Today begins the Third Annual International Cephalopod Awareness Days! Naturally, since it is the eighth of the month, we are honoring octopods today. This provides the perfect opening for my favorite cephalo-rant: the etymologically correct plural of octopus.

The rant is prefaced with a quick linguistic detour. Those familiar with declining nouns may skip this paragraph. Anyone who thinks I was talking about nouns becoming forgetful in their old age, please read on. You know how verbs conjugate according to person and tense? I run, you run, he/she/it runs, and so forth? In many languages, nouns also change form, declining according to what part of the sentence they are. In English, the only remnant of noun declension is in our pronouns: I and me are both first person singular, but one is the subject of a sentence and the other is the object. In German, Sanskrit, Latin, and Greek, all nouns decline all the time.

So. The word octopus is Greek, from the roots octo (eight) and pod (foot). When the root noun "octopod" is the singular subject of a sentence (The octopus eats a crab.) then it is declined thusly:

octopod + s

Contrary to expectation, this sum does not simply produce "octopods," because of a strange linguistic rule in ancient Greek:

d + s = s + prior vowel modification

In other words, according to the grammar of ancient Greek:

octopod + s = octopus

And you thought English was weird! Anyway, that's where we get "octopus." Now. How do we pluralize it? Let us first continue in the ancient Greek. If the noun is to be the plural subject of a sentence (The octopuses mate discreetly.) we go back to the root and decline it thusly:

octopod + es = octopodes

No strange linguistic rules come into play here, and we get the four-syllable plural "octopodes." Delightful! Etymologically correct! Unfortunately obscure and archaic, and no one uses it. What to do? If we consider that the singular form "octopus" was adopted as an English word, then it is perfectly reasonable that all further modification should follow rules of English grammar. So we can pluralize as we would any other English word ending in s:

octopus + es = octopuses

Perfectly reasonable, and just as etymologically correct, if you accept that English makes its living by stealing words from other languages and applying English grammar to them.

Wait, they always protest. What about octopi?
Fine, I weep. Fine, you want to know where octopi came from?

English has also pilfered many words from Latin, like cactus. Admittedly a large number of these words (again, like cactus) were originally Greek, but unlike octopus, they did legitimately become Latin words before being slurped up by English. Why does that matter? It matters because Latin noun declension is different from Greek noun declension. In Latin, cactus is the singular subject of a sentence (The cactus lives in the desert) and cacti is the plural subject of a sentence (The cacti are pollinated by bats). Probably because Latin had a much more recent influence on English than Greek, this delightful and etymologically correct pluralization is neither obscure nor archaic, but so commonly known that it has been adopted into the English language. Look it up in the dictionary: sing., cactus, pl., cacti.

Now.

Sigh.

I can hardly bring myself to say this--but go ahead. Look up octopus in the dictionary. There it is. Optional plural: octopi.

You see, enough silly people have made the error, assuming octopus to be as Latin as cactus, although it is as Greek as Aphrodite, that it is now in the dictionary. And that's how language evolves. Errors become popular enough that they turn into corrections. Alright is now an acceptable spelling of all right, and octopi is an acceptable plural of octopus.

But I don't have to like it.

Wait.
Now what?
You used "octopods" in the first paragraph of this absurdly long entry.
Wow, good memory.

See, if you want to be really pedantic, you can sidestep the issue with taxonomic terminology. Within Class Cephalopoda is Order Octopoda, containing about two hundred of these eight-armed beasties. And it's quite all right to refer to members of the Octopoda as octopods, just as it's quite all right to refer to members of the Pinnipedia as pinnipeds (by the way, that's a good trick for sounding erudite when you can't tell if it's a seal or a sea lion).