Bony fishes and the marsh frog’s external structure

Amphibians were the first animals to come onto land: they move on four legs, breathe with lungs and skin, have a three-chambered heart and lay eggs in water. The marsh frog lives in damp places and by water and never strays far from it; in winter it sleeps torpid in mud or a hollow. By day it waits motionless near the shore, notices only moving prey and catches it with a shot of its sticky tongue. The head is movably joined to the body and the neck is not visible. Movable eyelids protect and moisten the eyes; the nostrils pass air and a valve closes them when diving. The hearing organ is the eardrum behind the eye; males have vocal sacs that amplify sound. The front legs have four toes, the hind legs are long and strong with webbing between the toes: it jumps on land and swims in water. The skin is kept moist with mucus. The skeleton resembles a fish’s but the vertebral column joins the skull movably through one neck vertebra, there are no ribs or rib cage, and the tail bone is single. The limbs connect to the spine via the shoulder and pelvic girdles and have strong muscles.