Snakes occupy throughout the world except many islands and near the polar regions. The majority of snakes have a distribution in the tropics. They are well adapted to deserts as well as fresh and salt water bodies. Snakes are more closely related to lizards and can be distinguished by many distinctive features. The jaws are usually flexible and are loosely attached to the skull. With the exception of a few primitive snakes (where rudimentary hind limbs are present), all the snakes lack limbs. The body is covered with scales. The eyes are small and have no eyelids. The outer ear and tympanum are absent. There is no diaphragm. There is only one functional lung. The left lung is absent or vestigial in most snakes. Pythons, however, have developed left lungs. The long and cylindrical tongue is forked. Successor teeth lying below immediately replace the lost teeth. Teeth help in holding preys and assisting in swallowing. The poisonous snakes are characterized in having the highly specialized teeth in the upper jaw called fangs. The fangs are shed periodically and may be found in the faeces or on the bottom of the cage when they are kept in captivity. Reserve fangs are always present behind each functional fang. The fang has a poison gland (modified salivary gland) at its base and the venom produced by this gland flows through a groove or canal running through its centre. Another unique feature of the snake is the absence of urinary bladder. The c10aca holds the excretory products before voiding. Most species of snakes are non poisonous. The teeth of non poisonous snakes are short and solid and their bites leave a row of smooth marking.