One of the following exist as colony

Analyze the question: It asks which organism exists as a colony, meaning a group of individual cells living together as one organized unit.

Explain the concept: A colonial organism is made of many similar cells that live together, often in a fixed shape. Each cell can often survive alone, but the cells cooperate. This is different from:

• Unicellular organisms, which live as single cells.
• Filamentous organisms, which are chains of cells joined end to end.

Go through the options:

• A. Euglena: A single-celled organism with a flagellum and chloroplasts. It lives alone, so it is not colonial.
• B. Paramecium: A single-celled, slipper-shaped protozoan with cilia. It is also solitary.
• C. Spirogyra: A green alga with a long, unbranched filament of cells and spiral chloroplasts. It is filamentous, which is a multicellular thread rather than a true colony.
• D. Volvox: A green alga that forms a hollow, spherical colony of hundreds to thousands of flagellated cells. The cells are held together in a jelly-like matrix and beat their flagella together so the colony rolls through the water. It is the classic example of a colonial organism.

Answer: The correct option is D. Volvox.

Exam tip: Remember the key examples:

• Volvox = colony
• Spirogyra = filament
• Euglena and Paramecium = single-celled

Whenever "colony" shows up in a JAMB biology question, think Volvox first. Other colonial examples are Pandorina and Eudorina.