Technically speaking, Octopuses don't have tentacles at all. Those are arms. The morphology of "Octopus" is actually a really great example case for demonstrating paraconsistent logic.
Hmm so the name must be corrected to be 8-armed in Greek. Oktobratso? Oktooplizo? Oktocheiro?
It goes deeper than that! According to wikipedia, the tentacles, or appendages, are indeed called "arms", but they evolved from what in other molluscs is called a "foot"!
Deeper still, one of those arms on males contains their sexual organs call the hectocotylus.