The ophidian was a snake-like creature used by the Devidians to generate temporal vortices to 19th century Earth, where they harvested Human neural energy for sustanence. The ophidian functioned by creating minute distortions in the spacetime continuum, which the Devidians focused into a vortex using massive amounts of triolic waves.
In 2368, most of the command crew of the Enterprise-D followed the Devidians through a vortex to 1893, and were able to appropriate the ophidian, which the Devidians had disguised as a cane with a snake-head. Tuning a phaser to the approximate frequency of triolic waves and laying down a field burst, Lieutenant Commander La Forge was able to temporarily activate it. Later, having been stranded in the 19th century, Captain Picard was able to perform the same procedure to return to his own time. (TNG: "Time's Arrow", "Time's Arrow, Part II")