The freshwater snails are hosts, much like mosquitos in this diagram. They host platyhelminthes. The ascaris and tapeworms are also helminths. As far as I know anyway. I can understand that they wanted to show animals in the diagram because you can actually avoid those but avoiding the snails won’t actually help because the worms infect by being excreted by the snail and infecting another host(human or other), usually in water.