Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a
phylum
In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
of small marine
invertebrates that are widespread in mud or sand at all depths as part of the
meiobenthos
Meiobenthos, also called meiofauna, are small benthic invertebrates that live in both marine and fresh water environments. The term ''meiofauna'' loosely defines a group of organisms by their size, larger than microfauna but smaller than macrofau ...
. They are also called mud dragons. Modern species are or less, but
Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
forms could reach .
Anatomy
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. Like other ecdysozoans they do not have external
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
, 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
A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
layer, which secretes a tough
cuticle
A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
; 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.
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
The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...
at the posterior end of the trunk.
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.
The
nervous system consists of a
ventral nerve cord, with one
ganglion
A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...
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.
Reproduction
There are two sexes that look alike, although some
sexual dimorphism in
allometry has been reported. A pair of
gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
s 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
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...
, 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.
After having laid an egg, the female packs it into a protective envelope of mud and organic material.
Classification
Their closest relatives are thought to be the phyla
Loricifera and
Priapulida. Together they constitute the
Scalidophora.
Taxonomy
The two groups of Kinorhynchs are generally characterized as classes in Sørensen et al. (2015).
270 species have been described and this number is expected to increase substantially. 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).
[Sørensen, M. V. et al. Phylogeny of Kinorhyncha based on morphology and two molecular loci. PLoS One 10, 1–33 (2015).] The oldest known species is ''
Eokinorhynchus'' from the
Fortunian
The Fortunian age marks the beginning of the Phanerozoic Eon, the Paleozoic Era, and the Cambrian Period. It is the first of the two stages of the Terreneuvian series. Its base is defined as the first appearance of the trace fossil ''Treptichnus p ...
of China.
Phylum Kinorhyncha
*Class
Cyclorhagida
Cyclorhagida is a class of kinorhynchs, which are small marine invertebrates.
Orders and families
* Order Echinorhagata Sørensen et al., 2015
** Echinoderidae Zelinka, 1894Zelinka, C. (1894). Über die Organisation von Echinoderes. ''Verhan ...
(Zelinka, 1896) Chitwood, 1951
**Order
Echinorhagata
Echinoderidae is a family of kinorhynchs in the class Cyclorhagida
Cyclorhagida is a class of kinorhynchs, which are small marine invertebrates.
Orders and families
* Order Echinorhagata Sørensen et al., 2015
** Echinoderidae Zelinka, 18 ...
Sørensen et al., 2015
***
Echinoderidae
Echinoderidae is a family of kinorhynchs in the class Cyclorhagida
Cyclorhagida is a class of kinorhynchs, which are small marine invertebrates.
Orders and families
* Order Echinorhagata Sørensen et al., 2015
** Echinoderidae Zelinka, 18 ...
Zelinka, 1894
**Order
Kentrorhagata Sørensen et al., 2015
***
Antygomonidae
''Antygomonas'' is a genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biolog ...
Adrianov & Malakhov, 1994
***
Cateriidae
Cateriidae is a family of kinorhynchs in the class Cyclorhagida. It consists of a single genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus cla ...
Gerlach, 1956
***
Centroderidae
Centroderidae is a family of worms belonging to the class Cyclorhagida
Cyclorhagida is a class of kinorhynchs, which are small marine invertebrates.
Orders and families
* Order Echinorhagata Sørensen et al., 2015
** Echinoderidae Zelinka, ...
Zelinka, 1896
***
Semnoderidae
Semnoderidae is a family of worms belonging to the class Cyclorhagida
Cyclorhagida is a class of kinorhynchs, which are small marine invertebrates.
Orders and families
* Order Echinorhagata Sørensen et al., 2015
** Echinoderidae Zelinka, ...
Remane, 1929
***
Zelinkaderidae
Zelinkaderidae is a family of worms belonging to the class Cyclorhagida
Cyclorhagida is a class of kinorhynchs, which are small marine invertebrates.
Orders and families
* Order Echinorhagata Sørensen et al., 2015
** Echinoderidae Zelinka ...
Higgins, 1990
**Order
Xenosomata Zelinka, 1907
***
Campyloderidae Remane, 1929
*Class
Allomalorhagida Sørensen et al., 2015
** no order assigned
***
Dracoderidae Higgins & Shirayama, 1990
***
Franciscideridae Sørensen et al., 2015
***
Neocentrophyidae Higgins, 1969
***
Pycnophyidae Zelinka, 1986
References
External links
Introduction to the Cephalorhyncha*
{{Authority control
Animal phyla