The ultrastructure of the spermatozoon of the hoplonemertine,Emplectonema neesii

Summary Mature spermatozoa of the hoplonemertine, Emplectonema neesii were studied by light and electron microscopy. The spermatozoa are flagellate and motile, each gamete consisting of an elongate anterior head and a posterior flagellar tail. Three regions are identifiable in the head, the acrosome, a nuclear zone and a connecting piece containing two centrioles. The nuclear zone contains glycogen granules as well as an elongate, grooved nucleus and a large mitochondrion whose lobes interdigitate with the nuclear grooves. The flagellum has a typical 9 + 2 flagellar tubule organisation. Nemertine spermatozoan ultrastructure, as exemplified by that of E. neesii, is compared with that of platyhelminth male gametes and the supposed phylogenetic affinity of the two taxeis reexamined in the light of the results of this comparison.My thanks are due to Miss A. Harding and Miss J. Webb for collecting sexually mature E. neesii, and to Dr. R. G. Crump for laboratory facilities at the Orielton Field Centre, Pembrokeshire, Wales. The work was carried out during the tenure of a research grant from The Nuffield Foundation.