Animals
Various animals
Scientific classification
Domain: Eukaryota
Kingdom: Animalia

Animals are eukaryotic organisms with many cells. They do not use light to get energy as plants do. The study of animals is called zoology. Animals use different ways to get energy from other living things. They usually eat other living things, but some are parasites or have photosynthetic protists as symbionts.

Plants are also multicellular eukaryotic organisms, but 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.

Grouping animals

Sympetrum flaveolum - side (aka)
Yellow-winged darter, Sympetrum flaveolum

There are many 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. They think several million species exist but they 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:

Some invertebrates are:

Numbers and habitats

The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.

Phylum Example No. of
Species
Land Sea Fresh
water
Free-
living
Parasitic
Annelids Nerr0328.jpg 17,000 Yes (soil) Yes 1,750 Yes 400
Arthropods wasp 1,257,000 1,000,000
(insects)
>40,000
(Malac-
ostraca)
94,000 Yes >45,000
Bryozoa Bryozoan at Ponta do Ouro, Mozambique (6654415783).jpg 6,000 Yes 60-80 Yes
Chordates green spotted frog facing right 65,000
45,000

23,000

13,000
18,000
9,000
Yes 40
(catfish)
Cnidaria Table coral 16,000 Yes Yes (few) Yes >1,350
(Myxozoa)
Echinoderms Starfish, Caswell Bay - geograph.org.uk - 409413.jpg 7,500 7,500 Yes
Molluscs snail 85,000
107,000

35,000

60,000
5,000
12,000
Yes >5,600
Nematodes CelegansGoldsteinLabUNC.jpg 25,000 Yes (soil) 4,000 2,000 11,000 14,000
Platyhelminthes Pseudoceros dimidiatus.jpg 29,500 Yes Yes 1,300 Yes >40,000
Rotifers 20090730 020239 Rotifer.jpg 2,000 >400 2,000 Yes
Sponges A colourful Sponge on the Fathom.jpg 10,800 Yes 200-300 Yes Yes
Total number of described species as of 2013: 1,525,728

Life styles

Gombe Stream NP Mutter und Kind
In Gombe Stream National Park, male chimpanzees remain in their natal community while females disperse to other groups

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 mammal like a 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. The study of ancient life is called palaeontology.

Related pages

Images


Animal Facts for Kids. Kiddle Encyclopedia.