Snakes can be described as elongated, legless reptiles of the order Serpentes. Snakes are different from similar-looking reptiles, such as legless lizards, because they have no eyelids or external ears.
There are 18 snake families, all of which have unique traits. For instance, boas of the family Boidae have primitive characteristics, such as a nonmobile lower jaw and a pair of vestigal spurs near the anal vent. Sea snakes of the family Hydrophiidae are known for their ability to swim and their eel-like body structure. As of 2014, there are more than 2,900 known species of snakes, with only 375 species being venomous.