Hacklemesh Weaver

While spiders have muscles to flex their spindly limbs inward, they use hydraulic pressure to extend them outward. Almost all other limbed animals have both flexor and extensor muscles, which produce smoother, less jarring, and much less unsettling movements.

To extend their legs, spiders rapidly increase pressure in their cephalothorax -- the round, bulb-like midsection to which all the legs are connected. This increase in pressure sends hemolymph (blood) flowing to the extremities, causing the legs to stretch outward. When moving, spiders innately increase and decrease body pressure in fractions of a second to quickly skitter about.
http://www.realclearscience.com/blog/2013/02/spiders-their-amazing-hydr…