The spermatozoon of Oikopleura dioica Fol (Larvacea, Tunicata).

The spermatozoon of Oikopleura dioica is about 30 micron long, with a spherical head, about 1 micron wide, a 3 micron long and 1 micron wide midpiece, and a 25 micron long tail with a tapered end piece. The head contains a nucleus with the chromatin volume limited to about 0.1 micron3. A small acrosome is found in an anterior inpocketing, and a flagellar basal body in a posterior inpocketing of the nucleus. The midpiece contains a single mitochondrion with the flagellar axoneme embedded in a grove along its medial surface. The flagellar axoneme has the typical 9 + 2 substructure, and the basal body the typical 9 + 0 substructure. A second centriole and special anchoring fibres are absent.