What is the difference between parasitic protozoa and other unicellular animals?

As a rule, the protozoa leading a parasitic lifestyle are more complex in comparison with free-living ones. In multicellular organisms, on the contrary, degeneration is observed, for example, the digestive and nervous systems are often lost.

The simplest parasites often have special structures for attachment (for example, in lamblia, this is the attachment disk). Digestive vacuoles are lost, because the cell absorbs substances from the entire surface, organelles of movement may be absent. Also, parasites often have a rather complex life cycle with a change of hosts.