Sperm morphology in the Pterastericolidae (Platyhelminthes, Rhabdocoela): phylogenetic implications

The sperm morphology of five species of the Pterastericolidae was studied with transmission electron microscopy. The spermatozoa of all species have two axoncmes, which are incorporated in the sperm cell body for most of their length. The axonemes arc of the'9 + 1'pattern characteristic of the flatworm taxon Trepaxonemata. Outer dynein arms are absent from the microtubule doublets in the axonemes. Dense bodies are few, and occur only in the distal part of the spermatozoa, but small electron dense granules are numerous. A sister group of the Neodermata consisting of the Pterastericolidae and the Fecampidae is proposed. The monophyly of the taxon Dalyellioida is discussed