Nemerteans have simple reproduction, with many testes or ovaries built into the body wall so eggs and sperm are released directly to the outside via pores, or by breakages in the body wall. Most marine species have separate sexes, but freshwater and terrestrial species are often hermaphroditic. Usually fertilization occurs externally, with spawning induced by mating behaviors and pheremones. Mating balls can be seen containing many individuals responding to chemical cues and often coordinated spawning. Eggs develop either individually or in clumps protected in egg masses, or in a few species carried by the females in the ovaries until they hatch. In most nemertean classes development is direct, yielding oval shaped, cilia covered juveniles, which may have a pelagic lifestyle temporarily before settling into a more benthic one. There is some diversity in developmental strategies among the order Heteronemertea. Many of these species develop indirectly through a feeding, swimming larval stage called the pilidium, which looks like a swimming helmet with ciliated lobes. After swimming and feeding freely, the larval ectoderm separates to form a protective skin housing the metamorphosed juvenile inside, which lives planktonically before shedding the skin and settling on the benthos. A few species, including Micrura akkeshiensis goes through the Iwaka larval stage (similar to a pilidium larva but without the lobes). Those heteronemerteans that hatch from a benthic egg case undergo development through the Desor larval stage, which although usually classified as direct, does go through a metamorphosis. Some nemerteans can also reproduce asexually by splitting, but this is not usually a regular reproductive strategy. Many species can regenerate to various degrees, including regeneration of the proboscis.

(Kozloff 2003; Brusca and Brusca 1990)