Amphibians are cold-blooded tetrapods with moist skin that lacks, hair, feathers and scales. There are three living groups of amphibians, Anura (the frogs and toads), Urodela (the newts and salamanders) and Gymnophiona (the caecilians). See this page for more information on these groups.
The name amphibian comes from the Greek "amphi-bios" which literally means two or double life. Amphibians have a two-stage life cycle, a larval stage, usually spent in or around water when they use gills for respiration, and an adult stage, when lungs are used. These two phases are separated by a period of metamorphosis, or change. Another difference between the adults and the larvae is that the adults have a shortened gut and are usually carnivorous while the larvae tend to be herbivorous.
Fertilization occurs outside the body in amphibians. The female lays her eggs, usually in water or a moist environment, and the male then deposits his sperm on them. The egg then develops into the larvae. While this is the norm some species do not lay eggs and give birth to live young, often fully metamorphosed, such as in many caecilian species or in the frog genus Eleutherodactylus which gives birth to miniature adults.
Amphibians use three methods of respiration as adults:
As mentioned before, there is a fourth method of respiration, namely the use of gills. This is only seen in larval forms and a few adults, especially rare aquatic forms that retain a larval appearance even as adults.
Other amphibian characters:
Skin structure - Permeable to allow cutaneous gas exchange but it needs to be kept moist so also has mucus-producing glands. The skin also contains other glands including those that produce hormones involved in mate attraction as well as well as some that produce toxins. Amphibians that produce toxins are often quite brightly coloured as a warning to potential predators. The skin is also used in regulating water and salt balances as well as body temperature.
Pedicellate teeth (see left) - the crown and base of amphibian teeth are made of dentine and are separated by a narrow layer of uncalcified dentine. A few amphibians lack pedicellate teeth.
Operculum-plectrum complex - Two bones that are involved in transmitting sound to the inner ear.
Papilla amphibiorum - A special sensory area in the inner ear that is sensitive to frequencies below 1000 Hz.
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