Although amphibians spend a great deal of time on land as adults, they have never completely broken their bond to water and become true land vertebrates.
The Challenges of Leading a Double Life
As adults, amphibians are enclosed within a thin, porous skin that performs necessary respiratory functions. To function properly (gas exchange), the skin must remain moist at all times. However, this integumentary system presents serious environmental challenges to amphibians whose skin is exposed to air.
As land animals, amphibians are in constant danger of desiccation (drying) and suffocation (Their lungs are quite small and cannot provide all the oxygen required.) Thus, adult amphibians, with only rare exceptions, spend most of their time in or near water and are found on land only in very damp areas or close to standing water.
Eggs are almost always laid and develop in water. The larval form that hatches from the egg is more like a fish than an amphibian in that they have a two-chambered heart, possess a single fleshy fin, and breathe through gills.
How Many Kinds of Amphibians Exist?
The living (extant) amphibians are but a mere remnant of a once much larger group. Today there are three main groups of amphibians found on this planet:
Anura – approximately 5,600 species of frogs and toads
Caudata or Urodela – approximately 571 species of salamanders and newts
Apoda or Gymnophiona – approximately 175 species of legless caecilians
The General Characteristics of Amphibians
The living amphibians have very different body forms but the members of the various orders share the following characteristics:
- A bone endoskeleton with varying numbers of vertebrae; ribs present in some, absent or fused to vertebrae in others. None possess an exoskeleton and the notochord (a rod of tissue) does not persist in adults.
- A smooth, thin, porous skin containing both mucus glands and poison glands.
- Four limbs (tetrapod) which may vary in size with the forelimbs of some being much smaller than the hindlimbs; some are legless. Limbs have varying numbers of digits and webbed feet are often present; no true nails or claws.
- The mouth is usually large with small teeth in upper or both jaws; two nostrils open into the anterior part of the mouth cavity.
- Respiration is accomplished either separately or in combination by lungs, skin, and gills; some larval types possess external gills and these may persist throughout life.
- A three-chambered heart consisting of two atria and one ventricle.
- Body temperature regulation is ectothermic (cold-blooded) in nature. That is, the body is heated from without rather than from within (endothermic).
- Separate sexes (dioecious) with internal fertilization via spermatophore (a sperm packet on a stalk) in salamanders and caecilians, but external fertilization in frogs and toads. Larvae develop in water or very moist environments and undergo complete metamorphosis. Amphibians are the only vertebrates to undergo complete metamorphosis.
Pity the poor amphibian for they know not biologically what they are. They begin life as a legless animal with gills and tail like a fish and end up a with four legs and lungs like a reptile. Yet clearly they are neither fish nor reptile. They are amphibians, the only vertebrates that lead a double life.
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