For studio albums they sometimes used a bass player if the sound was too open, but the bassist would play very little so the part of their music would not be noticeable as another part, but as a whole with the rest of the music. Besides for that, Ray the keyboardist played bass for the band. Robby would play guitar as he normally would and Ray would use his right hand for melody and the main part of the songs.
The Door's sound also helped them to sound good with no bass, all of their playing is so solid a bass isn't necessarily needed to fill in the extra spaces. The Doors didn't really use all that many extra tracks. I know for their first album it was a four track, so that didn't really leave a lot of room for extras, but as they progressed they came slightly more involved with the different tracks (such as in The Soft Parade) In some concerts (like Boston) Robby would play bass and Ray would play guitar, this was just for fun though, it was usually used at the end of concerts. Using his left hand Ray would play a small keyboard bass. The keyboard for the bass is similar to this, however it is not the same. http://www.combo-organ.com/Rheem/RheemKeeBass.jpg t This picture isn't all too helpful but if you look closely you can see where the other keyboard is on the other one, there is a slight change in texture. http://z.about.com/d/classicrock/1/0/9/H/05-29njwf.jpg Sorry if my explanation was sort of confusing. This video can help show what I am talking about http://youtube.com/watch?v=slvPMU-7OyI for some reason the camera spends a lot of time on Ray's hands, but in this case it is good, and helps to understand how it works, everything that Ray plays with his left hand is bass, you will be able to tell by the smaller keyboard. Around 3:55 is where you can see Ray playing a main keyboard part with the base.