The nemerteans are elongate, often ribbon-like worms with bilateral symmetry. They are non-segmented and have a ciliated epidermis but unlike the flatworms have a separate mouth and anus. Dorsal to the gut, and separate from it, is a muscular, eversible proboscis used in the capture of food and in defence. It is from this structure that the animals get one of their common names, the proboscis worms. They are also known as ribbon worms. Nemerteans are carnivorous and feed on a wide variety of crustaceans, polychaete worms and molluscs, which are captured on the proboscis. In some cases the prey is ingested whole, while in others the body of the prey is torn by the proboscis, which in some species has one or more piercing stylets, and the nemertean feeds suctorially. Many species pursue their prey which is killed by powerful neurotoxins while others scavenge on dead and decaying animals. Despite the presence of toxins in the tissues, nematodes are themselves eaten by a wide range of fishes. Most nemerteans have separate sexes and fertilization is usually external. In some species the gametes are deposited in an egg sac where fertilization and development take place. Several different types of larvae have been described, including a ciliated pelagic larva known as a pilidium. Length of life is believed to be about one year but the larger species possibly live for several years.