[2] Modern caecilians are a clade, the order Gymnophiona /ˌdʒɪmnəˈfaɪənə/ (or Apoda /ˈæpədə/), one of the three living amphibian groups alongside Anura (frogs) and Urodela (salamanders).

Genetic evidence and some anatomical details (such as pedicellate teeth) support the idea that frogs, salamanders, and caecilians (collectively known as lissamphibians) are each other's closest relatives.

Some studies have instead argued that caecilians descend from extinct lepospondyl or stereospondyl amphibians, contradicting evidence for lissamphibian monophyly (common ancestry).

Rare fossils of early gymnophionans such as Eocaecilia and Funcusvermis have helped to test the various conflicting hypotheses for the relationships between caecilians and other living and extinct amphibians.

[14] This muscle system allows the animal to anchor its hind end in position, and force the head forwards, and then pull the rest of the body up to reach it in waves.

[17][18] Unlike other modern amphibians (frogs and salamanders) the skull is compact and solid, with few large openings between plate-like cranial bones.

The braincase is encased in a fully integrated compound bone called the os basale, which takes up most of the rear and lower parts of the skull.

Adults of species within the family Scolecomorphidae lack both a stapes and an oval window, making them the only known amphibians missing all the components of a middle ear apparatus.

These studies reveal that some caecilians exhibit a phenomenon wherein they provide their hatchlings with a nutrient-rich substance akin to milk, delivered through a maternal vent.

Among the species investigated, the oviparous nonmammalian caecilian amphibian Siphonops annulatus stood out, indicating that the practice of lactation may be more widespread among these creatures than previously thought.

As detailed in a 2024 study, researchers collected 16 mothers of the Siphonops annulatus species from cacao plantations in Brazil's Atlantic Forest and filmed them with their altricial hatchlings in the lab.

This milk-like substance, rich in fats and carbohydrates, is produced in the mother's oviduct epithelium's hypertrophied glands, similar to mammal milk.

The researchers observed the hatchlings emitting high-pitched clicking sounds as they approached their mothers for milk, a behavior unique among amphibians.

The presence of milk production in caecilians that lay eggs suggests an evolutionary transition between egg-laying and live birth.

In Southeast Asia, they are found as far east as Java, Borneo, and the southern Philippines, but they have not crossed Wallace's line and are not present in Australia or nearby islands.

[31] In 2021, a live specimen of Typhlonectes natans, a caecilian native to Colombia and Venezuela, was collected from a drainage canal in South Florida.

However, Apoda been used for groups of fishes and of sea cucumbers and is the name of a genus of moth, and its continued use in caecilian taxonomy is potentially confusing and unhelpful.

[41][42][43] †Eocaecilia micropodia †Rubricacaecilia monbaroni Amazops Rhinatrema Epicrionops Uraeotyphlus Ichthyophis Crotaphatrema Scolecomorphus Chikila Herpele Boulengerula ?Atretochoana ?Nectocaecilia ?Potamotyphlus Chthonerpeton Typhlonectes Oscaecilia Caecilia ?Idiocranium ?Indotyphlus ?Sylvacaecilia Gegeneophis Hypogeophis Praslinia Grandisonia Geotrypetes Schistometopum Gymnopis Dermophis ?Brasilotyphlus ?Microcaecilia ?Mimosiphonops Luetkenotyphlus Siphonops Little is known of the evolutionary history of the caecilians, which have left a very sparse fossil record.

[49] Chinlestegophis, a stereospondyl temnospondyl from the Late Triassic Chinle Formation of Colorado, was proposed to be a stem-caecilian in a 2017 paper by Pardo and co-authors.

[50] In 2024, Chinlestegophis was consistently recovered as a sister taxon of Rileymillerus within various positions of Stereospondyli outside Lissamphibia based on phylogenetic analyses and revisions.

Nevertheless, all of these ideas were refuted, and the most strongly supported hypothesis combined lissamphibians into a monophyletic group of dissorophoid temnospondyls closely related to Gerobatrachus.

[54][55][56] This form of parental care, known as maternal dermatophagy, has also been reported in two species in the family Siphonopidae: Siphonops annulatus and Microcaecilia dermatophaga.

In comparison to other amphibians, the extended parenting of caecilians can provide beneficial bacteria and fungi, but this transmission risks the spread of diseases like chytridiomycosis.

The stomach contents of wild caecilians include primarily soil ecosystem engineers[61] like earthworms, termites, lizards, moth larvae, and shrimp.

Some species of caecilians will opportunistically consume newborn rodents, salmon eggs, and veal in laboratory conditions, as well as vertebrates such as scolecophidian snakes, lizards, small fish, and frogs.

Caecilians in the Eastern Himalayas are colloquially known as "back ache snakes",[64] while in the Western Ghats, Ichthyophis tricolor is considered to be more toxic than a king cobra.

Kefa-ntie, a term also encompassing native moles and blind snakes, are considered poisonous, causing painful sores if encountered, contacted, or killed.

According to Oku tradition, the ceremony to cleanse the affliction involves a potion composed of ground herbs, palm oil, snail shells, and chicken blood applied to and licked off of the left thumb.

Colombian folklore states that the aquatic caecilian, Typhlonectes natans, can be manifested from a lock of hair sealed in a sunken bottle.

In southern Mexico and Central America, Dermophis mexicanus is colloquially known as the "tapalcua", a name referencing the belief that it emerges to embed itself in the rear end of any unsuspecting person who chooses to relieve themself over its home.