As far as I remember they have a circlip ,if your one doesn't it's probably fallen off or has been removed and not put back on because the wheel is stuck on.
In the pic below it looks to have a circlip.
People make pullers to get the wheels off.
I'd have to draw the way I remove them.the two plates the same go at the back of the wheel ,over the top of the other one, then use a bolt in the plate at the outside of the wheel to push on the axle, two holes must be drilled near the centre of the wheel and the 2 bolts go through.
You can also use CRC and heat.
The bolt end that pushes on the centre of the axle should be ground down so it's smaller than the axle otherwise it can mushroom out the axle end making it a lot more difficult to remove.
