Worms in my rat's cage?

I was cleaning my rats' cage today when I noticed, under the water bottle area, in the moist shavings (assumingly because one of the toys in the cage had pressed against it, triggering it to drip), were tiny clearish worms (about 10 of them) writhing around once I had disturbed them. I clean the cage one or two times a week and currently it just houses one adult male rat (he attacked his previous cagemates very violently they had to be separated). Anyway, they looked like little earthworms and while I do give my rat fresh fruits and veggies, I don't bring things out of the garden directly to his cage so I don't know how earthworms would've gotten in.

Also, just to note, the bottom of the cage has a slide-out pan so I don't believe Ripley ever came into contact with the worms, since they are under the bars, in a sawdust pit, basically.

They were very slimy looking, each about 3 cm long. I freaked out and immediately scrubbed the whole cage down. I've never seen these before. What might they have come from?