NEOTENY AS A SPECIAL MODE OF EVOLUTION - I - NEOTENY IN THE LOWER METAZOA, POLYCHAETA AND MOLLUSCA

Neoteny (sexual reproduction at the larval or early stage of developme nt with elimination of the following stages) is an adaptive phenomenon and arises in two situations: 1/when metamorphosis is impossible due to some external or internal factors or 2/when metamorphosis becomes n eedless due to the early onset of sexual activity (paedogenesis); it r epresents a particular case of paedomorphosis (preservation of juvenil e characters in adult animals). Examples of proposed and proved neoten y in Porifera, Cnjdaria, Ctenophora, Plathelminthes, Kamptozoa, Echiur ida, Annelida and Mollusca are considered.