What is Class Amphibia ?
Class Amphibia takes its name from the Greek words "amphi" and "bios." Amphi means "both sides," or "both types," and bios means "life." This name-both types of life-reflects the dual nature of these animals, which spend part of their lives in an aquatic habitat, and the other part in a terrestrial environment. Larval, or juvenile, stages of frogs are familiar to us as tadpoles. Tadpoles are totally aquatic filter-feeding herbivores that swim like fishes by using their tail; they have fish-like lateral line systems for sensing, and gills for breathing.
The amphibians are considered to be the first vertebrates that adapted to living on land successfully. They made the transition from water to land 360 - 350 million years ago during the Devonian Period. Lobe-finned fish and fishes with lungs like the extant (living today) coelocanths were adapted to survival in terrestrial conditions, and therefore are thought to be the ancestors of the tetrapods, or vertebrates with 2 pairs of limbs.
Besides being able to move about on land instead of swimming in water, ancestral amphibians had to evolve several other important adaptations in order to survive on land. For one thing, living out of the water meant having to breathe air with lungs, instead of using gills to absorb dissolved oxygen from the water. In addition, many amphibians rely on gas exchange through their skin surfaces to a large extent. Being out of the water meant also that the animals had to contend with gravity, since the support of water was no longer available. This necessitated the development of a more substantial, supportive skeleton.
Amphibians prevent excessive loss of body moisture in dry conditions by living relatively close to aquatic habitats or in rain forests. Most seek more humid shelter under moist leaves or by burrowing into mud. Amphibians such as frogs reproduce by laying eggs in the water, where they are fertilized by sperm. There are a variety of other methods of reproduction that include giving birth to young that develop inside the mother's body, but are not nourished by it; while others have young that are nourished inside the mother's body as they develop.
Whereas juvenile frog tadpoles are filter-feeders, adult frogs and amphibians in general are carnivores. Adult amphibians show evolutionary advances over the lobe-finned fishes and the lungfishes in their circulatory system. They have a circulation pattern that makes two circuits-one circulates blood between the three-chambered heart and lungs for oxygenation, and the other circuit carries oxygen-rich blood from the heart to the rest of the body. This circulation pattern provides extra time for the blood to be exposed to the oxygen in the lungs, making for more efficient gas exchange.
Amphibians have a pair of kidneys to rid their blood of wastes, and a bladder that stores urine to be excreted. Additionally, amphibians have well-developed eyes and effective camouflage to avoid detection from predators; some have toxic chemical poisons that afford large measures of protection. Often these species also have bright coloration that itself warns off potential enemies.