There are 2,700 known snake species, and the reptiles all share the following characteristics:
- They have thin, linear and limbless bodies.
- They are meat-eaters (carnivores).
- They are cold-blooded (ectothermic), meaning their inner temperature varies with the temperature in the environment.
Snakes look like legless lizards for a reason -- the two reptiles make up the order Squamata, which is divided into the suborders Sauria for lizards and Serpentes (or Ophidia) for snakes. Because of their long shape, snakes' organs are arranged linearly, but they are otherwise similar to those of other vertebrates, including people. The bone-encased brain and sensory organs are contained in the head, and snakes have almost all the senses people do -- with a few interesting modifications:
- Hearing - Although snakes are not equipped with outer ears like people, sound waves from the air hit their skin and are transferred from muscle to bone. When the sound reaches the ear bone beneath the skull, it sends vibrations to the inner ear, and the sound is processed by the brain.
- Sight - Snakes do not see colors, but their eyes are equipped with a combination of light receptors: rods that provide low-light but fuzzy vision, and cones that produce clear images. The complexity of the eyes varies among species because of their different lifestyles. For instance, snakes that live primarily underground have smaller eyes that only process light and dark, but snakes that live above ground and hunt by sight have crystal-clear vision and good depth perception. Some species, specifically boas and pythons, have a second visual tool: Pit organs on their heads see heat sources in their surroundings like infrared goggles -- an effective ability for nocturnal hunters of warm-blooded animals.
- Smell - Like humans, snakes breathe airborne smells into nasal openings that lead to an olfactory chamber for processing; but snakes have a secondary system, as well. When a snake flicks its tongue, it is gathering odor particles for transfer to two fluid-filled sacs at the roof of the mouth -- Jacobson's organs -- that lead to a second, smaller olfactory chamber. The tongue is used only to assist in this process; snakes do not have a sense of taste.
The digestive tract runs nearly the entire length of the body and includes the mouth, esophagus, stomach, small intestine, large intestine and anus -- all stretchable to digest prey larger than a snake's diameter (See Feeding section). When the snake's mouth is full, it has to extend its trachea (breathing tube) below the food and out in order to keep breathing. Snakes do not have a diaphragm like people do, so they circulate air in and out of the lungs by narrowing the rib cage to push air out and then widening it again to create a vacuum to suck air in. After each breathing cycle, snakes experience apnea -- a stop in breathing -- that lasts from a few seconds to as long as a few minutes. To process the oxygen, all snakes have an elongated right lung; many also have a smaller left lung, and a few even have a third lung along the trachea.