Apostrophes

Hi - can you help me with an apostrophe issue?

I'm not sure if I should use and apostrophe for 'cats' in the sentence below. I know you use apostrophes for omission and possession but don't think 'cats' in the sentence below is either. I think 'cats' are plural - what do you think.

The cats were purring as we stroked them.

Thanks for you help.