A double helix is <span>a pair of parallel helices intertwined about a common axis, especially that in the structure of the DNA molecule. or in other words a double spiral.</span>
Answer:
Structures that are still present but currently serve little to no purpose
Explanation:
<em>Vestigial structures/organs are general defined as structures/organs that are present but poorly developed without any functional use in the present living organisms but thought to have been fully developed and functional in their ancestors.</em>
Fully developed and functional organs gradually become less developed and non-functional across generations as the environment keeps changing and organisms found lesser use for the organs. An example includes the <u>caudal vertebrae</u> found in man which is kind of useless but thought to have been fully developed and functional in the ancestors of man - the monkeys.
<u>Vestigial organs are often used as one of the morphological evidence of the evolution of living organisms.</u>
<span>Invertebrates means having no backbone.</span>
