Arrow worm. Image credit: NOAA.
Chaetognatha is a phylum of marine worm-like animals, not closely related to any other group with narrow bodies divided into head, trunk, and tail, also known as arrow worms. The trunk bears lateral fins. Larval chaetognathans are free-swimming; adults are planktonic or tube-dwellers.
Chaetognaths are between 3 millimeters and 10 centimeters (0.125–4 inches) long, and have bristly jaws for seizing their prey (plankton). Although invertebrates, they swim like fish, by means of fins. They are hermaphrodite. Some species are useful indicator organisms because they are sensitive to the temperature, salinity, and depth of the water in which they live.