is there a way to plot only the continental outlines of rworldmap in R (no countries, no data)? 

There are two other questions close to this one, but they are to fill the continents with colors, and the commands are to plot data. 
[How can I plot a continents map with R?](https://stackoverflow.com/questions/20146809/how-can-i-plot-a-continents-map-with-r)

I only want to overlay the continental outlines without country lines and without requiring other data.