show the most elaborated forms of behaviour, e.g. attraction to light, avoidance of noxious chemicals, sensing of smells and temperature and are morphologically the most divers of all organisms. They have developed a compact outer form and big inner surfaces for gas and material exchange. They range from tiny microbes, which can only be seen in the microscope and which form the heterotrophic fraction of organisms that passively drift, maintained in suspension by water current or float or swim weakly (the plankton) to the whales which are the largest animals today. The members of most animal phyla inhibit shallow waters. Probably more than 99.9% of all the animal species that have ever lived are extinct. Of all organisms, active flight evolved only in animals. From our skewed perspective animals are often divided into invertebrates (= without backbone, 98% of all living animals) and vertebrate (= with backbone, only one phylum: Craniata).
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Editors: Chief Editor; Lydia King
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added: 28 April 2003
updated: 24 June 2003
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