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insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are around 7.77 million animal species. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology

Most living animal species are in

fossil record as marine species during the Cambrian explosion, which began around 539 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago

Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between taxa. (Full article...)

classification, habits, and distribution of all animals, both living and extinct, and how they interact with their ecosystems. Zoology is one of the primary branches of biology. The term is derived from Ancient Greek ζῷον, zōion ('animal'), and λόγος, logos ('knowledge', 'study'). (Full article...

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articles, which meet a core set of high editorial standards.

common ancestor
species that was already capable of flight.

However, there are alternate hypotheses which conclude that bats are
polyphyletic. The flying primate hypothesis was created in the 1980s stating that, based on morphological evidence, the Megachiroptera evolved flight separately from the Microchiroptera, although genetic evidence supports the monophyly of bats. This model states that fruit bats and primates share several anatomical features not seen in microbats, and are thus more closely related; for example, their brains show a number of advanced characteristics that link them to primates. (Full article...
List of selected articles

  • ...that six new species of marine slugs in the genus Phyllodesmium (Sp. kabiranum pictured) have been described in the last two years?
  • ...that
    Maui's dolphin
    is the most endangered species of dolphin in the world, with only about 110 left?
  • ...that the land snail Euglandina rosea is a significant threat to Hawaiian freshwater snail known as Newcomb's snail (Erinna newcombi), because the predatory Euglandina is able to hunt Erinna under water?
  • ...that the body of the "
    X-ray fish" (Pristella maxillaris ) is so transparent that it is possible to see its backbone
  • ...that even though the lancelets are classified as chordates, they lack a true backbone and well-defined head?

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The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species,[1] along with their principal habitats (terrestrial, fresh water,[2] and marine),[3] and free-living or parasitic ways of life.[4] 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.[5] 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.[6][7][a]

Phylum Example No. of
Annelids 17,000[1] Yes (soil)[3] Yes[3] 1,750[2] Yes 400[4]
wasp 1,257,000[1] 1,000,000
94,000[2] Yes[3] >45,000[b][4]
Bryozoa 6,000[1] Yes[3] 60–80[2] Yes
green spotted frog facing right 65,000[1]


Yes 40
Cnidaria Table coral 16,000[1] Yes[3] Yes (few)[3] Yes[3] >1,350
7,500[1] 7,500[1] Yes[3]
snail 85,000[1]


Yes[3] >5,600[4]
25,000[1] Yes (soil)[3] 4,000[5] 2,000[2] 11,000[5] 14,000[5]
29,500[1] Yes[14] Yes[3] 1,300[2] Yes[3]




Rotifers 2,000[1] >400[16] 2,000[2] Yes
10,800[1] Yes[3] 200-300[2] Yes Yes[17]
Total number of described extant species as of 2013: 1,525,728[1]


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  1. ^ The application of DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000 insect species for Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (Cecidomyiidae).[8]
  2. ^ Not including parasitoids.[4]

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