Amphibians (class Amphibia; from Greek αμφις "both" and βιος "life") are a taxon of animals that include all living tetrapods (four-legged vertebrates) that do not have amniotic eggs, are ectotherms, and generally spend part of their time on land. Most amphibians do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). There are around 6,000 described, living species of amphibians. The study of amphibians and reptiles is known as herpetology. Amphibians are able to breathe through their skin, making them very sensitive to anything they come in contact with, including human hands. When observing amphibians, one should never touch them.