Hi Bernhard,
A really interesting and nicely detailed photo.
Not for the first time has one of your postings introduced me to a completely new concept (for which I thank you). However although it's been an interesting journey, so far I have been able to find out very little about the process of encystation.
Out of interest, I came across
this website which contains quite a few photos of testate amoeba including one of an encysted arcella.
Bruce