“Journey to the Microcosmos: Making Decisions Without a Brain” explores how single-celled protists respond to their environment without a nervous system or brain. Through microscope observations and the research of Herbert Spencer Jennings and Jeremy Gunawardena, the video explains stimulus response, avoidance behavior, photosensitivity, cilia movement, and species variation in organisms such as Stentor, Paramecium, Trachelophylum, and Euglena. Key concepts include unicellular organisms, protists, electrical changes in the plasma membrane, environmental adaptation, and behavioral hierarchies. This biology and microbiology content supports life science curricula by showing how cells and microorganisms sense, react, and survive in changing conditions, while also highlighting scientific observation, experimentation, and the importance of replication in research. Part of the "Journey to the Microcosmos" series.