SPERMIOGENESIS AND SPERM ULTRASTRUCTURE IN CYLINDROSTOMA-FINGALIANUM (PLATYHELMINTHES, PROLECITHOPHORA) WITH NOTES ON A PROTOZOAN (KINETOPLASTID) SYMBIONT CLOSELY ASSOCIATED WITH ALLOSPERM

The only positive synapomorphy uniting members of the platyhelminth tu rbellarian taxon, Prolecithophora, is a peculiar membranous system pre sent in much of the shaft of the sperm. This investigation followed th e development of this membrane system during spermiogenesis in Cylindr ostoma fingalianum and its appearance in the mature sperm. Formation b egins with invaginations of the early spermatid shaft outer cell membr ane. The invaginations enlarge and the membrane becomes increasingly f olded. They appear to be subsequently supplemented with membranous mat erial supplied by an extensive vacuolar system originating from the Go lgi apparatus in the main cytoplasmic mass of the spermatid. Throughou t development, and in mature sperm, the membranous system forms a part of the outer cell membrane and is also intimately associated with the elongate mitochondrion that lies between the membranous column and th e nucleus. The nucleus is highly lobed and the sperm cell lacks flagel la or axonemes or the dense bodies found in most other turbellarian ta xonomic divisions. A kinetoplastid protozoon present in considerable n umbers amongst allosperm is also described.