Some animals lost organs over time, and it appears that tapeworms have lost their digestive system that would have enabled them to survive without relying on a host's pre-digested nutrients.
So - evolution in action then?
Still though, if a tapeworm had at some point a different digestive system then, presumably, it did not reside and feed in the guts of its hosts, as us currently the case - so in what was was it a tapeworm in its previous guise?
I think you need to explain how you know this, else we might think you are talking bollocks.