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. Their size is roughly 3-5 cm (1-2 in), about the length of a fingernail, though some individuals may be as large as 10 cm (4 in). There are only about 120 species of chaetognaths known in 20 genera, but despite the low species count, their abundance is difficult for the human mind to imagine.
We can give a rough estimate for the number of arrow worms worldwide. Measured densities range from about 1 to 30 individuals per cubic meter of illuminated water, with an average of five. As the World Ocean has an area of about 340 million square kilometers, not including Arctic and Antarctic waters, and the pelagic zone is 200 m deep, we can estimate a world total of approximately 340 billion, which is probably within an order of magnitude of being correct. Interestingly, this also allows us to estimate that the human biomass exceeds the arrow worm biomass.
Because chaetognaths are transparent, scientists can put them under a microscope and observe the entire process of their digestion. They estimate that arrow worms eat between 3 and 50 prey items per day, including numerous types of larvae, copepods and other small crustaceans, and other arrow worms. They pierce the thin protective tissue layers of these organisms with their chitin spines, then inject them with toxins, such as the neurotoxin tetrodotoxin, to cause death.
Arrow worms are considered a good model for an early bilaterian. They are thought to be basal protostomes (one of the two major divisions of animals), even though elements of their embryological development are reminiscent of deuterostomes. This is thought to be because they represent a very early branching off of protostomes from deuterostomes. This event likely occurred an extremely long time ago, in the Ediacaran period. Molecular and morphological studies have indicated that chaetognaths are most closely related to nematodes, and may in fact be related the common ancestor of ecdysozoans (the group that includes every animal that sheds its cuticle).