Chaetognatha
Arrowworm, common name for certain small, transparent, free-swimming marine animals. They are 1 to 8 cm (0.5 to 4 in) long, arrow-shaped, and streamlined, with one or two pairs of lateral fins and a caudal fin that propel the body forward in rapid, darting movements. The rounded head bears a set of bristles on each side, used in catching small animals. A retractable hood can be drawn over the bristles and mouth, which streamlines the head while swimming. Almost all arrowworms live in the open ocean waters. They occur in great numbers along coasts and continental shelves, where they constitute an important part of the food web as predators and as food for other animals. Arrowworms locate prey by detecting vibrations. All are hermaphrodites, containing both ovaries and testes, and some can fertilize themselves. The phylum is not very diverse, having only about 50 species in a few families. Arrowworms are quite unlike any other animals, but possibly may be related to echinoderms and chor...