Caecilian

Caecilians (/sɪˈsɪliən/; New Latin for 'blind ones') are a group of limbless, serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. All modern caecilians and their closest fossil relatives are grouped as a clade, Apoda, within the larger group Gymnophiona, which also includes more primitive extinct caecilian-like amphibians. Caecilians are mostly distributed in the tropics of South and Central America, Africa, and southern Asia. Their diet consists of small subterranean creatures such as earthworms. Caecilians completely lack limbs, making the smaller species resemble worms, while the larger species, with lengths up to 1.5 m (5 ft), resemble snakes. Their tails are short or absent, and their cloacae are near the ends of their bodies. Their skin is smooth and usually dark, but some species have colourful 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 some other living amphibians, the skin contains glands that secrete a toxin to deter predators. The skin secretions of Siphonops paulensis have been shown to have hemolytic properties. Caecilians' vision is limited to dark-light perception, and their 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 bones in the skull are reduced in number 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, and the largest of their kind. The representatives of this family have a fleshy fin running along the rear section of their bodies, which enhances 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 other creatures. These are more highly developed in the most efficient burrowers among the caecilians, and appear to help keep the skull and jaw rigid. 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. The middle ear consists of only the stapes and the oval window, which transfer vibration to the inner ear through a reentrant fluid circuit as seen in some reptiles. The species within the Scolecomorphidae lack both stapes and an oval window, making them the only known amphibians missing all the components of a middle ear apparatus. Except for one lungless species, Atretochoana eiselti, all caecilians have lungs, but also use their skin or mouths 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.