Phylum Kinorhyncha (kinein, motion + rhynchos snout)
Kinorhynchs are small, elongate bilaterally symmetrical worms. Their size is less than 1 mm. They are found exclusively in marine environments. They live in mud and sand. They have no external cilia or locomotory appendages. Therefore, they simply burrow through the mud and sand with their snouts. Therefore, the phylum takes its name from this method of locomotion. The phylum Kinorhyncha contains about 150 known species.
The cilia are absent on the body surface of kinorhynch. It is composed of 13 or 14 definite units called zonites. The head, is zonite 1. It bears mouth, an oral cone. and spines. The neck is Anire 2. It contains spines called scalids and plates called placids. The head can he retracted into the neck. The trunk consists of the remaining 11 or 12 zonites. It ends with the anus. Each trunk zonite bears a pair of lateral spines and one dorsal spine.
The body wall consists of a cuticle, epidermis and two pairs of muscles. These muscles are dorsolateral and ventrolatcral. The pseudocoelom is large and contains ambeboid cells.
Feeding and digestive system
A complete digestive system is present in them. It consists of a mouth, buccal cavity, muscular pharynx, esophagus, stomach, intestine and anus. Most kinorhynchs feed on diatoms, algae, and organic matter.
Other organ systems
Apair of protonephridia is present on zonite 11. The nervous system consists of brain and single ventral nerve cord. This nerve chord has a ganglion in each zonite. Some species have eyespots and sensory bristles.
Reproduction and development
Kinorhynchs are dioecious with paired gonads. Several spines surround the male gonopore. These spines are used in copulation. The young hatch into larvae. The larva does not have all of the zonites. As the larvae grow and molt, the adult morphology appears. Molting no longer occurs in adult.