In the Classic period, earth and sky are visualized as horizontally extended serpents and dragons (often bicephalic, more rarely feathered) which serve as vehicles for deities and ancestors, and make these appear from their maws. Other serpents, shown as vertically rising, seem to connect the various spheres, perhaps to transport the subterranean or terrestrial waters to the sky. Dragons combine features of serpent, crocodile, and deer, and may show 'star' signs; they have been variously identified as the nocturnal sky and as the Milky Way.