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| Kinorhyncha | |
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| Echinoderes hwiiza | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| Clade: | Protostomia |
| Superphylum: | Ecdysozoa |
| Clade: | Scalidophora |
| Phylum: | Kinorhyncha Reinhard, 1881 |
| Orders | |
Kinorhyncha/kaɪnoʊˈrɪŋkə,kɪnə-/ (Ancient Greek:κινέω,romanized: kīnéō,lit. 'I move',ῥύγχοςrhúnkhos "snout") is aphylum of small marineinvertebrates that are widespread in mud or sand at all depths as part of themeiobenthos. They are commonly calledmud dragons. Modern species are 1 mm (0.039 in) or less, butCambrian forms could reach 4 cm (1.6 in).[2]
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 thepanarthropoda, with a segmented body. Juveniles have eight or nine segments, depending on genus, with the last two or three being added later during growth.[3] A Cambrian species, Eokinorhynchus rarus, had about twice as many segments as present forms.[4] Like other ecdysozoans they do not have externalcilia, but instead have a number of spines along the body, plus up to seven circles of spines around the head.[5] These spines are used forlocomotion, withdrawing the head and pushing forward, then gripping the substrate with the spines while drawing up the body.
The body wall consists of a thinsyncitial layer, which secretes a toughcuticle; 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 calledplacids when retracted.[6]
Kinorhynchs eat eitherdiatoms or organic material found in the mud, depending onspecies. The mouth is located in a conical structure at the apex of the head, and opens into apharynx and then anoesophagus, both of which are lined by cuticle. Two pairs ofsalivary glands and one or more pairs of "pancreatic glands" connect to the oesophagus and presumably secretedigestive 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 ananus at the posterior end of the trunk.[6]
There is nocirculatory system, although the body cavity (pseudocoelom) is well developed, and includesamoebocytes. The excretory system consists of twoprotonephridia emptying through pores in the final segment.[6]
Thenervous system consists of aventral nerve cord, with oneganglion in each segment, and an anteriornerve ring surrounding the pharynx. Smaller ganglia are also located in the lateral and dorsal portions of each segment, but do not form distinct cords. Some species have simpleocelli on the head, and all species have tiny bristles on the body to provide a sense of touch.[6]
There are two sexes that look alike, although somesexual dimorphism inallometry has been reported.[7] A pair ofgonads 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 incopulation, although the details are unknown. Individual spermatozoa can reach a quarter of the total body length.[8] Thelarvae are free-living, but little else is known of their reproductive process.[6] After having laid an egg, the female packs it into a protective envelope of mud and organic material.[9]
Their closest relatives are thought to be the phylaLoricifera andPriapulida. Together they constitute theScalidophora.
The two groups of Kinorhynchs are generally characterized as classes in Sørensen et al. (2015).[10] 270 species have been described and this number is expected to increase substantially.[11][12] 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).[10] Phylogenomic data has shown Allomalorhagida and Cyclorhagida to be divided in three and two major clades respectively.[13]
The oldest known species isEokinorhynchus from theFortunian of China.[14]
Five genera,Echinoderes,Cephalorhyncha,Fissuroderes,Meristoderes, andPolacanthoderes, 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 theEchinoderes, that holds more than 100 species.
GloballyEchinoderes are the most diverse, abundant and commonly encountered of all kinorhynch genera.
PhylumKinorhyncha
Media related toKinorhyncha at Wikimedia Commons
Kinorhynch families and genera | |||||||
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| Stem-group |  | ||||||
| Cyclorhagida |
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| Allomalorhagida |
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| Authority control databases: National |
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