These organisms literally live in glass houses. Live in salt water. They all have flagella. Most are photosynthetic. They may have other pigments that make them appear other colors.
Plankton includes aquatic protistans and animal larve that drift or swim weakly in the water.
They are a major link in the food chain. One type produces a neurotoxin that is poisonous to man and to fishes. BUT, people who eat the mollusks are - some die. The covering of many is composed of cellulose plants. Protozoan means first animal.