When animals colonized terrestrial habitats, they had to adjust to the fluctuating temperatures, the replacement of water with air and the increased level of oxygen. Terrestrial animals adapted to these challenges by developing different metabolic systems, employing thermoregulatory behaviors, developing desiccation-resistant skin or exoskeletons. Additionally, terrestrial animals generally use different locomotor strategies than aquatic organisms, although there is some overlap. For instance, some fish “walk” along the bottom of ocean.
Amphibians, whose name means “dual-life,” are excellent examples of animals that are simultaneously adapted to terrestrial and aquatic existences. Frogs, for instance, have skins that are best suited for the aquatic environment, and most species must either live near the water or develop external secretions that prevent dehydration. Frogs are skilled swimmers, but they've evolved very efficient locomotor methods for use on land; some even live in the trees. Finally, many frogs breathe oxygen through both the air and the water, which enables them to live in both ecosystems.
The two types of animals most successful in colonizing terrestrial habitats were vertebrates and arthropods. Arthropods derive support from their strong exoskeletons, which enabled them to overcome the difference in density between water and air. Air is much less dense than water, so the body must be more rigid.