Answer: The word X represents the organs which is no longer used by the species.
Explanation:
The word vestigial shows the characters that is not used by the species anymore. Example is appendix which is considered as a vestigial organ in human beings as it is no longer useful for the human body.
This organ was used when there was only vegetarian food available for the human beings to eat.
But now that animals and vegetable both are available there is no need for the enzyme required the digestion of cellulose.
So, this organ is vestigial for the human body.