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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) and have complex interactions with each other and their environments, forming intricate food webs. The category 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 the echinoderms and chordates (including 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.

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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Golden toad
Credit: Charles H. Smith, USFWS

The golden toad (Bufo periglenes) is an extinct species of true toad that was once abundant in a small region of high-altitude cloud-covered tropical forests, about 30 km2 (12 sq mi) in area, above the city of Monteverde, Costa Rica. The last reported sighting of a golden toad was on 15 May 1989. Its sudden extinction may have been caused by chytrid fungus and extensive habitat loss.

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When the fox dies, fowls do not mourn.

—Anonymous

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A male Tasmanian devil in an aggressive posture
The Tasmanian Devil is a carnivorous marsupial found exclusively on the Australian island of Tasmania. At the size of a small dog, but stocky and muscular, the Tasmanian Devil is the largest carnivorous marsupial in the world. The devil is characterised by its black fur, offensive odour when stressed, extremely loud and disturbing screech, and vicious temperament when feeding. Known to hunt, as well as to scavenge carrion, communal eating is one of the few social activities in which the usually solitary devil participates. The Tasmanian Devil became extinct on the Australian mainland about 400 years prior to European settlement in 1788. The people of Tasmania saw devils as a threat to livestock and hunted them until 1941, when the animals were officially protected. Since the late 1990s devil facial tumour disease has reduced the devil population significantly and threatens the survival of the species. The impact of the disease on devil population may lead to listing of the devil as an endangered species.

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