Caecilians (Gymnophiona) are a group of slender-bodied, limbless amphibians that—at first glance—resemble snakes, worms or eels. Yet caecilians are only distantly related to such animals. Instead, their closest cousins include the better known amphibian groups: frogs and toads and newts and salamanders.
Most species of caecilians have lungs that enable them to take in oxygen from the surrounding air, although they also absorb additional oxygen through their mouth and skin. Two species of caecilians have no lungs and therefore rely entirely on the air they obtain through their skin and mouth.
Some species of caecilians are aquatic and have a fin that runs along their back that enables them to move through water efficiently. Other species of caecilians are primarily terrestrial and spend much of their time burrowing underground as they hunt using their sense of smell.
Modern caecilians have little use for their sense of sight and many species have partially or entirely lost their vision. They have a skull that is pointed and consists of strong, fused bones—both adaptations that enable caecilians to bore through mud and soil. Their skin appears to be segmented due to a series of ring-like folds, or annuli, that encircle their body.
Like all amphibians, caecilians are vertebrates and have jaws and teeth. These armaments enable caecilians to lead a carnivorous lifestyle, feeding on insects, worms and other invertebrates they encounter.
Caecilians are found primarily in wet tropical regions of South America, Southeast Asia, and Central America. They are most widespread throughout South America where they inhabit eastern Brazil, northern Argentina and northern South America.
Caecilians reproduce sexually. They are the only group of amphibians to undergo internal insemination. Although some species of caecilians lay eggs, most species give birth to live young.
Caecilians are divided into three groups, beaked caecilians, fish caecilians and common caecilians.
Caecilians are not well represented in the fossil record and consequently not much is known about caecilians of the past. The earliest known fossil caecilian is Eocaecilia, a primitive creature that lived during the Jurassic and had tiny limbs.