Caecilian


 * Apoda redirects here. For the moth genus, see Apoda (moth).
 * For the bishop of Carthage, see Caecilianus.

The caecilians are an order (Gymnophiona or Apoda) of amphibians that superficially resemble earthworms or snakes. They mostly live hidden in the ground, which makes them the least explored order of amphibians, and widely unknown.

Anatomy
Caecilians completely lack limbs, making the smaller species resemble worms, while the larger species with lengths up to 1.5 m resemble snakes. The tail is short and the cloaca is near the end of the body. Their skin is smooth and usually dark-matte, but some species have colorful skins. Inside the skin are calcite scales, which suggests that they may be related to the fossil Stegocephalia. However the scales are now believed to be a secondary development, and not directly inherited from Stegocephalia. Owing to their underground life the eyes are small and covered by skin for protection, which has led to the misconception that they are blind. This is not strictly true, although their sight is limited to simple dark-light perception. All caecilians share two tentacles at their head, which are probably used for a second olfactory capability in addition to the normal sense of smell based in the nose.

Except for one lungless species — Atretochoana eiselti, only known from two specimens collected in South America — all caecilians have lungs, but also use the skin or the mouth for oxygen absorption. Often the left lung is much smaller than the right one, an adaptation to body shape that is also found in snakes.

Distribution
Caecilians are found in most of the tropical regions of South-East Asia, Africa, the Seychelles islands and South America, except the dry areas and high mountains. In South America their distribution extends well into the temperate zone in the north of Argentina. They can be seen as far south as Buenos Aires, when they are carried by the flood waters of the Parana river coming from farther north. No studies have been made in central Africa, but it is likely that caecilians are found in the tropical rainforests there. The northernmost distribution is of the species Ichthyophis sikkimensis of Northern India. In Africa caecilians are found from Guinea Bissau (Geotrypetes) to Northern Zambia (Scolecomorphus). In South-East Asia the Wallace-Line is not crossed and they are not found in Australia or the islands in between. Ichthyophis is also found in South China and North-Vietnam.

Reproduction
Caecilians are the only order of amphibians which only use internal insemination. The male caecilians have a penis-like organ, the phallodeum, which is inserted into the cloaca of the female for 2 to 3 hours. About 25% of the species are oviparous (egg-laying); the eggs are guarded by the female. For some species the young caecilians are already metamorphosed when they hatch; others hatch as larvae. The larvae are not fully aquatic, but spend the daytime in the soil near the water.

75% of the species are viviparous, meaning that they give birth to already developed offspring. The fetus is fed inside the female with special cells of the oviduct, which are eaten by the fetus with special scraping teeth.

The egg laying species Boulengerula taitanus feeds its young by developing a special outer layer of skin, high in fat and other nutrients, which the young peel off with similar teeth. This allows them to grow by up to ten times their own weight in a week. The skin is consumed every three days, the time it takes for a new layer to grow, and the young have only been observed to eat it at night. It was previously thought that the juveniles subsisted on a liquid secretion from their mother, but the BBC documentary 'Life In Cold Blood' filmed the bizarre feeding ritual for the first time to reveal the surprising truth.

Some larvae, such as those of Typhlonectes, are born with enormous external gills which are shed almost immediately. Ichthyophis is oviparous and is also known to show maternal care.

Diet
The diet of caecilians is not known well, though it seems it mostly consists of insects and invertebrates found in the habitat of the respective species. The stomach contents of 14 specimens of Afrocaecilia taitana consisted of mostly undefinable organic material and plantal remains. Where identifiable remains were most abundant, they were found to be termite heads. While it was suggested that the undefinable organic material shows that the caecilians eat detritus, others believe these are in fact the remains of earthworms. Caecilians in captivity can be easily fed with earthworms, and worms are also common in the habitat of many caecilian species.

Origin of the name
The name caecilian derives from the Latin word caecus = blind, referring to the small or sometimes non-existing eyes. The name dates back to the taxonomic name of the first species described by Carolus Linnaeus, which he gave the name Caecilia tentaculata. The taxonomic name of the order derives from the Greek words γυμνος (gymnos, naked) and οφις (ophis, snake), as the caecilians were originally thought to be related to snakes.

Taxonomy
Taxonomically the caecilians are divided into 6 families. The species numbers are approximate and many of these species are identified on the basis of only one specimen. It is likely that not all species have been described yet, and that some of the species described below as different may be combined into one species in future reclassifications.
 * Beaked Caecilians (Rhinatrematidae) - 2 genera, 9 species
 * Fish Caecilians (Ichthyophiidae) - 2 genera, 39 species
 * Indian Caecilians (Uraeotyphlidae) - 1 genus, 5 species
 * Tropical Caecilians (Scolecomorphidae) - 2 genera, 6 species
 * Aquatic Caecilians (Typhlonectidae) - 5 genera, 13 species
 * Common Caecilians (Caeciliidae) - 26 genera, 99 species

Recent discoveries
A Swiss researcher, Daniel Hofer, has recently found that certain caecilians produce potent skin poisons from specialized poison glands. The poison prevents predation and the poison of the bright yellow caecilian of São Tomé Island (Schistometopum thomense) in West Africa kills other animals kept in the same tank within a few days. The chemical contents of caecilian poison have not been well studied. Werner Himstedt has shown that the skin glands of Ichthyophis protect the eggs from germs and fungi. It has also been shown that these poisons are very different from those from other amphibians such as the Poison dart frog.

British nature expert David Attenborough found that the young can increase their weight up to ten times in one week, post birth. When on filming the nest the young had teeth. It was then discovered that the young did not just diet on secretion/ milk but that the mother was able to reproduce fatty skin every three days for the young to feast on. This was documented on his BBC programme Life in Cold Blood and is the only currently known instance of dermatotrophy, or deriving necessary nutrients from the consumption of skin.