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Caecilian
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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.
ilians completely lack limbs, making the smaller species resemble worms, while the larger species with lengths up to resemble snakes. The tail is short or absent, and the cloaca is near the end of the body.
Their skin is smooth and usually dark-matte, but some species have colorful skins.
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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 resemble snakes. The tail is short or absent, 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. Because of these scales, the caecilians were once thought to be related to the fossil Stegocephalia, but they are now believed to be a secondary development, and the two groups are most likely unrelated.
The skin also has numerous ring-shaped folds, or annuli, that partially encircle the body, giving them a segmented appearance. Like other living amphibians, the skin contains glands that secrete a toxin to discourage predators. The skin secretions of Siphonops paulensis have been shown to have hemolytic properties.
Caecilian anatomy is highly adapted for a burrowing lifestyle. They have a strong skull, with a pointed snout used to force their way through soil or mud. In most species, the number of bones in the skull are reduced and fused together, and the mouth is recessed under the head. Their muscles are adapted to pushing their way through the ground, with the skeleton and deep muscles acting as a piston inside the skin and outer muscles. This 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. In water or very loose mud, caecilians instead swim in an eel-like fashion. Caecilians in the family Typhlonectidae are aquatic as well as being the largest of their kind. The representatives of this family have a fleshy fin running along the rear section of their body, which enhanced propulsion in water.
All but the most primitive caecilians have two sets of muscles for closing the jaw, compared with the single pair found in all other vertebrates. These are more highly developed in the most efficient burrowers among the caecilians, and appear to help keep the skull and jaw rigid.
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 possess a pair of tentacles, located between their eyes and nostrils. These 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, they do not cross the Wallace-Line, and they are not found in Australia or the islands in between. Ichthyophis is also found in South China and North Vietnam. They are also found in New Zealand.
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 cells of the oviduct, which they eat with special scraping teeth.
The egg laying species Boulengerula taitanus feeds its young by developing an 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.
Some larvae, such as those of TyphlonectesTyphlonectes cunhai...
, are born with enormous external gills which are shed almost immediately. Ichthyophis is oviparous and known to show maternal care, with the mother guarding the eggs until they hatch.
Etymology
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 ??µ??? (gymnos, naked) and ?f?? (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 almost certain 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.
Evolution
Little is known of the evolutionary history of the caecilians, which have left almost no fossil record. What few fossils exist suggest that they have changed little in millions of years. The earliest fossil known comes the Jurassic period. This primitive genus, Eocaecilia, had small legs and well-developed eyes.
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