Final answer:
Monogenea is the group of flatworms that are mainly ectoparasites of fish, using hooks or anchors to attach to the gills of their hosts.
Step-by-step explanation:
The group of flatworms that are primarily ectoparasites of fish is the Monogenea. These organisms are parasitic flukes, typically located on the gills of fish, and are known for their direct life cycles, meaning they do not require an intermediate host to complete their life cycle. Their defining characteristics include having hooks or anchors, which they use to attach themselves to the host. An example of a monogenean is Dactylogyrus spp., commonly known as gill flukes, which are about 0.2 mm in length and utilize two anchors to latch onto the gills of their host fish.