The animals have a head, mouth and digestive system as well as a lorica. The armor-like lorica consists of a protective external shell or case of encircling plicae. There is no circulatory system and no endocrine system. Many of the larvae are acoelomate, with some adults being pseudocoelomate, and some remaining acoelomate. The animals are hermaphrodites and probably oviparous. They have a very complex life cycle. The species which live in the deep sea are able to reproduce via parthenogenetic or paedogenetic reproduction. They are not known to be present in the fossil record.
Their closest relatives are thought to be the Kinorhyncha and Priapulida with which they constitute the taxon Scalidophora. The three phyla share four characters in common – chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retracts. A group called Introverta is formed with Kinorhyncha, Priapulida, Nematoda, and Nematomorpha.