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any of the approximately 100 species of microscopic marine invertebrates of the class Kinorhyncha (Echinodera; phylum Aschelminthes), widely distributed in the world's oceans. Kinorhynchs live mostly in the muddy bottoms of shallow seas and in the sand of seacoasts. They are rather bristly or spiny and range in length from 0.2 mm (0.008 inch) to 1 mm (0.04 inch). The yellowish or brownish jointed body consists of 13 or 14 segments. The first segment, which is enlarged and bears a conspicuous crown of curved spines, is called the head and bears the creature's mouth. When at rest, a kinorhynch withdraws its head into the front end of its body. The alternate thrusting forward and withdrawing of the head produces a squirming movement by means of which the animal burrows through mud or sand. Kinorhynchs feed on diatoms, certain protozoans, and fine organic debris. The sexes are separate