Which of these organisms exists as a filament?

Step 1: Analyze the question The question asks which organism grows as a filament, meaning long, thread-like chains of cells joined end to end.

Step 2: Review each option

• A. Diatom: A single-celled alga with a silica shell. Some form chains, but it is typically unicellular.
• B. Euglena: A unicellular, flagellated protist. It is not filamentous.
• C. Paramecium: A unicellular, slipper-shaped protozoan covered with cilia.
• D. Spirogyra: A green alga ("pond scum") whose cylindrical cells join end to end in long, unbranched filaments. Each cell has a distinctive spiral chloroplast.
• E. Volvox: A colonial alga. Its cells form a hollow sphere, not a filament.

Step 3: Answer Spirogyra is the classic example of a filamentous alga, so the answer is D.

Tip: The name "Spirogyra" points to its spiral chloroplast. Remember the shapes this way:

• Spirogyra = filament
• Volvox = sphere (colony)
• Euglena, Paramecium = single cells