Protozoa are one type of critter which are easy to cultivate at home by soaking plant fragments in a jar of water. An old trick if you want to cultivate protozoans is to place some individuals of the species of interest into a water container. Let the water stand beforehand to allow traces of chorine to disappear and add ONE rice grain. Rice provides food for the bacteria, and protozoans eat the bacteria.
Ciliated protozoans are probably the best known but other amazing families exist.
Some species build a sort of home called lorica, in which individuals can take refuge. Protozoa can be found in ponds, pools, or in any place where water is rather stagnant. But it's sometime difficult to believe that they also live in the sea, with species quite similar to those in freshwater but perfectly adapted to a marine environment. But nowadays, salt water species cannot to survive if they are placed into fresh water. To remain manageable, just free swimming and attached species are presented.