Spermatozoon of Callorhynchocotyle callorhynchi, anterior part. A-T, transverse sections, in antero-posterior sequence. A-C, anteriormost sections, showing a few microtubules. D, E, centriole and first axoneme. F, two sections with single axoneme surrounding a section with two axonemes. G, one fully formed axoneme and centriole of other axoneme. H-L, sections with two axonemes, peripheral microtubules on dorsal and ventral part, and additional microtubules. M-P, sections with peripheral microtubules on dorsal and ventral part, progressive disappearance of additional microtubules, appearance of sections of mitochondrion and nucleus. Q-T, “typical” sections of spermatozoon, showing two axonemes, one section of mitochondrion, one section of nucleus, ventral and dorsal peripheral microtubules and a few scattered additional microtubules. Scale in S, valid for all figures: 500 nm.