The live lobster near the beginning probably blew the budget. Later gargons are played by what looks like a black-painted vaguely lobster-shaped cardboard cutout. (They probably intended to do some process work to replace the silhouette with an actual lobster image, but didn't have the budget.)
All of the human skeletons are played by a single-bolted-joint specimen straight from a school science lab (which means that Alice's body accidentally has male hips). In the shot where Prof. Simpson's body is discovered, the aperture is clearly visible.
On the other hand, when you consider the next-to-nothing budget the film had, you have to be impressed that the movie's non-special effects work. For example, the entire movie was shot wild track: every single sound was dubbed in later — and unlike certain other B-movies, you can't really tell.
Tear Jerker: Derek's line of, "You... knew your brother?" to Betty. It's the one line in the film where Derek realizes that Earth contains a familial society he had only read of in books and grows to love it enough to the point of pulling off a willing Heroic Sacrifice to protect it.