Why do smaller annelid worms not need gills to obtain sufficient oxygen?

I got this question in a Biology past paper and tried to find the answer on the internet too, so i thought i best answer it for other people :P

i settled with the species are so small that they have a large surface area to volume ratio, meaning they take in quite a lot of oxygen, and because they're so small they don't use up all of this oxygen as quickly as lugworms and other larger species of annelids.

if it's wrong, sorry, i basically just took knowledge and made a logical guess