A parasitic animal lives symbiotically in or on its host for at least part of its life cycle and benefits at the expense of the host. Parasites feed repeatedly on the bodies of their hosts but do not destroy them as a predator would its prey. In fact, even though parasites often harm their hosts, the parasites and the host sometimes reach a state of balance. This state of balance is of great value to the parasite because it can not live with a dead host.
The greatest problem a parasite faces is "house hunting". Since it lives in some organ of the host animal, when the host finally dies, the parasite has to find a new host. It is like a fish having to find a new lake to live in every generation. Parasites respond to this problem by producing large numbers of eggs. The chance that any one egg or larva will reach the right kind of host be 1 in 100,000, but if a parasite lays a million eggs, it is reasonable certain that a few will make it. For this reason, they all reproduce in great quantity.
Parasites have become extremely well adapted for their environment and have lost many structures commonly found in their free-living relatives. Like cave animals, internal parasites rarely have eyes. Apparently not having eyes is no disadvantage if one always lives in the dark, so there have been no selective pressures for eyes.