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Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.

Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 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 animal taxa.

Humans make use of many other animal species for food, including meat, milk, and eggs; for materials, such as leather and wool; as pets; and as working animals for power and transport. Dogs have been used in hunting, while many terrestrial and aquatic animals are hunted for sport. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.

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Minke whale penises

The Icelandic Phallological Museum, in Reykjavík, Iceland, houses the world's largest display of penises and penile parts. The collection of 280 specimens from 93 species includes samples from whales (pictured), seals and land mammals. Exhibits are preserved in formaldehyde and displayed in jars or are dried and hung or mounted on the museum's walls and in display cases. The largest item on display once belonged to a blue whale; the smallest, from a hamster, can only be seen with a magnifying glass. The museum claims that it has specimens from elves and trolls that cannot be seen at all since, according to Icelandic folklore, these creatures are invisible. In July 2011, the museum obtained its first human specimen, but the preservation process did not go according to plan and the museum hopes to acquire a "younger and a bigger and better" example. Founded in 1997 by a retired teacher, it attracts thousands of visitors a year—the majority of them women—and has received international media attention. According to its mission statement, the museum aims to enable "individuals to undertake serious study into the field of phallology in an organized, scientific fashion".

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White-necked petrel
Credit: JJ Harrison

The white-necked petrel (Pterodroma cervicalis) is a seabird in the family Procellariidae; adults measure some 43 centimetres (17 in) in length, with a wingspan of 95–105 centimetres (37–41 in). Although the species is found in much of the South Pacific, it breeds on only three islands and is thus considered vulnerable by the IUCN.

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When the fox dies, fowls do not mourn.

—Anonymous

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African elephant
Elephants are large mammals found in sub-Saharan Africa, and South and Southeast Asia. Traditionally, two species are recognised, the African elephant (pictured) and the Asian elephant, although some evidence suggests that African bush elephants and African forest elephants are separate species. The largest living terrestrial animals, male African elephants can reach a height of 4 m (13 ft) and weigh 7,000 kg (15,000 lb). Distinctive features include the trunk, used for many purposes, and tusks, which serve as tools and weapons. Females (or "cows") tend to live in family groups; males (or "bulls") leave their family groups when they reach puberty, and may live alone or with other males. Adult bulls mostly interact with family groups when looking for a mate. Elephants can live up to 70 years in the wild, and their intelligence has been compared to primates and cetaceans. African elephants are classed as vulnerable, while the Asian elephant is classed as endangered. Elephants are threatened by poaching for the ivory trade, habitat destruction and conflicts with local people. They are highly recognisable and have been featured in art, folklore, religion, literature and popular culture

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