The spermatozoon of Myzostomum cirriferum (Annelida, Myzostomida).

The Myzostomum spermatozoon has several unique features: The chromatin material is condensed into a variable number of dense balls (usually 42); there is an incomplete myelin-like cover rather than a regular nuclear envelope; a palisade of microtubules surround two mitochondria which extend along the 30-microns-long cell body; an axoneme of the 9+0 formula originates at one end of the cell body, runs intracellularly for a short distance (less than 1 micron), then continues outside the cell body although attached to it by means of a "flagellum-cell body junction," which consists of two sets of strands connecting the membrane of the flagellum to that of the cell body. Two "dorsal rods" in the microtubular palisade evidently are connected to these strands. The flagellum also has a free portion (around 70 microns) which terminates in a pointed spine, called the "sting." No typical centriole and no acrosome have been found. The spermatozoon usually swims with flagellum foremost but may reverse the direction. The cell body also undulates but with an independent wavelength. Presumably some of these peculiarities are specializations for locomotion within body tissues rather than in water; Myzostomum has "dermal copulation."