Question

The reduced nictitating membrane is an example of
(a) Homologous organ
(b) Vestigial organ
(c) Analogous organ
(d) Functioning organ

Answer 1

Hint: Nictitating membrane is the third transparent or translucent eyelid that protects and moisturizes the eye, without losing visibility. It appears to be a nonfunctional residual part of the ancestor.

Complete answer:
The reduced nictitating membrane is an example of Vestigial organ. Vestigial structures are parts of the body that were reduced or lost their original function during the evolution of a species. They are found in numerous animals, including humans. Vestigial structures in the ancestors of these species (and in the homologous structures of other existing species) were fully functional, but at present, their function is practically useless or has changed. Some reptiles, birds, and sharks have full nictitating membranes; there remains a small, vestigial portion of the membrane at the corner of the eye in many mammals. Some animals have complete nictitating membranes, such as camels, polar bears, and whales. It is not functional in all organisms. So, it's not an example of a functioning organ.
So the correct answer is vestigial organs.

Additional Information:
- It is prevalent in amphibians, reptiles, and birds. Among the primates, only lemurs and lorises are interactive.
- The plica semilunaris is a remnant of the nictitating membrane in humans. Obviously, we can't move it, but it still has a lacrimal drainage feature and helps to lift the eyes.


Note: In many animals, the nictitating membrane function is protective — for example, keeping the eye clean and moist, or hiding the iris from predators. The membrane is sufficiently translucent in certain species to allow for vision when it is underground or underwater. Although it is uncertain why a nictitating membrane is lost in humans, changes in habitat and eye physiology may have made the tissue redundant.