Is it a rotifer ?

Yes it is. This was confirmed by Dr. Oscar Nomdedeu who identified the genus as Collotheca. Thanks.

This is of the size of a rotifer. Below is a small movie showing the mouth region, then the basal body where something as mastax can be seen. Not shown here but the expansion of the mouth region after a contraction of the the body is rapid and impressive.

Your suggestions are welcome. Thanks. Martin