Octopuses are notorious for being able to escape from almost any tank. The ability of an octopus to escape almost anything is only matched by backslashes in a unix tc shell. All it takes is an opening the size of the octopus's beak (which is usually rather small) and the octopus will be able to squeeze through. This is because octopuses have no bones, and no internal shell. The only solid part of an octopus is its beak. Here is an amusing story of an octopus that decided that it had had enough of life in a New Zealand aquarium. This octopus escape story also briefly mentioned the recently discovery that octopuses use two of their arms as legs. The article implies that this is true of all octopuses, but my understanding is that this behaviour has only been observed in two species, and octopus experts are uncertain if this is a general behaviour, or just an adaption by some species.