Reproductive biology

Most nemerteans are dioecious, although there are a few hermaphroditic hoplonemerteans (class Enopla). Palaeonemerteans are traditionally regarded as direct developers, whereas het-eronemerteans all produce pilidium larvae (free-swimming, planktotropic larvae), or larvae that seem to be derived from a pilidium. These heteronemerteans are broadcast spawners, but there are also heteronemerteans (Lineus rubber and L. viridis) known to reproduce by a kind of pseudocopulation in which the

Unsegmented ribbon worm (Lineus sp.) seen near Panama. (Photo by Kjell B. Sandved/Photo Researchers, Inc. Reproduced by permission.

fertilized eggs are deposited in a gelatinous cocoon producing pilidium-like larvae known as Schmidt's and Desor's larvae, respectively. Only a few species are known to reproduce vegeta-tively by fission (a few species in genus Myoisophagus).

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