Chaetognatha is a small 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, each worm containing both ovaries and testes. Reproduction is by self-fertilization andthe first stages in the development of the embryo take place inside teh body of the adult worm.

 

Some species are useful indicator organisms because they are sensitive to the temperature, salinity, and depth of the water in which they live.

 

Arrow worm (Sagitta elegans).