... contains 80% of all the animals on earth?

(oops! Should have posted to Zoology Trivia.
Can't delete this question, either.)
'

Do you mean 80% of individual animals or 80% of known species?

A quick internet search of "phylum" didn't turn up much on relative numbers. I can think of a book or two to look in, but my entire library is in temporary storage. That leaves guessing.

Since "animals" excludes bacteria, I would guess Arthropoda, which includes all insects, spiders, and crustaceans. I wouldn't be surprised, either, if the answer turned out to be Nematoda, the roundworms.

Can I hedge my bet by naming a superphylum? Ecdysozoa merges the two I mentioned above, along with six other phyla.

Arthropoda, containing the classes insecta and crustacea.

Edited / PS: and it appears Professor may have beaten me to it!.

...and by less than a minute! (finally, a worthwhile use of the razz! Sorry it had to be you, 'fuse!)

I hope you weren't just guessing, too.

80% of all the individual animals. Perf's backup response was right. Nematodes.

We're not aware of how many there are because they are tiny and most live in the soil so we don't see them. But collectively they make up a huge mass of protoplasm.

(Source: Edward O. Wilson, Discovery Magazine, June 2006 issue)