Presented below is a photomicrograph of stomach tissue of a common frog, Rana temporaria. Amphibian skin is thin, slippery, and permeable, and among the most unique in the animal kingdom. Most frogs rarely need to drink because their special skin easily absorbs the water they need, and a few have stomach skin so porous that it soaks up water from moist earth.

This smooth, delicate amphibian skin is also air-permeable, allowing oxygen to pass through. Although frogs have lungs, the ability to absorb oxygen from moist surroundings is especially useful to frogs that burrow deep or hibernate in underwater mud. Covered with mucus-secreting glands that help to keep their slippery skin stay moist and pliable, some frogs can also secrete a waxy substance to keep body water from evaporating.

Frogs are very edible and a favorite food in the ecosystem. In order to protect themselves, many secrete sticky, horrible-tasting or foul-smelling mucus, and some even secrete poisonous toxins. Indians in Central and South America coat the tips of blowpipe darts and arrowheads with toxins taken from such amphibians as the poison-arrow frog. Currently, scientists are investigating pharmacological uses of these toxins in treating human illnesses.

The earliest frog-related fossil dates back 245 million years and, after outliving dinosaurs, frogs have changed very little from their ancestors who roamed the earth 150 million years ago. These little creatures can be found almost everywhere on earth, atop freshwater lily pads, among the canopies of tropical rainforests, hidden in moist rock crevices on the forest floor, burrowed deep into the sands of the Australian desert, and hibernating in Arctic soil. Only the climate of Antarctica has proved too harsh for these cold-blooded vertebrates. Their skin, so adaptable to many environments has served them well. However, the lovely permeability of their skin also leaves them vulnerable to environmental toxins, such as pesticides and chemicals, which they easily soak up as embryos, tadpoles, and adults.