Lord Shiva doesnit wear snake. Vasuki surrendered to Shiva, by yogic control the naga clan learned to convert poison (venom) and turned into diamond (mani). the heat from poison in Shiva's throat is again channeled into body of vasuki. Likewise there are other snakes that form anklets and bracelets 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 other than patanjali yogi who transformed into snake for sake of lord vishnu.