Class Amphibia and the Frog Type Study

Class Amphibia and the Frog Type Study

Class Amphibia occupies a transitional position between the wholly aquatic fishes and the wholly terrestrial reptiles, and its members typically show a life cycle divided between an aquatic larval stage and an air-breathing, largely terrestrial adult stage (as in the frog's tadpole-to-adult metamorphosis).

Several structures make their first appearance, among the classes surveyed in this chapter, in Amphibia: a functional tongue (in many frogs, used to capture prey), eyelids and a nictitating membrane protecting the eye, a middle ear (tympanic membrane) for aerial hearing, and true, functional lungs, alongside the retained ability to respire through the moist skin (cutaneous respiration) — all structures adapted to a partly terrestrial existence, absent or undeveloped in the wholly aquatic Pisces.

The amphibian heart is three-chambered — two atria and a single ventricle — a change from the essentially two-chambered fish heart, reflecting the need to handle both a pulmonary (lung) and a systemic (body) circuit once lungs are present, even though the single ventricle still allows some mixing of oxygenated and deoxygenated blood. This differs from the reptilian heart, which is typically also three-chambered but usually shows a partial (in some reptiles, such as crocodilians, an almost complete) septum dividing the ventricle, reducing the mixing of oxygenated and deoxygenated blood compared with Amphibia — reptiles are thus a step further toward the fully separated, four-chambered hearts of birds and mammals. …