Kinorhyncha (, ' "snout") is a phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are commonly called mud dragons. Modern species are or less, but Cambrian forms could reach .

Anatomy

thumb|left|Living Echinoderes specimen, showing movement and head retraction

Kinorhynchs are limbless animals, with a body consisting of a head, neck, and a trunk of eleven segments. They are the only members of Ecdysozoa, except from the panarthropoda, with a segmented body; a feature that probably evolved independently from them. A Cambrian species, Eokinorhynchus rarus, had about twice as many segments as present forms. Like other ecdysozoans they do not have external cilia, but instead have a number of spines along the body, plus up to seven circles of spines around the head. These spines are used for locomotion, withdrawing the head and pushing forward, then gripping the substrate with the spines while drawing up the body.

The body wall consists of a thin syncitial layer, which secretes a tough cuticle; this is molted several times while growing to adulthood. The spines are essentially moveable extensions of the body wall, and are hollow and covered by cuticle. The head is completely retractable, and is covered by a set of neck plates called placids when retracted.

left|thumb|Myoanatomy of the body of [[Echinoderes|Echinoderes spinifurca]]

Kinorhynchs eat either diatoms or organic material found in the mud, depending on species. The mouth is located in a conical structure at the apex of the head, and opens into a pharynx and then an oesophagus, both of which are lined by cuticle. Two pairs of salivary glands and one or more pairs of "pancreatic glands" connect to the oesophagus and presumably secrete digestive enzymes. Beyond the oesophagus lies a midgut that combines the functions of a stomach and intestine, and lacks a cuticle, enabling it to absorb nutrients. The short hind-gut is lined by cuticle, and empties into an anus at the posterior end of the trunk. A pair of gonads are located in the mid-region of the trunk, and open to pores in the final segment. In most species, the sperm duct includes two or three spiny structures that presumably aid in copulation, although the details are unknown. Individual spermatozoa can reach a quarter of the total body length. The larvae are free-living, but little else is known of their reproductive process. There are six juvenile stages, during which segments are added and morphology is slightly modified.

Classification

Their closest relatives are thought to be the phyla Loricifera and Priapulida. Together they constitute the Scalidophora.

Taxonomy

[[File:Kinorhyncha tree 2022.jpg|thumb|350px|Phylogenetic tree of Kinorhyncha, from Herranz et al. 2022 Morphological data has been collected for systematic phylogeny from dozens, and the integration of this with molecular data has led to a new systematic paradigm featuring the order Allomalorhagida (with Homalorhagida being retired). Phylogenomic data has shown Allomalorhagida and Cyclorhagida to be divided in three and two major clades respectively.

The oldest known species is Eokinorhynchus from the Fortunian of China.

Five genera, Echinoderes, Cephalorhyncha, Fissuroderes, Meristoderes, and Polacanthoderes, are currently recognized within the family. The latter four genera are rather small and accommodate one to nine species only, whereas a vast majority of the diversity is contained in the Echinoderes, that holds more than 100 species.

Globally Echinoderes are the most diverse, abundant and commonly encountered of all kinorhynch genera.

Phylum Kinorhyncha

  • †Eokinorhynchus <small>Zhang et al., 2015</small>
  • Class Cyclorhagida <small>(Zelinka, 1896) Chitwood, 1951</small>
  • Order Echinorhagata <small>Sørensen et al., 2015</small>
  • Echinoderidae <small>Zelinka, 1894</small>
  • Order Kentrorhagata <small>Sørensen et al., 2015</small>
  • Antygomonidae <small>Adrianov & Malakhov, 1994</small>
  • Cateriidae? <small>Gerlach, 1956</small> (following Sørensen et al.)
  • Centroderidae <small>Zelinka, 1896</small>
  • Semnoderidae <small>Remane, 1929</small>
  • Zelinkaderidae <small>Higgins, 1990</small>
  • Order Xenosomata <small>Zelinka, 1907</small>
  • Campyloderidae <small>Remane, 1929</small>
  • Class Allomalorhagida <small>Sørensen et al., 2015</small>
  • Pycnophyidae <small>Zelinka, 1986</small>
  • Order Anomoirhaga <small>Herranz et al., 2022</small>
  • Cateriidae? <small>Gerlach, 1956</small> (following Herranz et al.)
  • Dracoderidae <small>Higgins & Shirayama, 1990</small>
  • Franciscideridae <small>Sørensen et al., 2015</small>
  • Neocentrophyidae <small>Higgins, 1969</small>

References

  • Introduction to the Cephalorhyncha
  • Drawings of Kinorhyncha