With reference to the sense organs of annelids, how many functional eyes are present in an earthworm?

Introduction:
Earthworms are common soil dwelling annelids that play an important role in soil formation and aeration. Many school level questions focus on their body structure and sense organs. It is easy to imagine that an animal must have eyes to sense light, but earthworms are a useful counter example. This question asks you to recall how many eyes an earthworm actually has.

Given Data / Assumptions:
- The organism is an earthworm, a segmented worm belonging to phylum Annelida. - The question is about true eyes as separate light sensing organs. - Options include one eye, many eyes, no eye and two eyes. - We assume typical species of earthworm studied in school biology.

Concept / Approach:
Earthworms do not have true eyes like vertebrates or many arthropods. Instead, they possess light sensitive cells or photoreceptor cells scattered in their skin, especially towards the anterior region. These cells help the worm detect light and darkness, allowing it to avoid strong light at the surface. Because there are no organised eye structures, the correct description is that earthworms have no eyes, even though they do perceive light to some extent through their skin.

Step-by-Step Solution:
Step 1: Recall the external morphology of an earthworm. The body is cylindrical and segmented with no obvious eyes on the head. Step 2: Remember that earthworms live in soil and prefer dark, moist environments, emerging mainly at night or during rain. Step 3: Understand from zoology lessons that they possess light sensitive receptor cells in the epidermis, but not structured eyes. Step 4: Interpret the question as asking about actual eyes, not just light receptors, and conclude that earthworms have no eyes.

Verification / Alternative check:
Zoology textbooks and laboratory manuals consistently describe earthworms as eyeless animals. Diagrams of earthworm external features label anterior segments, clitellum, setae and openings, but no eyes. The nervous system description also mentions epidermal receptor cells for light and touch, not separate eye organs. This confirms that no eye is the appropriate answer for exam purposes.

Why Other Options Are Wrong:
One: There is no single eye structure present at the anterior end of the earthworm. Many: Although many photoreceptor cells are scattered in the skin, they do not form distinct eye organs that can be counted as many eyes. Two: Earthworms do not have a pair of eyes like many other animals such as insects, fishes or mammals.

Common Pitfalls:
Students sometimes misinterpret the presence of light sensitive spots as many eyes and choose that option. Others assume that most animals must have two eyes and automatically select two without checking earthworm structure. A good strategy is to recall that earthworm diagrams never show eyes and to differentiate between true sense organs and scattered sensory cells. This keeps the concept clear during exams.

Final Answer:
An earthworm has No eye in the form of distinct visual organs.