Thank you all for your help, but I don't know how to derive an equation of the form x(t) = |insert function of t|, y(t) = |insert function of t| from an equation in terms of x and y. Sorry, my friends; I'm going to have to ask for the explicit equations.
Cartesian to polar to parametric form ...
http://online.redwoods.cc.ca.us/inst...en/CalcPpr.pdf