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 the head to extend it 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.
Kinorhyncha /kaɪnoʊˈrɪŋkə, kɪnə-/ (Ancient Greek: κινέω, romanized: kīnéō, lit. 'I move', ῥύγχος rhúnkhos "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 also called mud dragons. Modern species are 1 mm (0.039 in) or less, but Cambrian forms could reach 4 cm (1.6 in).[1]
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. Juveniles have eight or nine segments, depending on genus, with the last two or three being added later during growth.[2] 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.[3] 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.[4]
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.[4]
There is no circulatory system, although the body cavity (pseudocoelom) is well developed, and includes amoebocytes. The excretory system consists of two protonephridia emptying through pores in the final segment.[4]
The nervous system consists of a ventral nerve cord, with one ganglion in each segment, and an anterior nerve 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 simple ocelli on the head, and all species have tiny bristles on the body to provide a sense of touch.[4]
There are two sexes that look alike, although some sexual dimorphism in allometry has been reported.[5] 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.[6] The larvae are free-living, but little else is known of their reproductive process.[4] After having laid an egg, the female packs it into a protective envelope of mud and organic material.[7]
Their closest relatives are thought to be the phyla Loricifera and Priapulida. Together they constitute the Scalidophora.
The two groups of Kinorhynchs are generally characterized as classes in Sørensen et al. (2015).[8] 270 species have been described and this number is expected to increase substantially.[9][10] 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).[8] Phylogenomic data has shown Allomalorhagida and Cyclorhagida to be divided in three and two major clades respectively.[11]
The oldest known species is Eokinorhynchus from the Fortunian of China.[12]
Phylum Kinorhyncha
Kinorhyncha /kaɪnoʊˈrɪŋkə, kɪnə-/ (Ancient Greek: κινέω, romanized: kīnéō, lit. 'I move', ῥύγχος rhúnkhos "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 also called mud dragons. Modern species are 1 mm (0.039 in) or less, but Cambrian forms could reach 4 cm (1.6 in).
