Etymology:- From the Greek Kinema for motion and Rhynchos for snout.
Meaning animal with a movable snout.
Characteristics of Kinorhyncha:-
1)Bilaterally symmetrical, and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a pseudocoel.
4)Body possesses a through gut with an anus.
5)Body possesses in a cuticle and longitudinal muscles.
6)Has a nervous system with an anterior nerve ring and a double nerve chord.
7)Has no circulatory system (no blood system)
8)Reproduction normally sexual and gonochoristic.
9)Feed on diatoms and organic detritus.
10)All live in marine environments.
The Kinorhyncha are a smallish phylum (100 species) of marine worms. They are recognised by the 5 or 6 rings of recurved spines on their heads and the segmented appearance of their bodies. The segmented nature of their bodies is a reflection of the rings of cuticular plates that surround the the body, it is not homologous with the segmentation of Annelid worms or Arthropods.
The Kinorhynchan body is comprised of a head, a neck, and a trunk, the trunk has 11 segments or zonites each with a single dorsal plate (tergite), and two ventral plates (sternites). The head which bears the recurved spines or scalids can be retracted into the trunk and is therefore called an 'introvert'. The head also bears an oral cone or mouth surrounded by more spines called stylets. The gut passes straight along the body and their is an anus at the posterior (tail) end of the body. Metabolic excretion and water balance are performed by a pair of protonephridia in the 10th zonite, wastes are eliminated through duct opening through the 11th zonite called a nephridiopore. Kinorhynchans have a reasonably well developed nervous system composed of a nerve ring or brain which encircles the anterior (front) end of the pharynx, a double nerve cord which runs down the ventral side of the body and various other nerves and ganglia.
Kinorhyncha are not a very speciose phyla with only about 150 species currently known to science. They live in marine sediments ranging from coastal to depths of up to 5000 metres and have been found in all the worlds seas and oceans. They can not swim, and move by pushing their heads forward into the mud, extending their scalids then retracting the head into the body, because the head is held in place by the spines this has the effect of dragging the body forward. the spines can then be closed and the head pushed forward once more. They feed on diatoms and other organic matter they find in the mud.
The Kinorhyncha have the two sexes separate though to us they often look the same. Very little is know of their reproduction, the eggs are eventually fertilised, perhaps by a spermatophore attached to the females cuticle. The larvae are free living and go through at least 6 moults of their cuticle before they reach maturity.
No kinorhyncha fossils have yet been found.