There are about 3100 species of snakes throughout the world. Only the Arctic regions, Antarctica, Iceland and Ireland, along with a few other very small oceanic islands lack the presence of snakes. The suborder Serpentes belongs in the order Squamata that contains both the lizards and snakes. There are about 18 recognized families of snakes containing over 400 genera (http://www.reptile-database.org/db-info/SpeciesStat.html; accessed 7/09). Snakes come in a variety of shapes and sizes but all have scales, are cylindrical in shape and lack legs and external ear openings. They range in size from about 10 cm to over 10 meters.
Anatomy and physiology
1. Most snakes have six rows of teeth, four are in the upper jaw attached to the palatine and maxillary bones and two in the lower jaw, one attached to each of the mandibles. All teeth can be periodically shed and replaced (including venom fangs).
2. Snakes lack "true" eyelids, which fuse during development to form a transparent structure that covers the eye called the spectacle. This spectacle or "lens cap" is shed when the snake under goes ecdysis.
3. Snakes lack an external ear opening, tympanum, and middle ear, but do have an inner ear that aids in balance. Snakes can detect substrate vibrations and at least some species can hear airborne sounds.
4. Some snakes, including the boids and vipers, possess heat-sensing pits located on the external surface of the face and jaws.
5. Snakes do not see particularly well but are good at detecting movement.
6. Snakes can extend their glottis anteriorly when swallowing prey. This adaptation for breathing is possible because of an elastic ligament at the mandibular symphysis.
7. Snakes frequently flip their tongues as a means of chemosensory detection. Most snakes possess "taste buds" in the roof of their mouth in a structure commonly referred to as "Jacobson's organ." The tip of the tongue flips scent particles up to this organ.
8. Most species have one functional lung (the right) and there is no diaphragm. The vestigial left lung forms an air sac in some species.
9. The stomach is fusiform (other reptiles have an "S" shaped stomach).
10. The liver is fusiform (other reptiles have bi-lobed livers) and like all reptiles a gall bladder is present. The pancreas is adjacent to the duodenum and the spleen may be attached to the pancreas.
11. Snakes have more than 200 vertebra (some more than 400). Each vertebra has a pair of ribs that protect the internal organs.
12. Snakes have two kidneys with the left kidney lying behind the right.
13. Locomotion is accomplished by moving the large ventral scales in groups, by lateral undulatory musculoskeletal contractions, or in some cases, both methods are employed.
a. More details can be found by reading: Funk RS. 2006. Snakes. In: Mader D.R. Reptile Medicine and Surgery, Second Edition. Elsevier/Saunders Co., Phila, 42-58.