Amphibian
Tagged: cat, fishAny member of the class Amphibia, vertebrates distinguished by their ability to exploit both aquatic and terrestrial habitats. They include the frogs and toads, salamanders and newts, and caecilians. The name, derived from the Greek amphibios meaning “living a double life,†reflects this dual life strategy. Despite this distinction, however, some species are permanent land dwellers, while other species have a completely aquatic mode of existence. Being the earliest tetrapods to adapt to a terrestrial existence, primitive amphibians are regarded as intermediary life-forms between fishes and reptiles. Modern amphibians are not, however, strictly transitional in their morphology; during their successful radiation throughout the world, they have achieved a variety of modifications that do not exemplify this intermediate status but are specific adaptations to their environment. One such example is the skin, which is kept moist by mucus-secreting glands and is involved in respiration and maintenance of water balance.
Members of the three extant orders of amphibians, Anura (frogs and toads), Caudata (salamanders and newts), and Apoda (or Gymnophiona of some authors; caecilians), differ markedly in their structural appearance. Frogs and toads (anurans) are tailless, somewhat squat amphibians with long, powerful hindlimbs modified for leaping. Salamanders and newts have tails and two pairs of limbs of roughly the same size and have less-specialized structures than the other two orders. Caecilians are limbless, wormlike, and highly adapted for a burrowing existence.
Traditionally, these orders have been united in one class by the feature unique to them among all tetrapods, the anamniotic egg. Other general defining characteristics include glandular skin that lacks epidermal structures such as hair or feathers, two lungs, a three-chambered heart, and a biphasic lifestyle common to most groups in which aquatic larvae metamorphose into adult forms.
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