j Ne matology, 200 I, Vol. 3(6), 491-502 On the histological anatomy of Benthimermis me gala Petter, 1987, a giant nematode from the Norwegian deep-sea (Nematoda: Benthimermithidae) Dmitry M. MILJUTIN and Alexei V. TCHESUNOV * Department of invertebrate Zoology, Faculty of Biology, Moscow State University, Moscow 119899, Russia Received: ll December 2000; revised: 20 April 2001 Accepted for publication: 20 April 2001 Summary - Free-living adult stages of Benthimermis mega/a Petter, 1987, the biggest species (body length 78- 148 mm) of the genus, have been found in bottom sediments of the deep-sea off the Norway coast and studied histologically. The head region includes six subcuticular sensilla, four minute cephalic setae and pore-like amphids. Numerous tiny sensilla are distributed throughout the lateral si des of the body. An axial spine is present on the posterior body terminus. Hypodermal glands are associated with the somatic sensilla. There are eight chords in the hypodermis. Mouth opening is absent. Vestigial pharynx is glandular and devoid of an internal lumen. Midgut is a trophosome made up of large radial cells and a very thin axial internal lumen. The trophosome ce ll s are Ill led with various inc lu sions. which are reduced in si ze and number with age. A few cuticular grains are present in a vestigial rectum. The nerve ring is embedded in the anterior trophosome. The female genital system is amphidelphic. The germinal zone of the ovaries extends the length of the gonad (hologonic ovary), whereas the ovaries of sma ll er Benlhimermis species are telogonic. Neither spermatozoa nor spermatheca in fe male ducts were identified. No males of B. mega/a were found. Obviously, larval stages parasitise benthic invertebrates, while non-feeding adult stages dwell freely in sediment and reproduce. Keywords- cuticle, hologonic ovary, marine nematodes, parasitic nematodes, taxonomy, trophosome. The nematodes of the family Benthimermithidae were discovered comparatively recently (Hope, 1977; Petter, 1980). They are khown as rare and poorly studied para- sites of a wide range of deep-sea benthic invertebrates: polychaetes, priapulids, various crustaceans, holothuroids and even free-living nematodes (Hope, 1977; Petter, 1980, 1983b; Rubzov, 1980; Chesunov, 1988). As in fresh-water and soil mermithids (Mermithida), the larval stages of benthimermithids parasitise the host body cavities and in- ternal organs. Adult or late larval worms leave the host to the environment for a nonparasitic phase where they reproduce but evidently do not feed (as inferred from a strongly modified alimentary tract including lack of a mouth). The morphology of benthimermithids includes some features of marine free-living nematodes (head and nu- merous body seta, amphids situated far behind the anterior end). In addition, they have a strongly modified alimen- tary tract. However, internal structures of benthimermi- thids are poorly studied. We examined the anatomy of *Corresponding author, e-mail: tchesunov@mtu-net.ru © Koninklijke Brill NV, Leiden, 2001 the adu It female of Benthimermis megala Petter, 1987, the largest species of Benthimermis. Neither larvae nor males, nor hosts of this species are known hitherto. B. mega/a was originally collected in the Norway Sea at depths 2668 - 2904 m and briefly described by Petter (1987). Material and methods Adult free-living nematodes were collected in the Nor- way Sea during monitoring investigations with the vessel 'Academician Mstislav Keldysh' near the site of the 1989 wreck of the Russian atomic-powered submarine 'Komso- moletz'. Nematode spec imens were found between 1989 and 1995 among other macrobenthic organisms in trawl catches in the area 73°42' - 73°47'N and 12°57' - l3 °24'E at 1478 - 1780 m deep. Every year the trawlings were conducted in the period from May to September. All ne- matode specimens were fixed on board, initially with 4% formaldehyde in seawater. 491