O. vulgaris!!!!! Goooooood little octo!
The plural as far as I can tell is Octopusses because it if from the greek meaning foot mouthed or something similar (They have their feet round their mouths) and the greeks didn't have plurals with ii at the end like it is in latin.
Andy, the best explanation I've heard thus far. I'd never given this consideration until the matter was raised many months ago (this is, to a certain extent, thread or at the very least subject duplication). It is covered somewhere online, and I think you are correct in the spelling 'octopusses' - though for all I know on the subject matter I could be speaking through a hole in my head (I depend on the likes of you folk to ciorrect me in such matters).
Actually, "octopusses" is not really a proper pluralization of octopus, since the word is not Latin (if it was it would be "octopi") but Latinized Greek. If you want to be pedantic, it should be pluralized as "octopodes."