The skin is smooth, usually some shade of green or brown, and often spotted. The bulging eyes have nictitating membranes to keep the eyes moist. Adult frogs have lungs, but their breathing mechanism is poorly developed. At rest they breathe mainly through the mouth lining, filling the lungs only occasionally. When in wet places they also absorb much dissolved oxygen through their skins.
Frogs have true voice boxes and are noted for their various sounds.
Frogs capture insects and worms with their sticky, forked tongue, attached at the front of the lower jaw. Some large tropical species eat small mammals and snakes. A few frogs have skin glands that can produce irritating or poisonous secretions. Most frogs hibernate in underwater mud and lay their eggs in early spring. With few exceptions fertilization is external.