Stentor coeruleus is a large aquatic protozoan found in fresh water. It is one of the largest protozoa, sometimes larger than some small multicellular organisms. Its shape is trumpet-like when it is attached to a substrate and oval when it is free swimming.
Stentor coerulus ingest algae and form a photosynthesis relationship with them which gives them a blue or green color. They are referred to as heterotrichs because they have multiple cilial structures on different parts of their body. Cilia mostly cover the rim of the trumpet. A major characteristic of stentor is the rapid stretching and contraction of its body