Solved. In the back of my mind I knew that this movie had a one-word title, and yesterday I saw a DVD in the local cult video shop. The title stood out at me and suddenly I realized that this was the movie I wanted to identify. The movie is Equinox (1970), and is a lot more low budget and way more cheesy than I expected!
It started as a short student film that was made for $6500 in 1967. Other companies picked it up and shot extra footage that turned it into a feature length production, and it would have gone unknown had the director not gone on to do special effects work for George Lucas and Steven Spielberg.
Here's the trailer in all of its late 60's glory:
It's interesting that this movie was in the same book as movies like The Fly, The Devil's Rain, The Thing with Two Heads and The Man with X-Ray Eyes. The editors of the book obviously deemed this movie important 'enough' to include.