Marsh frog internal structure and amphibian reproduction

In spring frogs wake up and males call loudly. The female lays 5000–10,000 eggs in water and the male pours sperm over them – fertilisation is external, in water. A tadpole hatches: fish-like with a tail, breathing with external (later internal) gills, with a two-chambered heart and a single circuit. In development the hind legs appear first, then the front legs, gills are replaced by lungs, the circulation is rebuilt, the tail disappears and the young frog leaves for land; this is metamorphosis. Frogs mature in three years. The class has thousands of species (over 8000 by current counts), divided into tailless (frogs, toads) and tailed (newts, salamanders) orders; tailed ones are not found in Uzbekistan. The green toad is active at night, has rough skin that makes a smelly toxic fluid, and lays eggs in strings. Frogs and toads help by eating pest insects. Amphibians arose from lobe-finned fishes about 370 million years ago (in the Devonian period).