That's right, octopuses don't have tentacles, they have arms. Squids have 8 arms and 2 tentacles.
So how are tentacles different from arms? They are usually longer and, in general, only have suckers at their ends.
If the cephalopod doesn't have tentacles, it's most likely an octopus. If you want to get even more confused, check out why the Vampire squid isn't quite a squid or an octopus here.
I hate discovering that things that I've believed since I was a child are wrong! Sometimes I'm just not in a mood to be humbled, I guess. In search of knowledge, the road is windy and endless.
Apparently the arms and tentacles of the cephalopods that we know and love fall under the term "muscular hydrostat". According to Wikipedia, a muscular hydrostat is:
"a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton. The principle behind the hydrostatic skeleton is that water is effectively incompressible at physiological pressures. Thus, a fiber-wound chamber full of water will act as a constant-volume system. What makes the muscular hydrostat unique is that it relies on the same principle, but there is no water-filled cavity. Instead, the bulk of the organ is made up of muscle, which also has constant volume and is effectively incompressible, its main material being water. Thus, instead of a cylinder wrapped with muscle and connective tissue that changes its shape, a muscular hydrostat is a cylinder made of muscle."
So what is a muscular hydrostat? The bodies of worms, the trunk of an elephant, the arms and tentacles of cephalopods, and the tongues of animals.
So even now that I know this, I don't think it's necessarily important for me to correct others who say that octopuses have 8 tentacles, just as I don't think it's important that people stop using the term "Great White Shark". After all, even if you use these incorrect terminology, people will understand what you are talking about.
Language, in itself, is an imperfect metaphor for us to make sense and communicate these ideas of our perception of reality, so isn't being understood more than sufficient most of the time? If not, most people wouldn't care to debate the issue anyways...
I guess I'm still experiencing lingering annoyance at my ignorance, but since I've typed this out of my system, I feel much better!
Here's a story about the largest known creature that has muscular hydrostats:
link (via my Mom)
"(Mesonychoteuthis hamiltoni a.k.a. the Colossal Squid) attains a size larger than the giant squid. Giant squid is no longer the largest squid that's out there. We've got something that's even larger, and not just larger but an order of magnitude meaner."
On a lighter note, take a look at this Conservapedia entry on the Pacific Northwestern Tree Octopus. Ah, good stuff...