Arrow worms are small, planktonic, marine predators. They are slender, transparent and just a few centimeters long. There are over a hundred species, mostly found in warm, shallow seas but also occuring in polar regions. They are most common in the Indo-Pacific region where they may form dense swarms and are important predators in the plankton. Their name suggests their basic shape: worm-like body with a small head, two sets of lateral fins and a tail fin that give the worm its arrow-like appearance. The fins are formed from epidermis strengthened by rays.
The head has a ventral depression called the vestibule which leads to the mouth. There are both anterior and posterior rows of teeth plus bristles around the sides of the head, which give the phylum its name, Chaetognatha, which means bristle-jawed. There is a hood which folds over the head when the animal is not feeding which protects the teeth and bristles. There are a pair of well developed compound eyes for finding their prey. There is also a complex nervous system to control a complicated musculature and enable them to lead their active predaceous lifestyles.
The trunk is mainly given over to the digestive system, muscles and reproductive system. Chaetognaths have no respiratory or circulatory systems and gas exchange is simply across the tissues. There is also no excretory system.
Chaetognaths are different enough to deserve their own phylum but show relationships in their development with the deuterostomes. The blastopore becomes the anus and a different pore opens to become the mouth, as in the other deuterostome phyla, the Chordata and the Echinodermata. They are an old phylum, appearing in the fossil record some three hundred million years ago.
Arrow worms are voracious predators, attacking many small planktonic organisms such as crustaceans but also small fish that can be almost as large as the arrow worm. Most swim actively, searching for prey and pouncing with quick movements and a rapid snap of the raptorial spines on their heads. Others attach to a substrate and wait for their prey. The pharynx secretes a glue which helps to entangle the prey and may contribute to digestion as well.
Arrow worms are hermaphrodites. Paired testes lie near the tail. Sperm are formed into spermatophores. Ovaries like at the end of the trunk and there are double oviducts leading to the seminal receptacle. Most species are self-fertilising, with the spermatophores coming out and attaching to the body wall and then the sperm entering the vagina and moving to the seminal receptacle where they fertilise the eggs. A few species outbreed. Fertilised eggs break through the body wall and develop in the plankton. Newly hatched arrow worms are tiny versions of the adults and do not go through metamorphosis, just growing until they are large enough to mature sexually.
Arrow worms are important members of ocean communities. In some planktonic swarms, they are one of the most numerous members, being outnumbered usually only by the copepods. In some deep sea communities, they are the most numerous members. They are an important part of the food chains, not only feeding on smaller animals but being fed on in turn by fish, squid and sea birds.
References: Buchsbaum, R. 1968. Animals without Backbones. Penguin Press edition. Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://academic.evergreen.edu/t/thuesene/chaetognaths/chaetognaths.htm