I have been reading over the two theories, and cannot figure out what Einstein meant when he termed something a 'mollusc'. Can someone please explain?
He's just talking about arbitrary noninertial coordinate systems, maybe he used the word "mollusk" to suggest a curvy shape like the body of an octopus or a slug. Take a look at http://www.aei.mpg.de/einsteinOnline/en/spotlights/background_independence/index.html [Broken], particularly the part where they talk about "diffeomorphism invariance", for more on the idea that the equations of GR work in arbitrary coordinate systems...the last animated diagram on the page shows some rather "mollusk-like" curvilinear coordinate systems.For this reason non-rigid reference-bodies are used which are as a whole not only moving in any way whatsoever, but which also suffer alterations in form ad lib. during their motion. Clocks, for which the law of motion is any kind, however irregular, serve for the definition of time. We have to imagine each of these clocks fixed at a point on the non-rigid reference-body. These clocks satisfy only the one condition, that the “readings” which are observed simultaneously on adjacent clocks (in space) differ from each other by an indefinitely small amount. This non-rigid reference-body, which might appropriately be termed a “reference-mollusk,” is in the main equivalent to a Gaussian four-dimensional co-ordinate system chosen arbitrarily.