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Caecilians are long, worm-like legless amphibians in the order Gymnophiona (sometimes known as Apoda, meaning without legs). There are 163 species of caecilians, in 35 genera. Little is known about these animals, and few species have common names. Most of the caecilians are tropical or sub-tropical, and occur in Central and South America, Africa, and south and southeast Asia.

Caecilians grow up to 5 ft (1.5 m) in length in the case of Caecilia thompsoni of Colombia. Caecilians are virtually all body, with almost no tail. Caecilians do not even have rudimentary leg or girdle bones, and have probably been a distinct amphibian lineage for a very long time. However, caecilians have almost no fossil record, so little is known about their evolutionary history.

Caecilians are generally a uniformly or mottled gray in color, and somewhat lighter beneath. They have a small mouth, nostrils, and small eyes incapable of movement, covered by skin, and are probably only able to sense changes in light intensity. Caecilians have numerous ring-like, segmental grooves along their body, which enhance their superficial resemblance to earthworms.

Most caecilians live secretively in burrows made in moist soil or in forest litter, often near streams and wetlands. Some species occur in aquatic habitats, where they also burrow in soft substrates. Caecilians feed on invertebrates, some species specializing in earthworms or termites. The skull of caecilians is heavily boned, and the skin adheres to the skull—both of these are adaptations to the chisel-like burrowing methods of these animals.

Caecilians have internal fertilization, a relatively uncommon trait among amphibians. About one-half of caecilian species lay eggs that are guarded by the female, which coils around them until hatching occurs. The other species of caecilians are viviparous, meaning the eggs are retained within the reproductive tract of the female, where they develop and hatch into miniature adults. The larvae feed on their egg yolk, on a rich maternal secretion known as "uterine milk," and by scraping nutritious material from the lining of the reproductive tract of their mother. After a relatively long gestation period of 9-11 months, the baby caecilians emerge as fully metamorphosed but miniature replicas of the adults.

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