How do I clean this koi pond water pump?

Does the plastic screen come off? Is there some kind of filter inside it? I can feel it vibrating when I touch it (it is submerged in a pond) but when I feel around the top half of it and the sides I feel no suction at all. Does anyone know how these things are built? I haven’t been able to find a schematic drawing.

Here is an image of it (or at least of something that looks similar) – http://www.lagunakoi.com/Laguna-Pumps/Max-Flo-900-p-490.html