Ultrastructure of the postcorpus of the esophagus of Teratocephalus lirellus (Teratocephalida) and its use for interpreting character evolution in Secernentea (Nematoda)

The ultrastructure of the postcorpus of the putative outgroup of Secernente a (Nematoda). teratocephalus lirellus (Teratocephalida), is compared with p revious observations of representative species Zeldia punctata (Cephalobina ), Caenorhabditis elegans (Rhabditina), and Diplenteron sp. (Diplogastrina) in order to interpret the evolution of feeding structures within Secernent ea. The postcorpus of T lirellus consists of 6 marginal, 13 muscle, 3 gland , and 11 nerve cells. In both T. lirellus and Z. punctata, one duct from ea ch of two subventral glands opens into the esophageal lumen at the junction of the isthmus and the basal bulb, whereas in C. elegans and Diplenteron s p., homologous openings are at the posterior end of the median bulb. Caenor habditis elegans and Z punctata each have two additional glands that open w ithin the basal bulb. The postcorpus of each taxon has four anterior-to-pos terior layered sets of radial muscle cells, except in Diplenteron sp., whic h lacks a grinder and has homologs to the anterior two sets only. The anter ior set of muscles of T. lirellus and Z punctata includes six mononucleate cells, whereas the homolog in C. elegans and Diplenteron sp. includes three binucleate cells. Evaluation of character polarity defines Rhabditina and Diplogastrina as sister taxa, and suggests that the character of five gland s may result from functional convergence.