I presume you are referring to biological structures (e.g. the appendix), if so then the word you are looking for is 'vestigial'.
Body parts that are reduced in size with no apparent function would be vestigial limbs. Evolution could be at the root of the development of vestigial limbs. A good example would be a T-Rex with his tiny arms.
Phys
With time, if an organ or structure is no longer needed, natural selection may reduce the size of the organ until it completely disappears from the species or is very reduced in size. It is suggested that some vestigial organs may remain in a reduced state because they have a minor function within the body.
pineal gland
vestigal organ
Vestigial Organs
vestigial organs
A body part which is reduced in size and seems to have no function is called a vestigial. One example is the cecum. This is connected to the appendix.
an organelle is a small cell structure that carries out a function within the cell. I believe that the function is determined by the shape and size of it because the organelle needs to be a certain size to carry out all of the functions that need to be preformed.
Groups tend to develop a more formal social structure
Vestigial structures. Like the appendix in humans.
Vestigial structures. Like the appendix in humans.