AMPHIBIA, the term applied to one of the six classes of vertebrate animals. This class includes the frogs, toads, sala manders and coecilians, forms which stand in the zoological scale midway between the fishes and the reptiles. Many aquatic ani mals, such as crocodiles and seals, are amphibious, but, as their organization shows, belong to some other class of vertebrates.
The term Batrachia is still employed by many students of sys tematic zoology, although Stejneger made it clear that this name is a pure synonym of the much older term Salientia of Laurenti. The arguments for and against the use of the term Amphibia have been given in detail by Boulenger but his conclusion in favour of the term Batrachia has not been accepted by the ma jority of herpetologists to-day.
Amphibia are cold-blooded vertebrates having a smooth or rough, but not scaly skin, very rich in glands. Most have four legs and live on land, but, unlike other tetrapods, they respire to a large extent through their skin. The eggs are usually laid in the water, the larva passing through an aquatic existence bef ore metamorphosing into the gill-less adult. The eggs, even when laid on land, are without a calcareous shell, and do not develop the amnion or allantois of higher vertebrates. Modern Amphibia are readily distinguished from reptiles by many differences of skele ton, but the most primitive fossils were so similar to contem porary reptiles that no sharp distinction can be made between them. Recent palaeontological finds have also tended to bridge the gap between Amphibia and the fishes, but here a complete series of intermediate forms is still lacking. Modern Amphibia are the specialized descendants of a great group of more or less aquatic forms living in late Palaeozoic and Triassic times. It is these extinct forms which represent the actual ancestors of rep tiles and to them one must turn for the ground plan of skeletal organization from which all higher types evolved.