Amphibians (classAmphibia) are a group of animals that include all tetrapods (four-legged vertebrates) that do not have amniotic eggs. Amphibians generally spend part of their time on land, but they do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes). There are about 3000 living species of amphibians.
Amphibians developed with the characteristics of pharyngeal slits/gills, a dorsal nerve cord, a notochord, and a post-anal tail at different stages of their life. They have persisted since the dawn of tetrapods 390 million years ago in the Devonian period, when they were the first four-legged animals to develop lungs. During the following Carboniferous period they also developed the ability to walk on land to avoid aquatic competition and predation while allowing them to travel from water source to water source. As a group they maintained the status of the dominant animal for nearly 75 million years. Throughout their history they have ranged in size from the 15 foot long Devonian Ichthyostega, to the slightly smaller 6 foot long Eryops, and down to the tiny 1 centimeter long Psyllophryne didactyla, commonly known as the Brazilian Gold Frog. Amphibians have mastered almost every climate on earth from the hottest deserts to the frozen arctic, and have adapted to climatic change with ease.
Solomon Berg Martin, Biology
Duellman/Trueb, Biology of Amphibians
Traditionally the amphibians are taken to include all tetrapods that are not amniotes. Recent amphibians all belong to a single subgroup of these, called the Lissamphibia. Recently there has been a tendency to restrict the class Amphibia to the Lissamphibia, i.e. to exclude tetrapods that are not more closely related to modern forms than they are to modern reptiles, birds, and mammals.
Authorities disagree on whether Salientia is a Superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. In effect Salientia includes all the Anura plus a single Triassic proto_frog species, Triadobatrachus massinoti. Practical considerations seem to favour using the former arrangement now.
For the purpose of reproduction all amphibians are bound to water. Several species have adapted to arid and semi-arid environments, but most of them need water to lay their eggs. The larvae breathe with exterior gills. After hatching they start to transform gradually to the adult's appearance. This process is called metamorphosis. Typically, the animals then leave the water and become terrestrial adults, but there are some exceptions to this general way of reproduction.
The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on land. But there are several other changes:
The gills are replaced by other respiratory organs, e.g. lungs.
The skin changes and develops glands to avoid dehydration
The eyes get eyelids and adapt to vision outside the water
9;The Urodela are Amphibia with elongated bodies and relatively short limbs, devoid of scales or pectoral plates, with numerous præ-caudal vertebræ, and with amphiclous, or opisthoclous, vertebral centra.
The pectoral arch in the Amphibia is distinguishable into a scapular, a coracoidal, and a præcoidal region, although the extent to which these parts of the primitive cartilaginous arch become separately ossified varies very much in the different members of the group.
On the other hand, it is the male Alytes obstetricans which twists the strings of eggs laid by the female round his hind-legs and, thus cross-gartered, retires into seclusion until the young are ready to be hatched, when he resorts to the water in which the tadpoles are to perform their further metamorphoses.
AMPHIBIA, a zoological term originally employed by Linnaeus to denote a class of the Animal Kingdom comprising crocodiles, lizards and salamanders, snakes and Caeciliae, tortoises and turtles and frogs; to which, in the later editions of the Systema N aturae he added some groups of fishes.
In this he was followed by Dr J. Gray; but Dumeril and Bibron in their great work,' and Dr Gunther in his Catalogue, in substance, adopted Brongniart's arrangement, the Batrachia being simply one of the four orders of the classReptilia.
Huxley adopted Latreille's view of the distinctness of the Amphibia, as a class of the Vertebrata, co-ordinate with the Mammalia, A y es, Reptilia and Pisces; and the same arrangement was accepted by Gegenbaur and Haeckel.
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