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Chaetognatha(kētôgnăth`ə), phylum of predominantly pelagic marine animals commonly known as arrowworms. Arrowworms have slender, transparent bodies, usually under 1 in. (2.5 cm) long. Lateral and caudal fins propel the animal in sudden darting movements. The well-developed head bears eyes and other sense organs, grasping spines used in the capture of prey, and rows of teeth flanking the mouth. A protective hood can be folded down over the bristles and teeth. The digestive system includes a glandular pharynx, a straight intestine, and a short, muscular rectum. The nervous system centers in a bilobed, dorsal brain and several other nerve ganglia. Although widely distributed, arrowworms prefer warm, shallow seas and are particularly plentiful in the Indo-Pacific region. They are voracious predators; some feed on freshly hatched fish nearly as large as themselves. They are influential planktonic consumers when abundant.
A phylum of abundant planktonic arrow-worms. Their bodies are tubular and transparent, and divided into three portions: head, trunk, and tail. The head possesses one or two rows of minute teeth anterior to the mouth and usually 7–10 larger chaetae, or seizing jaws, on each side of the head. One or two pairs of lateral fins and a caudal fin are present.
Nine genera and about 42 species are recognized by some specialists. Most species belong to the genus Sagitta, which can be recognized by the presence of two pairs of teeth and two pairs of lateral fins.
Chaetognaths are cosmopolitan forms which live not only at the surface but also at great depths; however, no one species is found in all latitudes and at all depths. One of the Arctic species, Eukrohnia hamata, may extend to the Antarctic by way of deep water across the tropics. A few species are neritic and are not found normally beyond the continental shelf. Their food consists principally of copepods and other small planktonic crustaceans; however, they are very predacious and will even eat small fish larvae and other chaetognaths on rare occasions.
Studies have shown them to be useful as indicator organisms. Certain species appear to be associated with characteristic types or masses of water, and when this water is displaced into an adjacent water mass, the chaetognaths may be used as temporary evidence for such displacement.
(arrowworms), a phylum of marine invertebrates having a transparent elongate body that measures 5 mm to 9 cm in length. There are lateral fins and a caudal fin. The head bears sickle-shaped bristles that serve to capture prey. The coelom is divided by transverse septa into a head, trunk, and tail. The digestive tract is straight. There are no circulatory or excretory systems. The nervous system consists of an epipharyngeal brain and a ventral ganglion that are connected by long longitudinal cords. Chaetognaths are hermaphrodites. They are predators that live amid marine plankton. There are six genera, embracing about 30 species. Chaetognaths are usually assigned to the group of deuterostome animals; sometimes the invertebrates are regarded as an independent taxonomic category higher than a phylum.
REFERENCESFilatova, Z. A. “Klass shchetinkocheliustnykh.” In Rukovodstvo po zoologii, vol. 3, part 2. Moscow, 1951.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1–2. Moscow, 1964.
Dogel’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1975.
A. V. IVANOV