By Martin Sorensen, Niels Bohr Institute & Biological Institute, University of Copenhagen
Kinorhynchs are marine, benthic, microscopical metazoans that occur in the oceans and inland seas throughout the world. The body is characterized by having a head with a protrusible mouth cone and an eversible introvert, a neck consisting of small plates, the so-called placids, and a trunk with eleven segments that either consist of plates or closed cuticular rings. The introvert is equipped with several circles of small appendages, the so-called scalids, and many species have needle-like spines or tubules bilaterally arranged on their trunk segments. Their sizes may vary from about 120 µm up to almost 1100 µm. Kinorhynchs were originally grouped with the polyphyletic ‘Aschelminthes’ or ‘Nemathelminthes’, but are today considered part of the basal ecdysozoan taxa, probably most closely related with the priapulids. Today the phylum comprises about 166 species, based on descriptions of adults, but the number of newly discovered entities, both on species- and higher taxonomic levels are still increasing, which indicates that a great part of the kinorhynch biodiversity still remains to be revealed.
Species of Kinorhyncha are most often found in muddy sediments, but they may also be present in various other kinds of habitats, such as quartz- or calcareous sand, coralline algae or kelp holdfasts.