Spiny-headed worms, or arrow worms as they are also known, belong to the phylum Chaetognatha. Their bodies are shaped like a torpedo with distinct head, trunk, and tail regions, the latter which bears a pair of finlike projections that probably assist with balance. Although many spiny-headed worms can swim, they usually conserve their energy and instead drift with the water current. The body is usually transparent and slender. The head bears a pair of eyes and is adorned with a number of large curved spines, which can range from 4-14, depending on the particular species. These spines, which also fulfil a sensory role, are used for capturing small prey and, when not in use, are covered with a special hood that arises from a fold in the body wall. This may also help reduce resistance to the water current by streamlining the body even further. All of these species are active carnivores, feeding on plankton, small crustaceans, and even small fish. The usual hunting strategy is to lunge at prey once it is within reach, grab it with the smaller spines surrounding the mouth, and then crush it with the larger spines, while simultaneously pushing it in towards the mouth.
Some 65 living species are known, all of which are marine-dwelling. The majority of these are tropical species. With the exception of members of the genus Spadella, which are specialized benthic species, all remaining spiny-headed arrow worms are designed for a planktonic existence. The vast majority of these are small invertebrates, measuring approximately 1.2 in (3 cm) in length; some species, however, may reach a length of 3.9 in (10 cm).
Spiny-headed worms are hermaphroditic, with the male and female reproductive cells arising from the lining of the coelom. Some species reproduce by self-fertilization, the mature sperm being stored in special sacs known as spermatophores until such time as the eggs are ready for fertilization. Some species, however, exchange male gametes by coming together and cross-fertilizing each other. The eggs may be retained within the body for further development, or may be released to the ocean. The larvae, which resemble the adults, are free-swimming.