INTRODUCTION

With over 4000 species described, the class Amphibia represents a significant contribution to the diversity of vertebrate life on earth. Amphibians occupy an important ecological niche in which energy is transferred from their major prey item, invertebrates, to their predators, primarily reptiles and fish (Stebbins & Cohen 1995).

The first amphibian fossils date back approximately 350 million years. Current evidence indicates that they descended from a group of fish similar to the coelacanth (Latimeria chalumnae) (Boutilier et al. 1992; Wallace et al. 1991). These fish had functional lungs and bony, lobed fins that supported the body. Further refinements of these fea­tures allowed amphibians to be the first group of verte­brates to take on a terrestrial existence. The class name Amphibia (derived from the Greek roots amphi, meaning “both,” and bios, translated as “life”), refers to the dual stages of life: aquatic and terrestrial.

Multiple features support the role of amphibians as an evolutionary step between fish and reptiles. The 3­chambered heart represents an intermediary between the 2-chambered piscine model and the more advanced 3­chambered heart of the reptiles.

The trend toward terrestrial life is also evident in the respiratory system. Most species have aquatic larval forms where gas exchange occurs in external gills. Metamorphosis to the adult, usually a terrestrial form, results in the develop­ment of lungs. These primitive lungs are relatively ineffi­cient compared to those of other terrestrial vertebrates, and respiration is supplemented by gas exchange across the skin. Secretions of the highly glandular skin help to main­tain a moist exchange surface; however, amphibians are restricted to damp habitats.

Most amphibians are oviparous, similar to fish and most reptiles. Though their eggs must not be laid in completely aquatic environments, the ova lack the water-resistant membranes or shell of reptiles and birds, thus they must be deposited in very damp places to avoid desiccation.

Amphibians

The larval stages rely on fins to move through their aquatic environment, in a manner similar to fish. Metamorphosis includes the development of legs for terrestrial locomotion (Figs. 1.1-1.6). The dual life cycle remains evident as the limbs of many amphibians remain adapted, for instance with webbing between the toes, for aquatic locomotion.