The organelle which is contractile in function in Euglena is the

Step 1: Analyze the question The question asks which organelle in Euglena is contractile, meaning it can shorten or change shape to produce movement.

Step 2: Review the options

• A. Chloroplast: the site of photosynthesis. It is not contractile.
• B. Eyespot (stigma): a light-sensitive red pigment spot that helps the organism detect light. It is not contractile.
• C. Gullet: a canal-like invagination at the anterior end, where the flagellum arises. It is involved in ingestion and in discharging excess water from the contractile vacuole, but it is not itself contractile.
• D. Myoneme: a fine contractile fibril in the pellicle (the cell covering) of Euglena. Myonemes allow the characteristic "euglenoid movement" (metaboly), in which the body shortens, bends and changes shape.
• E. Paramylon: a carbohydrate food reserve similar to starch. It is for storage and is not contractile.

Step 3: Concept Euglena moves in two ways:

1. By its flagellum, which whips to propel the cell through water.
2. By euglenoid movement, a wave-like contraction and expansion of the body brought about by myonemes beneath the pellicle.

The word myoneme comes from "myo-", meaning muscle, which is a clue that it relates to contraction.

Step 4: Answer The contractile organelle is the myoneme.

Exam tip: Link "myo-" with muscle. Whenever you see "myo-" in a biology option, think of contraction, as in myoneme, myosin and myocardium.