Date thesis is presented jugr1gLL_1961 Title THE LIFE HISTORY OF NEOECHINORHYNCHUS BU_T.LLI (MUELLER, I78O) AND ITS DEVELOPMENT IN THE INTERMEDI-

an acanthocephalan parasite, which lives attached to the rnucosa of the intestine. Between July l96I and January L963, several hun-dredof these fish were exarnined and nearly every one was infected with this parasite, which was identified as Neochinor:hynchus rutili (Mueller, I780). Since relatively little is known about the life cycle and post-ernbyonic developrnent of N. rutili, it was decided to study these problerns in the laboratory. Of Z6Z ostracods (Cypria tyJneri) exarnined, 64 or 24 percent were found to harbor the cystacanth or juvenile stage. Sorne of these ostracods were fed to two srnall infection-free cutthroat trout. The first fish was fed five and the second was fed nine. Two weeks later one rnale acanthocephalan was found established in the Redacted for Privacy intestine of the first fish and fou-c, i"nc1t'-ding three fer:'rales in the second. It was possible to foLlow the developrnent of the parasite f rorn the ernergent ernbrvo to the infective cystacanth. Several infection-free ostracods frorn a 1ocal pond we're infected by allowing thern to feed on shelled ernbryos frorrr the body cavity of an adult worrn. The ostracods were then dissected periodically and the stages of developrnent were studied and d::awn. The life cycle can be surnrnarized in the following way. The shelled ernbryos are released by the adult worrn into the intestinaL lurnen of the fish and they pass or:.t with the feces. These ".gg"" are ingested by ostracods (Cypril tulneri). The ernbryo efiIerges as an acanthor and penetra.tes the wall " of the gut into the hernocoel where it rnetarnorphoses th.rough acantheLla to the cystacanth or juveniJ-e stage in 48 to 57 days. This fir:aL stage is infective when eaten with the ostracod by the fish