Why do vestigial structures not always disappear and still show up in organism? For example humans still have a tail bone, even though we don't have a tail. 1) The organism depends on them to survive

2) The structure is not harmful to the organisms survival

3)Sticks around just incase we need it again

4)The structure makes the organism look cooler

3 would be my best guess

Explanation:

we dont depend on it

its not harmful but that wouldn't be a reason to have it

it doesnt look cool

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

they deliver oxygen to the blood