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any member of the invertebrate phylum Chaetognatha, a group of small wormlike marine animals with transparent to translucent or opaque arrow-shaped bodies. The phylum consists of about seven extant genera and one fossil genus. There are more than 50 species, most of which are in the genus Sagitta. The size of arrowworms ranges from about 3 millimetres to more than 100 millimetres; species inhabiting colder waters generally are larger than those from tropical seas. Chaetognatha are hermaphroditic (having both male and female sex organs, or gonads). The body is divided into head, trunk, and tail by two transverse walls or membranes and has lateral fins and a tail fin. Respiratory, circulatory, and excretory systems are not properly developed