Caecilians: Gymnophiona

Caecilians (sih-SILL-yuhns) are long, legless amphibians with a dual-action jaw. Some have tails, but most do not. Amphibians (am-FIB-ee-uhns) are vertebrates (VER-teh-brehts), or animals with a backbone, that have moist, smooth skin; are cold-blooded, meaning their body temperature is the same as the temperature of their surroundings; and, in most instances, have a two-stage life cycle. Some species of caecilians are land dwellers, and some are water dwellers.

The most striking feature of caecilians is a series of rings that make these amphibians look like earthworms. The rings run the length of the body starting just behind the head. The rings are inside the body and attached to the vertebrae (VER-teh-bree), or the bones that make up the spinal column. Some caecilians have one ring per vertebra (VER-teh-bruh, the singular of vertebrae); some have two rings, especially toward the rear of the animal; and some have three. The skin is folded over the rings, making grooves between the rings. In some species the grooves go all the way around the body, and in some they only go part of the way around. The second and third sets of rings make shallower grooves than the main set. The species with three rings per vertebra have a short tail, and the tail also has rings. Some species of caecilians have scales just under their skin.

Between the nostrils and the eyes on each side of the head, caecilians have a tentacle that is pumped out and in as needed. The tentacles are sense organs that respond to chemicals and touch. Inside each tentacle is a fluid-filled channel that runs from the tip of the tentacle to a chamber that opens into the part of the brain that controls the sense of smell. There is a hole in the skull through which the tentacle pumps out and in. This hole can be anywhere between the eyes and the nostrils. Scientists use the location of the hole to tell different species of caecilians apart.

The eyes of caecilians are small and covered by skin or, in some species, bone. The eyes of many species have lost some or all of their muscles, and some have lost the lens and have only a small retina and optic nerve. Even with the lack of eye parts, most caecilians can distinguish light and dark.

Caecilians have a somewhat flat head with nostrils at the tip of the snout and a large mouth. In some caecilian species the lower jaw is the same length as the upper jaw, and the mouth opens at the front of the animal's head. In other species, the lower jaw is shorter than the upper jaw, a feature that improves burrowing ability. The mouths of these species open on the bottom of the animal's head. Caecilians have two rows of teeth on the upper jaw and one or two rows on the lower jaw. The tops of the teeth have different shapes among species, but all teeth are hinged and usually curve backward, apparently to prevent the loss of prey the animals have caught.

The skin of caecilians has glands that release a substance that is poisonous to many predators. Most caecilians are dark gray to grayish brown to deep purple, often with a lighter head and belly. Some caecilians, however, are brightly colored. For example, a species on the African island of São Tomé is bright yellow. A South American species is deep bluish purple with bright white grooves between its rings. Some species are dark gray to grayish brown to brownish black with bright yellow side stripes. Most adult caecilians are 12 to 24 inches (30 to 60 centimeters) long, but some species are much smaller or larger. The smallest adult caecilians are about 4.5 inches (11 centimeters) long. The largest are more than 63 inches (160 centimeters) long.