“Arrow worms” is a common name that refers to the members of the phylum Chaetognatha. The term “arrow worms” is descriptive of these organisms’ appearance, which do indeed look a bit like arrows. This phylum is composed of predatory worms that live in marine environments. Many of these worms are types of plankton, which are broadly defined as drifting organisms that inhabit the pelagic zone of bodies of water. However, not all arrow worms are plankton, as many live in the benthic zone, which is the lowest level in a body of water. The pelagic zone where plankton are found, in contrast, is defined as any part of a body of water that is not near the shore or bottom.