Turbellarian taxonomic database

Abstract / Notes

Systematics of the Acoela is particularly difficult because of the paucity of readily discernible
morphological features. In other soft-bodied worms, sclerotized structures, such as copulatory stylets,
provide important characters that can be seen in whole mounts, but acoels generally lack such features. Among
the few sclerotized structures in acoels are bursal nozzles-tubiform outlets on the seminal bursae that are
believed to be conduits (spermatic ducts) through which allosperm are transported to the oocytes. Early
classifications of the Acoela used features of the female reproductive system, including bursal nozzles, for
distinguishing major groups, but the current system essentially ignores them as too plastic to provide
higher-level distinctions. We used confocal and electron microscopy to further characterize bursal nozzles in
five acoel species, and found all composed of actin-reinforced extensions of stacked, flat mesenchymal cells.
In Notocelis gullmarensis, Aphanostoma bruscai, and Daku woorimensis, the nozzle is a stiffened region of the
same cells forming the wall of the bursa. By contrast, in Wulguru cuspidata cells forming the nozzle are
distinct from those of the bursa. The so-called bursal cap of A. bruscai and D. woorimensis has small
sclerotized disjunct units within it, also composed of stacked, flat, actin-reinforced cells. The nozzle of
W. cuspidata, prominent like that of other convolutid acoels, is relatively complex, its actin-reinforced
cells sandwiched with secretory cells and its base bearing a "sorting apparatus" of egg-shaped cells that
send narrow processes inside the spermatic duct. Cases of sperm inside the nozzle corroborate its assumed
role in reproduction. Whereas most nozzles sit at the end of the bursa facing the ovary, in species of
Pseud-mecynostomum and purportedly in a few other acoels, they sit between the female pore and the bursa,
constituting what we call a vaginal nozzle. All bursal nozzles of acoels show a common ground pattern
indicating common ancestry, but certain features discerned through electron and confocal microscopy show
promise of providing synapomorphies for grouping some species,

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