Spermatozoon of Callorhynchocotyle callorhynchi, posterior part. A-P, transverse sections, in antero-posterior sequence. A-C, similar to “typical” sections of spermatozoon, with one section of mitochondrion, one section of nucleus, ventral and dorsal peripheral microtubules and a few scattered additional microtubules, but with a single axoneme. The nucleus chromatin is not electron-dense. D, intermediary region in which the section is not flattened in the middle; one axoneme, ventral and dorsal peripheral microtubules, section of mitochondrion with very small diameter, and section of nucleus with electron-dense content. E-I, various sections in the flattened region, with single axoneme, ventral and dorsal peripheral microtubules, no mitochondrion, nucleus as small diameter electron-dense section. J-K, flattened region, distal extremity of axoneme; the section of the nucleus has a very small diameter or almost disappeared (K). L, side-by-side section in flattened region and distal comma-shaped section with axoneme reduced to a circle of single microtubules. M-O, distal comma-shaped section, no axoneme. P, distalmost section. Scale in E, valid for all figures: 500 nm.