Arrow Worms: Chaetognatha

Arrow worms position themselves at an angle in the water. When their side bristles detect something moving in the water, the worms quickly sweep their tail, swim rapidly in the direction of the prey, and grab the prey using hooks. Scientists believe arrow worms use venom to immobilize their prey after capturing it. Arrow worms that live in middle depths usually swim to the surface at night to feed and sink to deeper water during the day.

Arrow worms make both eggs and sperm. The male reproductive organs are in the tail, and the female organs are in the trunk. The sperm develop first. Fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, takes places inside the body after one worm places a sperm pouch on another worm, and the sperm move into the second worm's body. In some species the fertilized (FUR-teh-lyzed) eggs are released into the water. In deep-water species the young develop in sacs hanging from the worm. Arrow worms do not have larvae. Small arrow worms hatch from the eggs and then continue to grow.