Kinorhyncha (Gr. κίνηω,
kīneō 'move' + ρυνχος,
rhynchos 'snout') is a phylum of small (1 mm or less) marine
pseudocoelomateBy the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s that are widespread in mud or sand at all depths as part of the
meiobenthosMeiofauna 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 Macrofauna, rather than a taxonomic grouping...
. They are also called
mud dragons.
They are segmented, with a body consisting of a head, neck, and a trunk of eleven segments. 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, which they use for
locomotionAnimal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...
, withdrawing the head and pushing forward, then holding with the spines while drawing up the body. The spines are part of a
cuticleA 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. Cuticles are non-homologous, differing in their origin, structure and chemical composition.-Human anatomy:In human anatomy, cuticle ...
secreted by the
epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell...
; this is molted several times while growing to adulthood. The head is completely retractable, and is covered by a set of neck plates called placids when retracted.
Kinorhynchs eat diatoms and other things found in the mud.
There are two sexes that look alike, and the
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e are free-living, but little else is known of their reproductive process.
Their closest relatives are thought to be the phyla
LoriciferaLoricifera is a small phylum of marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca. 1 mm...
and
PriapulidaPriapulida are a phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian. Their nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora...
. Together they constitute the
ScalidophoraScalidophora is a group of marine pseudocoelomate invertebrates, consisting of the three phyla Kinorhyncha, Priapulida, and Loricifera. The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle...
.
The two groups of Kinorhynchs are still generally characterized as orders rather than classes, about 150 species are known.
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