Here's why:
* Vertebrates: Some parasites are vertebrates, like the lamprey, a fish that attaches itself to larger fish and sucks their blood.
* Invertebrates: Many more parasites are invertebrates, like tapeworms, roundworms, fleas, ticks, and many others.
The defining characteristic of a parasite is its relationship with a host, where it benefits at the host's expense. It's not about their own classification as vertebrate or invertebrate.