The group vertebrata is known to enclose class Amphibia that can be best represented by frogs, toad, salamanders, newts and caecilians. Amphibians are ectothermic tetrapods whose eggs are not surrounded by egg membranes. Most of the adults are known to lay eggs in water from which larvae with gills emerge out and undergo metamorphosis into lung bearing adult. However, mudpuppies and olms retain larval gills throughout their life while adults are also known to respire through the skin. The subclass Lissamphibia is known to contain three orders of modern amphibians namely Anura, Caudata and Gymnophiona. About 6,500 species of amphibians are known today. Superficially they resemble reptiles but repltiles along with birds and mammals have eggs surrounded by protective membranes. The study of amphibians is known as batrachology and they are good ecological indicators as a good proportion of them are now facing danger of being extinct. The early amphibians are believed to have evolved from Sacrcopterygians during the Devonian Period. They diversified and became dominant during the Carboniferous and Permian periods but were later replaced by reptiles, birds and mammals.
The word amphibian has descended from an ancient Greek word meaning dual mode of life. The term was initially used for the animals living both on land and water like otters and seals. Later in 1919, when scientific classification came into existence then the term was delivered to amphibians only. They are the vertebrates with four limbs. Typically they are known to inhabit freshwater not the marine environments except the one or two frogs that are known to dwell in the brackish water and mangrove swamps. They lay their eggs entangled in a gelatinous covering that generally swells when comes in contact with water. The larvae hatching out from the eggs are generally different in form when compared with adult. In frogs and toads they are known as tadpoles and are characterized by presence of a large head with dorsoventrally flattened tail. These tadpoles are herbivores and breathe through gills and lack limbs but later on these structures disappear and they undergo metamorphosis where they attain structures present in adults and become carnivorous. The newts and salamanders are long bodied creatures with feathery gills and are carnivorous with the fact that their front legs develop earlier than the hind legs. They lack metamorphosis like frogs and toads while caecilians either reproduce or produce eggs in damp burrows.
Amphibians are cold blooded organisms that maintain their body temperature above their surroundings which enable them to survive at higher temperatures. There is a great degree of sensitivity among the species living in different places. Many species are known to enter hibernation during the winter months in order to prevent their race from being extinct. In the colder climates the species are known to remain in a condition of hibernation for more than half of the year. In the hot summer months many species are known to undergo aestivation under the cool mud of the dried ponds in order to remain free from the heat of desiccation. Cold blooded nature enables them to remain alive during the periods of prolonged starvation.
The skin of amphibians is permeable to water and is loaded dual mode bluetooth module with numerous mucus glands that prevent the skin from drying. The skin also facilitates gaseous exchange allowing the amphibians to breathe when they undergo hibernation. The skin is prevented from damage by the predators many amphibians have evolved poison glands over the skin and the toxicity of the glands varies according to the species. The toxins secreted by some amphibians are fatal to human beings also but rest have a very little or mild effect. The glands responsible for the production of toxin are the paratoid glands that release bufotoxin and are located behind the ears of certain frogs and toads while in salamanders they are present just behind the eyes.
The integumentary structure is demarcated by the presence of certain dynamic structures typical of vertebrates for example, presence of highly cornified outer layers that undergo regular moulting and the process is controlled by the hormones released by the pituitary and thyroid glands. Warts or local thickenings are characteristics of toads. The outside of the skin is shed periodically in one piece while in mammals and birds it is shed in flakes and they are also known to eat the sloughed skin. Chromatophores also known as the pigment cells are responsible for the skin color of amphibians and are arranged in three layers. The three layers typically include the cells known as melaophores, guanophores and lipophores. Many species are also known to change the color of their skin and this is strictly under the control of pituitary glands. Very bright color typically indicates that the skin is loaded with poison glands.