Ribbon worms usually feed on scavenges and dead animals. Some live in symbiotic relationships with mussels, while other species are parasites on marine invertebrates. Food is located by chemical signals or simply by pure chance. Even species equipped with eyes do not necessarily use them to find food. Many species make use of a sharp, sometimes poisonous, tooth-like structure (stylet), in their hunt for food.
This phylum includes more than 1100 registered, mainly marine, species. They seem to have existed on earth for 500 million years, since the Cambrian period. Traditionally, the ribbon worms are classified into one of two classes, the Enopla
class, including ribbon worms with one or more stylets, while the Anopla
class is exclusively for those without. Modern molecular research does not support this classification. Again, the traditional method of sorting all life into orderly boxes, based on appearance, prove to be an impossible task.