Marimermithids (Nematoda):
Anatomy, Taxonomy, and Phylogeny

A. V. Chesunov

Department of Invertebrate Zoology, Biological Faculty, Moscow State University,
Vorob’evy gory, Moscow, 119899 Russia

Received May 16, 1996

Abstract—The organization of the Marimermithidae, Benthimermithidae, and Rhaptothyreidae (marine nem-
atodes with reduced or modified alimentary tract) is analyzed. Previously, these taxa were combined as part of
the order Marimermithida. It is shown that these families are unrelated to each other and should be assigned to
the particular orders of different subclasses of the Nematoda. The order Marimermithida sensu stricto (genera
, Australonema, Marimermis, and Thalassonema) is characterized by the pattern of the cephalic sensil-
lae in two circles (six papillae + ten setae), porelike amphid, presence of the body setae, normal cylindrical mus-
cular esophagus through the polycytic midgut, and straight paired female gonads. At the larval stage, they are
parasites of the body cavity of benthic marine invertebrates; adults are free-living and inhabit the ground, where
they mate and lay eggs. Based on the pattern of the cephalic sensillae, the location of the outlets of the esoph-
ageal glands in the anterior part of the esophagus, triangular internal opening of the cardium, and other charac-
ters, the order Marimermithida should be assigned to the subclass Enoplia. The order Benthimermithida
Tchesunov, 1995 [genera Adenodelphis, Benthimermis (= Abos syn. n.), and Trophomera] is characterized by
the following: cephalic sensillae that are composed of six plus four (papillae or short setae); rows of short setae
that are connected to the hypodermic glands that stretch along the body sides; amphids that are porelike or
round; the absence of a mouth; an esophagus that is reduced and modified into a glandular structure; intestine
that retains a cellular structure but is transformed into the trophosome (reserve of nutritious substance); the
reduction of the anus; and female gonads that are reflexive. At the larval stage, they are parasites of the body
cavity of benthic marine invertebrates; adults are free-living on the ground, where they do not feed but lay eggs.
Based on the pattern of the sensillae of the cephalic edge and medioventral supplementary organs of males, this
taxon should be assigned to the subclass Chromadoria. Judging from the structure of the early parasitic larvae
with stylet, they are probably phylogenetically close to the family Camacolaimidae. The order Rhaptothyreida
Tchesunov, 1995 (with one genus Rhaptothyreus) is characterized by huge amphids of unusual structure; there
is no mouth; the esophagus is rudimentary, and the intestine is modified. Apparently, they are free-living forms,
symbionts of the procaryotes filling the intestine.

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