Arrow Worms: Chaetognatha

Arrow worms are long, straight, narrow sea worms that use hooks for catching prey. Arrow worms that live near the surface are clear, which helps them avoid predators. The intestines of species that live in middle depths often are yellow or red because they eat prey of those colors. Species that live in deeper water are more muscular and less clear than those that live near the surface. Folds of the body wall in the neck region form a hood that folds over the head, helping the worms to swim smoothly. Tufts of bristles on the body are used for detecting prey. The worms have tail fins and one or two pairs of fins on the sides of the body.

Arrow worms are 0.1 to 6 inches (3 to 150 millimeters) long. The largest species lives in Antarctic waters and is about 3 inches (70 millimeters) long. The bottom-dwelling species are the smallest, one species reaching maturity at a length of about 0.1 inches (3 millimeters).

The body cavity of arrow worms is filled with fluid that is surrounded by muscles and a tough covering. The head has complex muscles that support the grasping hooks. Arrow worms have a mouth, one or two rows of teeth, and eyes.