Some monkeys use this as a fifth limb; it's prehensile, after all
Unfortunately, in some 
areas of its distribution, populations of the common marmoset are... [DOC] [TLE] Black spider monkey calling and hanging from branch by prehensile ... [PAR] The largest of the spider monkeys, the black spider monkey (Ateles paniscus) ... 
belly, long, spider-like limbs, and a long, prehensile tail, which is used like a fifth 
limb.