Kinorhyncha
A phylum of free-living marine invertebrates less than 0.04 in. (1 mm) long. They are segmented and lack external ciliation (see illustration). Kinorhynchs are benthonic, so-called because they generally dwell in mud or sand from intertidal to deep-sea habitats. Two orders are generally recognized, Cyclorhagida and Homalorhagida.
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The body is covered by a transparent cuticle secreted by an underlying epidermis. The cuticle is molted only in the process of juvenile growth. Three body regions are recognized: a head segment, a neck segment and an 11-segment trunk. The head is completely retractable. When everted, it extends its five to seven circles of recurved spines called scalids. The neck consists of plates called placids which function to close the anterior opening of the trunk when the head is retracted. Most kinorhynchs have a pair of adhesive tubes on the ventral surface of the third or fourth segment.
Kinorhyncha
[‚kin·ə′riŋ·kē·ə]Kinorhyncha
a class of marine worms, in some respects similar to the nemertines. The body of the Kinorhyncha is 0.1–1 mm in length, yellowish or brown, and bilaterally symmetrical. It is divided into 13 (rarely, 14) segments, or zonites, which form a head (first zonite), neck (second), and trunk. All of the zonites are covered with crowns of spines or hooks. The skin is of simple epithelium. There is no common musculocutaneous sac; the musculature consists of individual bundles of striated muscle. The intestinal tract is a simple tube. The nervous system consists of a cephalic peripharyngeal ring and an abdominal nerve trunk with aggregates of nerve cells in each segment of the trunk. The sensory organs consist of one or two pairs of pigmented ocelli and sensory bristles. Coelom and circulatory system are absent. There is a single pair of protonephridia, opening to the outside on the eleventh zonite.
Kinorhyncha are dioecious. The sex glands are paired. The worms develop by metamorphosis. The class comprises one order (Echinodera), with over 30 species. The Kinorhyncha are widely distributed; in the USSR, they are found in the coastal zone and at depths to 400 m in the Black, Baltic, Bering, and Far Eastern seas. They feed on unicellular algae and detritus.
REFERENCE
Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.B. E. BYKHOVSKII