We have a situation with two lavs back to back that are mirror images of
each other. The two tubs. toilets, and sinks are exactly back to back with
the plumbing between. Also, I believe the kitchen drain heads over that
way passing the tubs, and sinks before hitting the main sewage line which I
think is in alignment with the toilets (the vent stack is in the wall there
too).

Anyway, drainage from the tubs is sluggish and thus backs up into the
opposite tub if the drain is not closed. (Never notice anything from the
kitchen but there is a good length of large diameter pipe that would need
to fill first).

We get this rodded out every so often but it doesn't take long to slow down
again so they are probably not getting it thoroughly enough and any
remaining obstruction is going to start collecting into a partial clog
again.

I realize the solution here is to get someone to really give it a good
cleaning but naturally one won't know the results until years go by and it
stays free running.

My question here today is this. Usually when dirty water backs up into a
tub from emptying the opposite one it's just some dark dirt matter. But
today this happened and looking closely at it I noticed a few worms of some
kind. (All together now: Ewwww!) They were black, about a mm thick and
maybe 5-8mm long. What are these things? I immediately dumped liquid
bleach into the water on both sides even before beginning any cleanup but
I'd like to know what these are and how dangerous are they. Should one
dump some bleach or something else regularly down the drain?

These may have come up from the street sewer but obviously right then they
had to be living in the drain within inches of the tub to get washed over
from one to the other.

WTF?