I have always thought the word annulus to be exceptionally awkward. I’d like to know the relation between annulus and anus.
Geometrically, an annulus is a disk with a hole in it. The anal muscle (“sphincter”) has an annular shape.
I’m wondering how we got from annulus to anus.
You have the derivation slightly backwards. The Latin anus meant ring and thus also the anatomical anus.
Meanwhile anulus was a Latin diminutive of anus, so a small ring