by Sandra Petersen

Created on: November 21, 2007

Shh! Don't tell anybody, but that frog your son just picked up from the edge of the pond has led a double life. Frogs and their similar cousins, the toads, are the largest group of the animal class Amphibia. They share some unique characteristics with the other animals we call amphibians.

The "double life" mentioned in the first paragraph is reflected in the Greek words forming the name amphibian. "Amphi" means both and "bios" means life. Most amphibians, upon hatching from their gelatinous shell-free eggs, spend a few months in a larval stage in the water, a short time in the transitional stage called metamorphosis, and years on land as adults. No other animal leads this type of double life. No other animal lays eggs that are like this.

According to amphibiaweb.org, there are 6,239 species of amphibians worldwide, although other sources say the number is closer to 5000. The Class Amphibia includes the members of three distinct orders: the frogs and toads (Order Anura) which comprise about 80 per cent of all amphibians, the salamanders and newts or mud puppies (Order Caudata), and the caecilians, legless snake-like creatures (Order Gymnophiona, which means "naked snake").

Besides their lives on both water and land, all amphibians are ectothermic, or cold-blooded. Simply phrased, the internal body temperature of an amphibian is controlled by its external environment. When the temperature drops, the amphibian moves sluggishly because it can not create body heat. In fact, the amphibian may go into a hibernation type state called torpor. Cold-bloodedness is shared by other animals like insects and reptiles.

Most amphibians have skin that is thin, moist, and smooth, and that has no scales, feathers, or fur. One exception to this is the warty skin of a toad. The skin is thin so that water and oxygen can pass in and out of it. Most adult amphibians breathe through their skin besides their lungs in a process called cutaneous respiration. Amphibians have special glands in their skin that secrete a mucous to keep the skin moist. In some cases the skin serves as camouflage or acts as a defense against predators or an inhospitable environment.

Most adult amphibians are tetra-pods, that is they have four limbs. Their webbed feet have no claws. Of course, caecilians have no legs, although researchers speculate that at some time in its ancestry, this order had legs but lost them as they began living underground.

The heart of an amphibian is composed of three chambers: one ventricle

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