The spermatozoon from testes ofLoxosoma pectinaricolaconsists of a head region composed of a long tapering nucleus with homogeneous chromatin. In some mature spermatozoa the nucleus has an anterior acrosome-like extension. A deep basal invagination in the nucleus contains a centriole and the proximal part of the flagellar axoneme. The midpiece consists of two long mitochondrial rods that are connected to each other by a thin mitochondrial lamella. The mitochondrial material surrounds a non-membrane-bounded rod of electron-dense material which also partly surrounds the axoneme. This rod runs parallel to the axoneme. A manchette of microtubules surrounds the nucleus and the mitochondrial material during elongation and nuclear condensation. The tail consists of an axoneme with the usual 9+2 arrangement of microtubules bounded by the plasma membrane. Spermatozoa have been found in the intercellular space in the tissue of the tentacular membrane, indicating possible sperm transport to the ovaries.