Caecilians are limbless amphibians, not snakes or worms
Named from Latin for blind ones, caecilians are worm- or snake-shaped amphibians with tiny or absent eyes—order Gymnophiona (Apoda), the third living amphibian lineage beside frogs and salamanders. Fossils are scarce, so genes and odd teeth carry much of the evolutionary story.
These amphibians spend most of their lives out of sight in soil or streambeds across tropical South and Central America, Africa and southern Asia, which is why so few people have ever seen one. More than 220 living species sit in 10 families. Their range reaches from southern Mexico to temperate northern Argentina, where floods on the Paraná River sometimes wash them as far as Buenos Aires, and the most northerly species is Ichthyophis sikkimensis of northern India. Oddly, none have been found in the vast forests of central Africa.
Smaller kinds look like earthworms, while Caecilia thompsoni grows to about 1.5 meters and passes for a snake. Traces of limbs survive only as vestiges in Ichthyophis and as embryonic buds in Typhlonectes compressicauda. To burrow, the animal uses its spine as a piston inside a sleeve of skin-bound muscle, squeezing the body cavity to shove the head forward before hauling the rest along; in loose mud it swims like an eel. Nearly all breathe with lungs, the left one usually stunted as in snakes, but Atretochoana eiselti has none at all.
The skull is a compact, bullet-nosed ram with many bones fused, and most species keep their mouth tucked beneath an overhanging snout. The jaw is closed by two muscle sets rather than the single pair other amphibians use, and an extra inner row of up to 20 teeth runs beside the main one. Eyes register little more than light and dark, so a pair of tentacles between eye and nostril probably gives a second sense of smell. Scolecomorphid adults lack every part of a middle ear, unique among amphibians. Calcite scales lie hidden in the ringed skin of many species, and skin glands release toxins; in Siphonops paulensis the secretion destroys red blood cells.
Parental care can be remarkable. A 2024 study reported in Science filmed 16 Siphonops annulatus mothers collected from cacao farms in Brazil's Atlantic Forest; their helpless young suckled a thick white fluid from the mother's cloaca and grew fast during their first week. The same species carries dental glands that may match the venom glands of some snakes and lizards, though their purpose remains unknown.
Source: Caecilian
