Amphibian
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this.
The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. They are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia.
The three modern orders of amphibians are Anura (the frogs), Urodela (the salamanders), and Apoda (the caecilians). The number of known amphibian species is approximately 8,000, of which nearly 90% are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) South China giant salamander (Andrias sligoi), but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.
The word amphibian is derived from the Ancient Greek term ἀμφίβιος (amphíbios), which means 'both kinds of life', ἀμφί meaning 'of both kinds' and βιος meaning 'life'. The term was initially used as a general adjective for animals that could live on land or in water, including seals and otters.[3] Traditionally, the class Amphibia includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest sense (sensu lato) was divided into three subclasses, two of which are extinct:[4]
The actual number of species in each group depends on the taxonomic classification followed. The two most common systems are the classification adopted by the website AmphibiaWeb, University of California, Berkeley and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database "Amphibian Species of the World".[6] The numbers of species cited above follows Frost and the total number of known amphibian species as of March 31, 2019 is exactly 8,000,[7] of which nearly 90% are frogs.[8]
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(Andrias japonicus), a primitive salamander
(Triturus dobrogicus), an advanced salamander
- Mucous gland
- Chromatophore
- Granular poison gland
- Connective tissue
- Stratum corneum
- Transition zone
- Epidermis
- Dermis
(Ceratophrys cornuta)
- Internal gills where the blood is reoxygenated
- Point where the blood is depleted of oxygen and returns to the heart via veins
- Two chambered heart
- Right atrium
- Liver
- Aorta
- Egg mass
- Colon
- Left atrium
- Ventricle
- Stomach
- Left lung
- Spleen
- Small intestine
- Cloaca
- Jelly capsule
- Vitelline membrane
- Perivitelline fluid
- Yolk plug
- Embryo
(Ambystoma macrodactylum)
(Ambystoma macrodactylum)
(Ichthyosaura alpestris)
(Ambystoma gracile) eating a worm