1. How does it move? It swims or glides, and you can see a fringe of fine hairs or whirling crowns

One cell, or an animal with a jaw?

Focus inside it at 100–400×. A rotifer's jaw (mastax) pumps rhythmically; ciliates have nothing like it.

1. Many-celled: a pumping jaw inside, crowns that look like spinning wheels, a forked foot→ Rotifers
2. Many-celled, small, gliding on its belly, with a forked tail and often spines→ Gastrotrichs
3. A single cell: no jaw, no gut, cilia over the surface or in rows→ next question

Not sure? How to judge size by the field of view · This step looks wrong