Help identifying round worm or nematode?

[Tloc]

https://imgur.com/ZdaZmOv
https://imgur.com/ZC109qj
https://imgur.com/iD8P5uv
https://imgur.com/Xn3nunn

Shown at 100x. The worm had 6 bilaterally symmetrical folds with cilia which rapidly move water across the mouth. Obvious digestion tract, though no visible anus.

More than just finding out what this is called, I'm curious how I can go about learning how to identify animals like this myself. What traits can I look for (or look for the absence of) to narrow down the phylum or class?

[karhukainen]

I've only just started scoping these things myself, but to me this guy looks like some kind of a bristle worm, not a nematode. It has segments and bristles.

[Tom Jones]

Probably Dero digitata (Ward and Whipple, Fresh Water Biology, 2nd. Ed., pgs 530-531, and https://worms.myspecies.info/taxonomy/term/936)

Edit: Nice images, too!