Kinorhyncha  is a phylum of small (1 mm or less) marine invertebrates. They are widespread in mud or sand at all depths of the sea. They are also called mud dragons. About 150 species are known. Kinorhynchs eat either diatoms or organic material found in the mud, depending on species.
Kinorhynchans are segmented, limbless animals, with a body consisting of a head, neck, and a trunk of eleven segments. Unlike some similar invertebrates, 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. These spines are used for movement, withdrawing the head and pushing forward, then gripping the substrate with the spines while drawing up the body.
The spines are 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.
The nervous system is typical for invertebrates. It 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 (eye-spots) on the head, and all species have tiny bristles on the body to provide a sense of touch.
There are two sexes that look alike. 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. The larvae are free-living, but little else is known of their reproductive process.
- Order Cyclorhagida
- Order Homalorhagida
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