This is an incomplete list of organisms that are true parasites upon other organisms.
(endo = within; parasites that live inside their hosts)
These can be categorized into three groups; cestodes, nematodes and trematodes. Examples include:
(ecto = outside; parasites that live on but not within their hosts, for example, attached to their skin)
Monogeneans are flatworms, generally ectoparasites on fish.