Question

Nictitating membrane is vestigial in
A. Frog
B. Rabbit
C. Human
D. All the above

Answer 1

Hint: In scientific terminology, the nictitating membrane is known as plica semilunaris. It is often called a hawk or third eyelid. It is drawn across the medial canthus of the eyes.

Complete step by step answer:
This membrane is translucent or transparent that is drawn across the medial canthus of the eyes for protection. It also helps in keeping the eyes moist, while keeping sight. It can move horizontally across the eyeball, just like the upper and lower eyelids can.
Fish, birds, amphibians, reptiles, and some mammals have fully developed nictitating membranes that are rarely present in primates. Some mammals like camel, seals, polar bears, etc have fully developed, while many mammals including humans have a small vestigial portion of the membrane present at the corner of the eyes. In humans, this membrane is also called the semilunar fold.
Hence the nictitating membrane is vestigial in humans, so the correct answer is option C.

Additional Information:
Vestigial means form of a small remnant of something that was once more noticeable. The vestigial organ is a result of evolution. These organs lost some or all of their all function in a given species. Example of the vestigial structures is – island-dwelling birds lost their functional wings, snake and whale lost their hindlimb, and in humans the most common is an appendix. Apart from these examples, there are many more examples of vestigial organs in human like wisdom tooth or tailbone, etc

Note:
In some animals, the gland namely nictitans gland or harder’s gland are attached to the nictitating membrane and produced almost fifty per cent of the tear film. The nictitating membrane mostly turns on the land to remove the soil particles, dirt and other debris. In diving animals like crocodiles, underwater it protects the eyes from the water but also interrupts to focus on their prey.