Kingdom Protista

All protozoa are unicellular, so the whole organism is one mass of cytoplasm holding every structure a typical cell would have: they eat, move, hold their internal conditions steady and reproduce inside that one cell, with no tissue organisation, which is what separates them from the Metazoa. Most take food in by endocytosis, and vacuoles do the internal work, food vacuoles digesting engulfed material, contractile vacuoles pumping out the water that enters by osmosis, storage vacuoles holding waste aside.

The groups are defined by how they move, and there are three locomotory organs plus one absence. Amoebae lack flagella and move on pseudopodia, temporary cytoplasmic projections that also serve to feed. Foraminifera and actinopods move on pseudopodia too and build a shell or test with pores, calcareous in the foraminifera and siliceous in the actinopods. Zooflagellates have whip-like anterior flagella and feed by ingesting or absorbing. Ciliates such as Paramecium carry thousands of short hairs whose coordinated beat lets them reverse and turn, and they alone have two kinds of nucleus, a diploid micronucleus for the sexual process and a polyploid macronucleus running metabolism. Apicomplexans have no locomotory structure at all, they flex, they form spores, and many split their life between two hosts.