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Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.

Living snakes are found on every continent except Antarctica (and other polar regions), some large islands such as Ireland, New Zealand, and many small islands of the Atlantic and central Pacific. Fifteen families are currently recognized, comprising 456 genera and over 2,900 species. They range in size from the tiny, 10 cm-long thread snake to the Reticulated python of up to 8.7 meters (29 ft) in length. The fossil species Titanoboa cerrejonensis was 15 meters (49 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the mid-Cretaceous period, and the earliest known fossils date to around 112 Ma ago. The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma ago).

London Zoo is the world's oldest scientific zoo. It was opened in London on 27 April 1828, and was originally intended to be used as a collection for scientific study. It was eventually opened to the public in 1847. Today it houses a collection of 755 species of animals, with 16,802 individuals, making it one of the largest collections in the United Kingdom.

It is managed under the aegis of the Zoological Society of London (established in 1826), and is situated at the northern edge of Regent's Park, on the boundary line between City of Westminster and Camden (the Regent's Canal runs through it). The Society also has a more spacious site at ZSL Whipsnade Zoo in Bedfordshire to which the larger animals such as elephants and rhinos have been moved. As well as being the first scientific zoo, ZSL London Zoo also opened the first Reptile house (1849), first public Aquarium (1853), first insect house (1881) and the first children's zoo (1938).

ZSL receives no state funding and relies on 'Fellows', 'Friends', 'Members', entrance fees and sponsorship to generate income.

Reptiles (Reptilia) are members of a class of air-breathing, ectothermic (cold-blooded) vertebrates which are characterized by skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors. Modern reptiles inhabit every continent with the exception of Antarctica. Reptiles originated around 320-310 million years ago during the Carboniferous period, having evolved from advanced reptile-like amphibians that became increasingly adapted to life on dry land. Four living orders are typically recognized:

Aves (birds), which are not cold-blooded or scaly, are not included in this list. Crocodiles are more closely related to birds than they are to lizards, however, so some writers who prefer a more unified (monophyletic) grouping also include the birds: over 10,000 species.

Unlike amphibians, reptiles do not have an aquatic larval stage. As a rule, reptiles are oviparous (egg-laying), although certain species of squamates retain the eggs until hatching and a few are viviparous. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm (0.7 in) to the saltwater crocodile, Crocodylus porosus, which may reach 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).