TURBEVILLE, J. M.; Virginia Commonwealth Univ.: Ultrastructure of the Proboscis Pseudocnids of the Palaeonemerteans Cephalothrixcf.rufifrons and Carinomella lactea and an Assessment of their Phylogenetic Utility.

The ultrastructure of proboscis pseudocnids (= filament-core rhabdoids) of the palaeonemertines Cephalothrix cf. rufifrons and Carinomella lactea was examined to clarify the organization of these structures and to enhance evaluation of their phylogenetic utility. The pseudocnids of both species are situated within pseudocnid-forming cells, which constitute a portion of the glandular proboscis epithelium, and they are oriented parallel to the apical basal axis of these cells. Pseudocnids of C. rufifrons are club-shaped structures, measuring 4-5 μm in length with a bulbous lateral process present near their base. Pseudocnids of C. lactea are rod-shaped structures measuring 2 μm in length and they lack a lateral process. The structures possess a uniform electron-lucent cortex surrounding an electron-dense subcortical zone and a distinct, centrally-situated tubular core, extending from the apex of the pseudocnid toward its the base. No evidence of core eversion was observed in these species. Comparative analyses reveal that these secretory bodies are not cleptocnidae and neither rhabdite nor paracnid homologues, confirming earlier results. Position and ultrastructure of pseudocnids support the hypothesis that they are homologous among palaeo- and heteronemerteans, and they are provisionally interpreted as a synapomorphy of the Anopla, as suggested in recent studies. However, molecular phylogenies of nemerteans do not support anoplan monophyly, thus contradicting this primary homology statement. A rigorous interpretation of pseudocnid evolution awaits a simultaneous analysis of molecular and morphological characters inclusive of pseudocnids.