The extensor mechanism in the tibio-metatarsal and femoro-patellar joints has been examined in the legs of several species of spiders[long dash]Dugesiella californica, Delopelma helluo, and Aphonopelma cryptethus representing the suborder Mygalomorphae, and Agelena naevia, Miranda aurantia, Latrodectus mactans, and Aranea cavicata, representing the Dipneumonomorphae. The absence of extensor muscles is confirmed. Extension is intimately associated with changes in the volume of blood in the leg, i.e., with changes in the internal fluid pressure in the leg. This hydraulic mechanism involves either partial occlusion of the "venous" pathway for returning the blood from the appendage, or diversion of a part of the arterial flow into the sac formed by the ventral interarticular membrane. Probably both processes play a part in extension.