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Well, actually it must not move the joints at the hip area. Itīs enough to move to move only the "claws" on top of the legs. Itīs like the fingers. We canīt move every single finger from the hand join on, even if they are single bones in the hand. But we could walk with our hand 
In this case the three legs on both side may be able to move, but not simultaneously, a bit like the hand example indeed. However for a arachnoid lifeform it would look quite strange if he walks like a bi-ped although having 3 legs on each side. 
Anyways, enough about the legs.
Cheers
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