The corpus of the pharynx in the nematode Aphelenchus avenae (Nematoda: Tylenchomorpha) was threedimensionally reconstructed to address questions of phylogenetic significance. Reconstructed models are based on serial thin sections imaged by transmission electron microscopy. The corpus comprises six classes of radial cells, two classes of marginal cells, and 13 neurones belonging to eight classes. Between the arcade syncytia and isthmus cells, numbers of cell classes along the pharyngeal lumen and numbers of nuclei per cell class correspond exactly between A. avenae and Caenorhabditis elegans. The number of radial cell classes between the arcade syncytia and the dorsal gland orifice (DGO) in A. avenae is also identical with outgroups. Proposed homologies of the pharynx imply that expression of the anterior two cell classes as epithelial or muscular differs within both Rhabditida and Tylenchomorpha. Numbers of neurone cell bodies within the corpus correspond exactly to C. elegans, other free-living outgroups, and other Tylenchomorpha. Neurone polarity and morphology support conserved relative positions of cell bodies of putative neurone homologues. The configuration of cells in the procorpus, including the length of individual cell classes along its lumen, differs across representatives of three deep Tylenchomorpha lineages. Nonhomology of the procorpus challenges the homology of DGO position within the metacorpus, the primary taxonomic character for circumscribing 'Aphelenchoidea'. Comparison of A. avenae with Aphelenchoides blastophthorus shows that, despite gross pharynx similarity, these nematodes have several differences in corpus construction at a cellular level. The possibility of convergent evolution of an 'aphelenchid' pharynx in two separate lineages would be congruent with molecular-based phylogeny. Putative homologies and conserved arrangement of pharyngeal neurones in Tylenchomorpha expand the experimental model of C. elegans.

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The anterior sensory anatomy (not including amphids) of the nematode Aphelenchus avenae (Tylenchomorpha) has been three-dimensionally reconstructed from serial, transmission electron microscopy thin sections. Models, showing detailed morphology and spatial relationships of cuticular sensilla and internal sensory receptors, are the first computerized reconstruction of sensory structures of a Tylenchomorpha nematode. Results are analyzed with respect to similarly detailed reconstructions of Rhabditida outgroup nematodes, Acrobeles complexus (Cephalobomorpha) and Caenorhabditis elegans (Rhabditomorpha). Homologies identified in A. avenae demonstrate the general conservation of the anterior sensory system between freeliving nematodes and the largely plant parasitic Tylenchomorpha. A higher degree of similarity is shown between A. avenae and A. complexus, with common features including: the presence of a second, internal outer labial dendrite (OL1); a second cephalic dendrite in the female (CEP2/CEM); an accessory process loop of inner labial dendrite 1; and terminus morphology and epidermal associations of internal sensory receptors BAG and URX. Unique to A. avenae is a pair of peripheral, lateral neurons of unknown homology but with axial positions and intercellular relationships nearly identical to the "posterior branches" of URX in A. complexus. Knowledge of homologies and connectivity of anterior sensory structures provides a basis for expansion of the experimental behavioral model of C. elegans to the economically important nematodes of Tylenchomorpha.

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