Your lecturer has ignored acoelomate (which, since there’s no cavities, is a valid thing to ignore) and split the coelomates along your protostome-deuterostome line, based on how the mesoderm forms. So you may not be as confused as you thought you were.

As a true indicator of groupings, only the last two are any use. The pseudocoelomates are a bunch of different types lumped together on a fairly archaic basis.