MARIMERMITHID NEMATODES - ANATOMY, POSITI ON IN THE NEMATODE SYSTEM, PHYLOGENY

Structure and life cycles of marine nematode families Marimermithidae, Benthimermithidae and Rhaptothyreidae are analysed. These three taxa were united by some nematologists in the order Marimermithida Rubtzov 1980 regarded mainly by modified alimentary tract. However the order M arimermithida was considered to be artificial; its three families are not related and should be erected into separated orders. The order Mar imermithida s. str. (genera Ananus, Australonema, Marimermis, Thalasso nema) is characterized by the cephalic sensilla pattern (6 papilla + 1 0 setae), pore-like amphid, presence of body setae, normal cylindrical muscular esophagus, normal polycytous midgut, didelphic outstretched female tubes. Their larvae parasitize body cavity of marine benthic in vertebrates whilst the adults dwell freely and reproduce in sediment. According to cephalic sensilla pattern, position of the esophageal gla nd outlets at the anterior esophagus, triangular internal lumen of the cardia the order Marimermithida should be referred to Enoplia. The or der Benthimermithida Tchesunov 1995 (genera Adenodelphis, Benthimermis /=Abossyn. n./, Trophomera): cephalic sensilla 6 + 4 (papilla or short setae); short lateral setae associated with hypodermal glands along t he body; pore-like or rounded amphids; no mouth opening; esophagus ves tigial or transformed into a glandular structure; midgut cellular, wit h reduced internal lumen, evidently serving as a trophosome; anus redu ced; female tubes paired and reflexed. Like marimermithids, larvae par asitize body cavity and internal organs of marine benthic invertebrate s whilst adults live freely in sediments upon trophosome reserves and reproduce. Judging on cephalic sensilla pattern and male midventral su pplementary organs the Benthimermithida should be referred to Chromado ria. Young stilet-bearing parasitizing larvae show some features allow ing to hypothesize the phylogenetical relation of Benthimermithida to Camacolaimidae, free-living marine nematode family. The order Rhaptoth yreida Tchesunov 1995 (single genus Rhaptothyreus): big amphids of an unusual construction; no month opening; esophagus vestigial; midgut mo dified. Apparently they live free in sediment, feeding by interacting with procaryote organisms filling the midgut.