A few ribbon worms swim by undulations of the long body. The young and the smaller forms glide along, by means of beating cilia on the body surface, over a lubricating bed of secreted slime. In larger worms more use is made of muscular contractions for creeping. Some even spiral ahead at times by agile body contortions.
One may grasp several inches of a delicate, slimy nemertean and pull cautiously lest it break, yet have it slip from one's fingers and disappear down a crack in the rock. Worms that do break in escaping from would-be captors, human or animal, almost always replace a missing rear end; and certain species can regenerate a whole worm from any fragment that contains a portion of one of the lateral nerve cords. As in flatworms, the capacity for regeneration goes with the natural capacity of certain species for reproducing asexually by fragmentation of the body, especially during warm months. A large specimen of Lineus socialis, which lives gregariously under stones on the American Atlantic coast (or of Lineus vegetus on the west coast), may fragment into six to twenty or more pieces. After transforming into complete worms of smaller size, these grow again and later reproduce sexually. Most though not all ribbon worms are of separate sexes. The eggs are usually laid in gelatinous strings or masses, and the young hatch as juvenile worms. In some species of Lineus, in Cerebratulus, and in some of their relatives, the egg hatches as a gelatinous, helmet-shaped, free-swimming little larva, called a pilidium. It must feed on microscopic organisms and develop further before it takes on the structure of the adult.