Comprehensive ultrastructural investigation of spermiogenesis and the mature sperm in Thylacorhynchus ambronensis revealed a number of features valuable for cladistic analysis. Two basal bodies lie on either side of an intercentriolar body in the zone of differentiation of the spermatid, but only one develops into a normal flagellum while the other remains as a small bud, eventually disappearing. Structures within and surrounding the two basal bodies differ, and, contrary to observations in another monoaxonemal schizorhynch (Baltoplana magna), the two basal bodies become separated and only that of the normal flagellum is carried distally from the cytophore which unites an isogenic group of spermatids. A spiralling ridge develops on a projection of the spermatid that is distal to the flagellar basal body; the normal flagellum becomes incorporated into the shaft of the sperm, paralleling its long axis, in the distal to proximal orientation; and the nucleus and a string of mitochondria become tightly coiled around the axoneme. Cortical microtubules surrounding the nucleus, mitochondria, and axoneme are also helically wound around the shaft, except at the proximal end. Mature sperms are very long and filiform, with a corkscrew structure -7 µm long at the distal end. The nucleus extends throughout most of the length of the sperm, while mitochondria and the central axoneme terminate some distance from the proximal end. There are no dense bodies in any region of the sperm. Although terminal corkscrew structures and, separately, monoaxonemal sperms have been found in other platyhelminth taxa, evidence suggests that neither of these features is homologous between T. ambronensis and those taxa.
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