Ultrastructure of spermatogenesis in Phreodrilus (Phreodrilidae, Oligochaeta, Annelida)



Phreodrilus has a free filamentous spermatozoon of an advanced type, characteristic of the Oligochaeta but with distinctive features. As in the Lumbricidae and Megascolecidae the primary acrosome vesicle, enclosing the subvesicular space (secondary acrosome vesicle) is withdrawn deeply into the acrosome tube and the nucleus is non-spiral. The midpiece has six mitochondria as in these families, in the Lumbriculidae and in some Tubificidae and its radial septa are spiral as in the Lumbriculidae and Enchytraeidae. Among peculiar features are the great elongation of the acrosome and midpiece, the long basal acrosomal chamber distal to the axial rod and primary vesicle and spiralling (possibly transient) of the peripheral elements of the flagellar axoneme. Spermatogenesis accords with that in other oligochaetes but an anomalous and hither unknown type of sperm morula is present in addition to typical morulae. It shows multiplication of each cytophore-attached spermato-gonium to give many spermatids on each zonula collaris and more than 400 cells in a morula.