Seems pornographic anime has led you astray...
"What's the difference between an arm and a tentacle? Arms, like those on an octopus,
have suction cups the entire length of the limb. Tentacles only have suction cups near the end of the limb. Some Cephalopods have arms, some have tentacles, and some have both!"
Cephalopods, flies and even humans share genes needed to develop limbs, perhaps provided by a common ancestor.
www.nytimes.com
Still useful either way...