The duck is the definitive host for the acanthocephalan parasite.
From the en.wikipedia.org
Having been expelled by the female, the acanthocephalan embryo is released along with the feces of the host.
From the en.wikipedia.org
Acanthocephalan worm, Echinorhynchus truttae, in the invader Gammarus pulex, the parasite extending from the host's head to about half way along the body.
From the sciencedaily.com
Inside the intermediate host, the acanthocephalan penetrates the gut wall, moves into the body cavity, encysts, and begins transformation into the infective cystacanth stage.