Sciency Words: Tentacle

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


Believe it or not, octopuses do not have any tentacles. Zero. None. They have four pairs of arms, according to cephalopod experts.

When discussing cephalopod anatomy, arms are defined as shorter, more muscular appendages with suckers all the way along their length. Tentacles are longer and only have suckers at the “club-shaped” end. So octopuses have eight arms. Squid and cuttlefish have eight arms and two tentacles.

As a science fiction writer, I’ve created a few characters who have tentacles. Or at least, I think I have. But maybe my buddy Omglom here only has arms.

However, after doing further research I’ve found that this arms vs. tentacles thing is specific only to cephalopods. In a more generalized zoological sense, just about any boneless, flexible, elongated appendage can be referred to as a tentacle.

The word tentacle traces back to a Latin word meaning “to feel” or “to test” or “to probe.” This seems appropriate to me because in most cases tentacles aren’t really for grasping or manipulating objects. They’re sensory organs used for feeling, smelling, tasting, and even seeing (for example, the eyestalks of slugs and snails are considered to be tentacles).

There’s even a mammal with tentacles: the star-nosed mole, which has twenty-two tiny tentacles arranged in a star pattern around its snout. These tentacles are extremely sensitive feelers which help the star-nosed mole feel its way around as it burrows through the earth.

As for my friend Omglom… the gelatinoids of Rog aren’t cephalopods, so his tentacles can be called tentacles after all!

P.S.: It may sound strange, but the proper plural form of octopus is octopuses, not octopi. The cephalopod expert at the end of this video does an outstanding job explaining why.

4 thoughts on “Sciency Words: Tentacle

  1. I have to admit that the arm vs tentacle thing is something I never knew. In truth, I really just considered a tentacle an arm of something that had more than two of them. You live and learn!

    From what I’ve read, there is no fact of the matter on whether “octopuses” or “octopi’ is the correct plural form. I usually use “octopuses” because that’s what most biologists seem to use. But dictionaries list both forms as valid, indicating they see usage out there for both, and language usage defines language correctness.

    Liked by 1 person

    1. You’re right, of course, and actually I use both forms interchangeably. I guess the better way to put it is that octopuses is the more traditional form and octopi is a newer invention. But most people seem to think it’s the other way around.

      Liked by 1 person

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