The buccal capsule of Aduncospiculum halicti (Diplogasterina) is compared with that of Zeldia punctata (Cephalobina) and Caenorhabditis elegans (Rhabditina). Characters are mapped on an independent DNA-based phylogenetic tree (inferred from RNA polymerase II and rDNA sequences) to test evolutionary hypotheses. Irrespective of dimorphism, the buccal capsule wall of A. halicti consists of an anterior to posterior series of six cuticular structures classically termed rhabdions. These are defined according to their internal differentiations, discontinuities in profiles, and underlying tissues. Homologies of rhabdions 1 and 2 in A. halicti are proposed on the basis of position and association with adjacent tissues, consistent with those of Cephalobina and Rhabditina. Rhabdion 3 is associated with radial epithelial cells as is the mesorhabdion in C. elegans; this contrasts with Z. punctata, where a rhabdion in a similar position is associated with radial muscle cells. Dorsal and subventral teeth in A. halicti comprise rhabdions 4 and 5; this may be homologous with a corresponding region in Z. punctata but contrasts with C. elegans, where the corresponding region consists of a single metarhabdion. These characters, when mapped on the sequence-based tree, suggest that A. halicti and Diplogasterina share with C. elegans and other Rhabditina derived characters, including a mesorhabdion associated with epithelial cells, but retain some apparently primitive features shared with Cephalobina.
