What are Arrow Worms?
Arrow worms are a highly unusual group of “worms” not obviously related to any other worm or any other animal. They make up their own phylum, Chaetognatha, one animal phylum among about 37 total. In terms of numbers, arrow worms are among the most abundant phyla on the planet — only arthropods, nematodes, and a few others even come close. Arrow worms inhabit the water column, usually at the pelagic zone, within 200 m (656 ft), where most plankton lives, though they can be found as deep as the lower mesopelagic zone, 1000 m (3,280 ft). Alongside copepods, the crustacean “insects of the sea”, chaetognaths are the most common form of plankton. Their name — Chaetognatha — comes from the Greek chaite for long hair and gnathos for jaw. The long hair is a reference to the hooked, chitinous grasping spines found in pairs on their head, their primary hunting tool. The spines are held in a hood while they are swimming. Arrow worms are carnivores, eating up any plankton that they can find. The
