shiva doesnt wear snake. vasuki surrendered to siva, by yogic control the naga clan learned to convert poison (venom) and turned into diamond (mani). the ehat from poison in shiva's throat is again channeled into body of vasuki. liekwise ther are other snakes that form anklets and braclelets of shiva out of respect. in same way anata sesha bears the burden of earth on his hood and also bears vishnu. This anata is none othen than patanjali yogi who tansformed into snake for sake of lord vishnu.