The common name for Chaetognatha is arrow-worm. There are about 90 known species, and they range in length from 2.5 - 10 cm. They are soft-bodied and transparent worms with distinct head, trunk and post-anal tail (see Sagitta elegans right), although the trunk and tail fins have no muscles.

They tend to stay on the bottom during the day, and swim to the surface at night. They are important plankton predators, though they will consume small fish and other arrow worms. Prey is grasped by the moveable spines on either side of the mouth (see the drawing of a chaetognath head on the left), and killed by injecting a very potent poison. They have multi-faceted eyes giving them good vision. Sagitta elegans preys on larval molluscs, small crustaces and fish larvae, and can eat 1/3 of its body weight in a day. It is preyed on by jellyfish and annelids. S. elegans occurs in water of high salinity and purity, high in phosphate and rich in plankton. A closely related species S. setosa can tolerate water of lower phosphate content and with fewer plankton, so these two species can be used as indicator species.