Question

Number of eyes present on the prostomium of Nereis
A) Four
B) Three
C) Two
D) One

Answer 1

Hint: Eyes are one of the sense organs of Nereis. Eyes are present on the head’s dorsal side. Eyes are round, simple and pigmented.

Complete answer:
- The length of the body of Nereis dumerilii is between 7-8 centimetres. The colour is soft violet and reddish occurs in the areas of the body that are richly supplied by blood vessels. The organism is long, slender and cylindrical.
- The head can be divided into two sections: the prostomium and the peristomium. The anterior, thin, nearly conical lobe of the periosteum is the prostomium. It lies right in front of the mouth. It is not a true segment of the body. The lobe holds palps, tentacles, and eyes that act as sensory organs.

Now, let us find the solution from the options-
- They've got two eye sets that mean they have four eyes. Therefore, the options B, C and D are incorrect.
- Each eye is a structure which is cup-shaped and darkly pigmented. The concave side includes the retina, a circular aperture, a pupil and a gelatinous lens.
- The wall of the cup is shaped by several elongated and slender cells which are arranged in parallel.
- The cuticle region, which covers the eyes, serves as a cornea.

Thus, the correct answer is option (A) 4.

Note: Neiris' body is divisible into roughly 80 segments or metameres, and at the anterior end there is a distinct head. All segments except the head and the last segment carry parapodia which is laterally located, small, muscular and vertical, mobile paired appendages.