Atretochoana Eiselti

A. eiselti is the largest tetrapod to lack lungs, double the size of the next largest. Caecilians such as Atretochoana are limbless amphibians with a snake-like body, marked with rings like that of earthworms. It has significant morphological differences from other caecilians, even the genera most closely related to it, despite the fact that those genera are aquatic. The skull is very different from those of other caecilians, giving the animal a broad flat head. Its nostrils are sealed, and it has an enlarged mouth with a mobile cheek. Its body has a fleshy dorsal fin.
Most caecilians have a well-developed right lung and a relictual left lung. Some, such as Atretochoana's relatives, have two well-developed lungs. Atretochoana, however, entirely lacks lungs, and has a number of other features associated with lunglessness, including sealed choanae, and an absence of pulmonary arteries. Its skin is filled with capillaries that penetrate the epidermis, allowing gas exchange. Its skull shows evidence of muscles not found in any other organism. The Vienna specimen of Atretochoana is a large caecilian at a length of 72.5 centimetres (28.5 in), while the Brasília specimen is larger still at 80.5 centimetres (31.7 in). By comparison, caecilians range in length from 110 to 1,600 millimetres (4.3 to 63 in).