Monogenea is a class of parasitic flatworms (related groups include turbellarians, tapeworms and trematodes). Monogeneans, however, have a distinctive "haptor"–the large hooks you see surrounded by smaller marginal hooks–that they use to attach to the fish. This is lacking in the other flatworm groups. The monogeneans in this video are from the subgroup monopisthocotylea because they have hooks rather than clamps. More specifically, these parasites are from the family Gyrodactylidae. A cool fact: this family is often compared to Russian nesting dolls because adults contain fully developed embryos, which contain an embryo, ... (you can see the haptor of an embryo inside one of the adults at 1:23 in the video!). Pretty cool!