Why is there an observed loss of organs & structure in parasitic species?

Question

Why is there an observed loss of organs & structure in parasitic species?

Answer 1

Parasites have filled a lifestyle niche that allows them to make use of the organs of their host animals.  Example: Why maintain a stomach when your host digests your food for you.  The first critters to fill this sort of niche had fully functioning stomachs, but ate predigested food.  The stomach was no longer an important organ so the genes that controlled the formation of the stomach became unimportant.  

Normally, stomachs are necessary for animals.  If a deer is born with a deformed stomach, that animal will probably die before it can reproduce any offspring who share its mutant stomach genes (mutations are copy errors when a cell divides.  They are very common but don't usually effect important genes that would have an impact on the animal.  Sometimes they do and the result can be either helpful or hurtful to the animal born with that mutation).

Now, a deformed stomach in a parasite that eats predigested food is no big deal.  It lives and reproduces with no problem because the stomach is now unimportant.  Its offspring also have deformed stomachs and their offspring will too.  Since the "stomach genes" are not being maintained by being needed, they are prone to accumulating copy errors that break them even further from generation to generation, resulting in atrophied, vestigial organs.

Answer 2

Many parasitic species are able to rely on their hosts to supply the needs of those organs, and so the parasite is able to save metabolic energy by not bothering to develop those.