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 cystacant h of Acanthocephalus clavula within the hemocoel of naturally infected amphipods, Echinogammarus stammeri, from the River Brenta (northern I taly). A fully developed larva of this acanthocepbalan occupies a larg e portion of the host's hemocoel. The envelope bounding the parasite m aintains intimate contact with the internal organs and hemocytes of th e host. Intact amphipod hemocytes, as well as cells in varying degrees of degeneration, mainly on the outermost edge of the envelope of A. c lavula, were observed. No melanized larvae of A. clavula were seen wit hin the hemocoel. This envelope apparently protects the developing aca nthocephalan larva from cellular responses of the host. The present pa per reports light and transmission electron microscopical observations on this transparent acellular envelope surrounding the cystacanth of A. clavula. In the interface region of the parasite envelope with hemo cytes of E. stammeri, cells were found adherent to the outer surface o f the larval envelope and also between the inner edge of the envelope and parasite tegument.