one of the three major extant orders of the class Amphibia. Its members are known as caecilians (caecilian), a name derived from the Latin word caecus, meaning “sightless” or “blind.” The majority of this group of wormlike amphibians (amphibian) live underground in humid tropical regions throughout the world. Because of their relatively hidden existence, caecilians are unfamiliar to the layperson and are not usually considered in discussions about amphibians. They are nevertheless a fascinating group of highly specialized amphibians about which there is still much to be learned.

Caecilians have long, limbless, cylindrical bodies that abruptly end behind the cloaca or short tail. Annuli (primary grooves) in the skin encircle the body and form segments; in some taxonomic groups, secondary and tertiary grooves partially circumscribe the body. Within the tissue of the annuli, bony scales (scale) of dermal origin usually occur. The heads of caecilians are blunt, and their skulls are bony and compact. Centres of ossification have fused, which has reduced the number of independent cranial bones in caecilians in comparison with anurans (Anura) and salamanders (salamander); for example, a single bone, the os basale, forms both the floor of the braincase and the posterior part of the skull. Teeth are found on all jaw bones, and a palatal series of teeth appears in addition, medial to the maxillary series. A U-shaped facet, which articulates with the quadrate and also has a long retroarticular process that serves as an attachment site for three major jaw muscles, is located on the lower jaw. The vertebral column is made up of an atlas (the first vertebra of the neck) and 95 to 285 trunk vertebrae; no differentiated sacral vertebrae are present. Double-headed ribs are found on all vertebrae except the atlas and the terminal three to six vertebrae. Of the three amphibian orders, only caecilians have an axial musculature in which all the hypaxial components, excluding the subvertebral musculature, form an outer muscular sheath. This sheath, which is anchored to the skin by fibrous connective tissue, is all but disconnected from the vertebral musculature and thereby allows the skin and superficial muscles to move as a single unit. The degenerate eyes are covered with bone or skin. These adaptations make it possible for the caecilian to feed, reproduce, and avoid enemies within their subterranean realm. The features of aquatic caecilians of the family Typhlonectidae are representative of secondary adaptations.

Elongate limbless amphibians well adapted for burrowing; body segmented by annular grooves, with some species containing scales; tail, if present, short and pointed; skull compact, in which some elements are fused; eyes small, covered with skin or bone; teeth curved; left lung often primitive; projectable sensory tubercle between eye and nostril; protrusible copulatory organ in males; aquatic larvae (with gill slits but no external gills), direct development of terrestrial eggs, or viviparous; about 170 species.

Edward H. Taylor, The Caecilians of the World: A Taxonomic Review (1968), is the first major review of caecilians, and it served to ignite the interest of other scientists. Similarly, William E. Duellman and Linda Trueb, Biology of Amphibians (1986, reissued 1994), offers the first general review of caecilians' biology and anatomy. A more recent and less detailed review of caecilians is available in George R. Zug, Laurie J. Vitt, and Janalee P. Caldwell, Herpetology: An Introductory Biology of Amphibians and Reptiles, 2nd ed. (2001). The most comprehensive review of caecilians is Werner Himstedt, Die Blindwühlen (1996). The current classification is derived from Ronald A. Nussbaum and Mark Wilkinson, “On the Classification and Phylogeny of Caecilians (Amphibia: Gymnophiona), a Critical Review,” Herpetological Monographs, 3:1–42 (1989).William E. Duellman