ULTRASTRUCTURE OF THE SPERMATOZOON OF SANDONELLA-SANDONI (CESTODA, PROTEOCEPHALIDEA, SANDONELLINAE)

The mature S. sandoni spermatozoon exhibits an apical cone of electron -dense material about 1 mum long and a helicoidal crest-like body 50-1 00 nm thick. The cortical microtubules, varying in number from 0 to 18 depending on where the section is cut, are of different lengths, spir alized, and in close contact with the plasma membrane in regions III a nd IV of the spermatozoon where they form one or two fields of 1 to 9 units, which are associated with electron-dense subjacent material and are either laterally delimited or not by submembranous thickenings. T he cytoplasm contains amorphous electron-dense material in regions II, III and IV of the gamete. The nucleus is a fine cord coiled in a spir al around the axoneme. This is of the 9 + ''1'' pattern. Its posterior extremity is made up of only the axial element surrounded by nine sin glets formed by the A microtubules attached to the plasma membrane. Th is type of posterior extremity of the flagellum has never been describ ed in a platyhelminth. Similarly, submembranous thickenings and microt ubular fields associated with electron-dense subjacent material have n ever been observed in a cestode spermatozoon. Moreover we report for t he first time the existence of a spermatozoon with a single axoneme an d crest-like bodies in the Proteocephalidea.