Fine structure of the envelope surrounding the cystacanth of Acanthocephalus clavula (Acanthocephala) in its intermediate host Echinogammarus stammeri (Amphipoda)

We describe the ultrastructure of the envelope covering each cystacanth of Acanthocephalus clavula within the hemocoel of naturally infected amphipods, Echino- gammarus stammeri, from the River Brenta (northern Italy). A fully developed larva of this acanthocephalan occupies a large portion of the host's hemocoel. The envelope bounding the parasite maintains intimate contact with the internal organs and hemocytes of the host. Intact amphipod hemocytes, as well as cells in varying degrees of degeneration, mainly on the outermost edge of the envelope of A. clavula, were observed. No melanized larvae of A. clavula were seen within the hemocoel. This envelope apparently protects the developing acanthocephalan larva from cellular responses of the host. The present paper reports light and transmission electron microscopical observations on this transparent acel- lular envelope surrounding the cystacanth of A. clavula. In the interface region of the parasite envelope with hemocytes of E. stammeri, cells were found adherent to the outer surface of the larval envelope and also between the inner edge of the envelope and parasite tegument.