International Journal for Parasitology 18 "0888# 388Ð409 9919!6408:88:, ! see front matter Þ 0888 Australian Society for Parasitology[ Published by Elsevier Science Ltd[ All rights reserved[ PII]S9919!6408"87#99191!0 Ultrastructure of spermiogenesis and the spermatozoon of Mesocestoides litteratus "Cestoda\ Mesocestoididae# Jordi Miquel a\ \ Carlos Feliu a \ Bernard Marchand b a Laboratori de Parasitologia\ Facultat de Farma cia\ Universitat de Barcelona\ Av[ Joan XXIII s:n\ E!97917 Barcelona\ Spain b De partement d|E  cologie E  volutive\ Laboratoire Arago\ Universite  Pierre et Marie Curie "Paris 5#\ CNRS UMR 6517\ B[P[ 33\ F!55540 Banyuls!sur!Mer Cedex\ France Received 10 July 0887^ received in revised form 08 October 0887^ accepted 08 October 0887 Abstract This paper constitutes the _rst ultrastructural study of spermiogenesis and the spermatozoon of a cestode belonging to the family Mesocestoididae\ Mesocestoides litteratus[ Spermiogenesis in M[ litteratus is characterised by a ~agellar rotation and a proximodistal fusion[ The zone of di}erentiation presents striated roots associated with the centrioles and also an intercentriolar body[ The most interesting ultrastructural feature found in the mature spermatozoon of M[ litteratus is the presence of parallel cortical microtubules[ The spermatozoon also exhibits a single crest!like body and granules of glycogen[ The pattern of spermiogenesis and the parallel position of cortical microtubules reveal the lack of concordance between M[ litteratus and cyclophyllidean species studied to date in spermiogenesis and in the ultrastructural organisation of spermatozoon[ This study provides new spermatological data and calls into question the validity of the current systematic position of mesocestoidids[ Þ 0888 Australian Society for Parasitology[ Published by Elsevier Science Ltd[ All rights reserved[ Keywords] Ultrastructure^ Spermiogenesis^ Spermatozoon^ Mesocestoides litteratus^ Platyhelminthes^ Cestoda^ Mesocestoididae 0[ Introduction The ultrastructural characteristics of spermato! zoa and spermiogenesis are useful tools in the interpretation of relationships of Platyhelminthes[ Several studies ð0Ð3Ł provide evidence of this\ and numerous studies ð3Ð6Ł have been performed descri! bing the ultrastructure of the spermatozoon and spermiogenesis[ Mesocestoidids constitute a small group of cyclo! phyllideans within a single family "Mesocestoididae  Corresponding author[ Fax] ¦23!82!3913493^ e!mail] jmiquelÝfarmacia[far[ub[es[ Fuhrmann\ 0896# with only two subfamilies "Meso! cestoidinae Lu  he\ 0783 and Mesogyninae Chert! kova et Kosupko\ 0866#\ each of which presents a single genus "Mesocestoides Vaillant\ 0752 and Mesogyna Voge\ 0841\ respectively#[ Repre! sentatives of the family Mesocestoididae present certain characteristics that clearly di}erentiate these cestodes from the other cyclophyllidean taxa[ Among these features\ the species of the genus Mesocestoides present a life!cycle with three hosts\ a genital atrium located medioventrally\ and a bipartite vitelline gland ð7Ł[ These are biological and morpho!anatomical characters that are not present in the other cyclophyllidean cestodes[ A recent phy! logeny of the Eucestoda\ based on molecular