Subclass Spiruria comprises mostly parasitic secernentean nematodes. In an alternate classification, they are treated as suborder Spirurina, with the orders listed here being ranked as infraorders.The Ascaridida and the Oxyurida, which include worms that infect many mammals (including marine mammals), are sometimes placed in subclass Rhabditia. But that seems as spurious as the erstwhile placement of the Rhigonematida in subclass Tylenchia.