• Arcade syncytia;
  • buccal cavity;
  • cladogram;
  • Diplogastrina;
  • glottoid apparatus;
  • stem species pattern;
  • stoma

The Diplogastrina include about 290 species of free living nematodes. Traditional classifications of this taxon are not based upon hypotheses of phylogenetic relationships. The highly variable structures of the buccal cavity were examined in 21 species using light microscopy and SEM. The function of the stomatal structures was studied with the aid of video recordings of living worms. The morphological data were used to reconstruct a first outline of the phylogenetic relationships of the Dipolgastrina. A rhabditoid gymnostomatal tube which is longer than wide, a short stegostom and a small dorsal tooth as in Pseudodiplogasteroides belong to the stem species pattern of Diplogastrina. Diplogastrina with a ‘Rhabditis’-like gymnostomatal tube feed on bacteria and small fungal spores. A short and broad gymnostom as well as a right subventral tooth which forms a functional unit with the dorsal tooth were acquired step by step in the ancestral line leading to Mononchoides and Tylopharynx. The cuticularized cheilostom was divided into six plates connected by pliable regions twice independently within the Diplogastrina. The teeth-bearing posterior part of the buccal capsule can move forewards by pushing apart the plates of the cheilostom so that the teeth can get in contact with food items that are too big to be sucked into the buccal cavity. Diplogastrina with a divided cheilostom can feed not only on bacteria, but also on larger fungal spores, ciliates or other nematodes. Tylopharynx is specialized to rip apart the cell wall of fungal hyphae with the movements of a dorsal and a subventral tooth in order to suck out the contents of the fungus. This shows that the transformation of the buccal cavity in Diplogastrina is linked with an expansion of ecological niches.