One thing that can easily be demonstrated within the invertebrates is the evolution of the gut. In lower invertebrates such as Platyhelminthes we see a pocket gut, with but a single opening for food to enter and waste to exit. In the Cestoda this becomes a simple through gut, with a mouth at one end and an anus at the other. Moving higher in the group, we see the through gut becoming more complex, with pockets and pouches forming along the tube, all of which increase the efficiency of digestion.
Another milestone is the eye. Planarians have a light-sensing eyespot, which is not much more than a pigment patch. As you look at more evolved groups, the eye develops into a true eye with an internal lens; additionally, animals with more eye development have greater brain development and are more definitively cephalized.