Final answer:
The Monogenea class of flatworms are primarily external parasites of fish. They have a simple life cycle, involving a larval stage that attaches to fish and they are typically hermaphroditic, with cross-fertilization being common. Monogeneans are part of the wider group of helminths, which includes both free-living and parasitic species.
Step-by-step explanation:
The group of flatworms primarily known as external parasites of fish is the Monogenea. These creatures are a subclass within the larger grouping of Platyhelminthes, which encompasses both free-living and parasitic flatworms. Monogeneans are characterized by their simple life cycles, which involve a free-swimming larval stage that attaches to fish, initiating their transformation into the adult ectoparasitic form. Unlike other parasitic flatworms, such as trematodes and cestodes, monogenean parasites usually infect only a single host species during their lifetime.
Monogeneans attach to their hosts using specialized structures and feed on host tissues by producing enzymes or by grazing on mucus and skin particles. Many of these parasites exhibit hermaphroditism, possessing both male and female reproductive organs. However, cross-fertilization is commonplace in monogeneans as male gametes typically develop before female ones.
As part of the helminths, which also include roundworms and other flatworms like trematodes and cestodes, monogeneans represent an important group of species within aquatic ecosystems. Their presence can signal environmental and ecological changes, and their parasitic nature can have significant impacts on fish health and fisheries.