The Monogenea are a class of this phylum who mostly live as parasites on fish, typically inside the 
The Monogenea are a class of this phylum who mostly live as parasites on fish, typically inside the gills. The Turbellarians are the only class of this phylum that is not parasitic, while the most notable parasites within this phylum are in class Trematoda. Some species of this phylum are hermaphrodites that reproduce using a technique called "penis fencing". Members of this phylum remove waste using flame cells. Members of this phylum are bilaterally symmetric and lack a body cavity. Members of this phylum have a simple eyespot, and include flukes and planarians. For ten points, name this phylum of flatworms.
The Monogenea are a class of this phylum who mostly live as parasites on fish, typically inside the gills.
{Platyhelminthes} [prompt on “flatworms”]