Ultrastructure of the Stoma in Cephalobidae, Panagrolaimidae and Rhabditidae, With a Proposal for a Revised Stoma Terminology in Rhabditida (Nematoda)

Available information on the terminology and ultrastructure of the stoma in Rhabditida is reviewed, with new data on the cephalobids Seleborca complexa, Triligulla aluta and Zeldia punctata, as well as the panagrolaimid Panagrolaimus superbus. It is shown that the stoma of most examined species can be divided into six regions rather than five, on the basis of the cuticular differentiations and especially the surrounding structures and tissues. Probable stomatal homologies between different families are used as a basis for a revised buccal terminology, in which the following three main regions are distinguished: 1) cheilostom, surrounded by labial cuticle; 2) gymnostom, surrounded by arcade epidermis; 3) stegostom, surrounded by cells that lie enclosed within the peripharyngeal basal lamina layer. The stegostom can be divided further into (usually) four more regions, respectively called pro-, meso-, meta- and telostegostom, defined by the presence of three interradial or six adradial cell. Continued use of the term "rhabdion" as well as the traditional five-part buccal terminology is strongly discouraged, because they prove to be based on incorrect and incompatible anatomical assumptions.