You could start by contrasting the cell type and compare the structures which are found in exact type of cell. You could then go on to talk about cell division and structures that are present to enable cell division.
You could refer to sheer size, a visible worm clearly cannot be unicellular as we are in the wrong order of magnitude- unicellular organisms being measured in microns reparler them visible to the naked eye. You could use this as your opener, to them talk about the differences you would see under the microscope.
You could talk about body plans and how they have evolved differently.