Caecilians, the Snake-Like Amphibians

Caecilians are a group of slender-bodied, limbless amphibians.
Caecilians are a group of slender-bodied, limbless amphibians. Photo © Pedro H. Bernardo / Getty Images.

Caecilians are an obscure family of slender-bodied, limbless amphibians that—at first glance—resemble snakes, eels and even earthworms. Their closest cousins, however, are better-known amphibians like frogs, toads,newts and salamanders. Like all amphibians, caecilians have primitive lungs that enable them to take in oxygen from the surrounding air, but crucially, these vertebrates also need to absorb additional oxygen through their moist skin. (Two species of caecilians lack lungs entirely, and thus are completely dependent on osmotic respiration.)

Some species of caecilians are aquatic, and have slender fins running along their backs that enable them to move through water efficiently. Other species are primarily terrestrial, and spend much of their time burrowing underground and hunting for insects, worms and other invertebrates using their acute sense of smell. (Since caecilians need to stay moist to stay alive, they not only look but also behave much like earthworms, rarely showing their face to the world unless they've been uprooted by a spade or a careless foot).

Because they mostly live underground, modern caecilians have little use for a sense of sight, and many species have partially or entirely lost their vision. The skulls of these amphibians are pointed and consist of strong, fused bones—adaptations that enable caecilians to bore through mud and soil without doing any damage to themselves. Due to the ring-like folds, or annuli, that encircle their bodies, some caecilians have a very earthworm-like appearance, further confusing people who don't even know that caecilians exist in the first place!

Oddly enough, caecilians are the only family of amphibians to reproduce via internal insemination. The male caecilian inserts a penis-like organ into the cloaca of the female, and keeps it there for two or three hours. Most caecilians are viviparous--the females give birth to live young, rather than eggs--but one egg-laying species feeds its young by allowing the newborn hatchlings to harvest the outer layer of the mother's skin, which is well-stocked with fat and nutrients and replaces itself every three days.

Caecilians are found primarily in the wet tropical regions of South America, Southeast Asia, and Central America. They are most widespread in South America, where they are especially populous in the dense jungles of eastern Brazil and northern Argentina.

Caecilian Classification

Animals > Chordates > Amphibians > Caecilians

Caecilians are divided into three groups: beaked caecilians, fish caecilians and common caecilians. There are about 200 caecilian species overall; some undoubtedly have yet to be identified, lurking in the interiors of impenetrable rain forests.

Because they are small and easily degraded after death, caecilians are not well represented in the fossil record and consequently not much is known about the caecilians of the Mesozoic or Cenozoic eras. The earliest known fossil caecilian is Eocaecilia, a primitive vertebrate that lived during the Jurassic period and (like many early snakes) was equipped with tiny, vestigial limbs.

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Strauss, Bob. "Caecilians, the Snake-Like Amphibians." ThoughtCo, May. 22, 2018, thoughtco.com/caecilians-definition-129713. Strauss, Bob. (2018, May 22). Caecilians, the Snake-Like Amphibians. Retrieved from https://www.thoughtco.com/caecilians-definition-129713 Strauss, Bob. "Caecilians, the Snake-Like Amphibians." ThoughtCo. https://www.thoughtco.com/caecilians-definition-129713 (accessed May 23, 2018).