Around 150 species of kinorhynchs have been described since this group was first discovered on the northern coast of France in 1841, nearly all of them less than 1 mm long. They have been collected as far north as Greenland and as far south as Antarctica, as well as in the Black Sea. Most live in marine sand or mud from the intertidal zone to a depth of 5000 to 8,000 meters, but some are known from algal mats or holdfasts, sandy beaches, and brackish estuaries and others have been found living on hydrozoans, bryozoans, or sponges. The general morphology of kinorhynchs is fairly homogeneous. The body includes a head (which can be retracted into the anterior portion of the neck), with both anteriorly and posteriorly directed spines, and a trunk. The body is divided into 13 distinct "zonites" (the head and neck being formed by the first two zonites and the trunk by the remaining 11), which are viewed as true segments by many specialists. The anus is on the last segment and is usually flanked by strong lateral spines. Kinorhynchs apparently lack a coelom (distinct body cavity). Like arthropods (and most or all other groups in the superclade Ecdysozoa), kinorhynchs periodically shed their chitinous cuticle as they grow, but apparently do not molt as adults. Kinorhynchs lack circular body muscles and do not swim. They move and burrow by forcing body fluid into its head to extend the head into the substratum, anchoring the anterior spines, then pulling the rest of the body forward. Little is known about kinorhynch feeding, but they are likely direct deposit feeders, ingesting the substratum and digesting the organic material or consuming unicellular algae (they have been found with their guts full of benthic diatoms). Kinorhynchs have separate males and females, which are generally externally indistinguishable. Little is known about the reproduction or embryology of kinorhynchs. Fertilized eggs are deposited in egg cases and development is direct (no distinct larval stage), with juveniles emerging from egg cases with 11 of the 13 body segments already formed, the last 2 segments being added during the juvenile molts (Brusca and Brusca 2003 and references therein; Margulis and Chapman 2010)
The Kinorhyncha are believed to be closely related to both the Priapula and Loricifera, with which they are often grouped in a clade referred to as Scalidophora; some authors include the Nematomorpha as well in a clade referred to as Cephalorhyncha (Aleshin et al. 1998 and references therein; Halanych 2004 and references therein). Both morphological and molecular data indicate that the phylum Priapulida is the sister group to the Kinorhyncha, although it is possible that the Loricifera are more closely related to one of these groups than the other (e.g., Kristensen 2002; Mallat and Giribet 2006; Telford et al. 2008). Sørensen et al. (2008) present data that they argue supports a sister relationship between Loricifera and Nematomorpha, which would render the Scalidophora paraphyletic. As of 2010, relationships among phyla within the Ecdysozoa remain poorly resolved, so it is difficult to know which groups will eventually win wide acceptance by specialists as convincingly monophyletic.