Evolution of Zooparasitic Groups in the Phylum Nematoda, with Special Reference to Host-Distribution

Abstract

There has been repeated evolution of parasitism from free-living groups of Nematoda, ranging today from a parasitic order down to parasitic spp. in basically free-living genera. In the class Phasmidea the major parasitic lines are the order Spirurida and the suborders Strongylina and Ascaridina of the order Rhabditida. In addition there are a number of parasitic families in the suborder Rhabditina and the order Tylenchida. In the class Aphasmidea, Chromadorida has no true parasites, although a few commensalistic forms are known. In Enoplida there are 3 major lines [long dash]the superfamilies Mermithoidea and Trichuroidea of the suborder Dorylaimina and the suborder Dioctophymatina. Parasitism is more strikingly developed in the Phasmidea, and this seems correlated with the fact that the primitive members of the class are soil-forms, whereas the bulk of the aphasmideans are marine. Soil-inhabitation provides a greater opportunity for development of parasitic habits. The suborders Rhabditina and Strongylina are treated in particular detail. The former is basically free-living, the latter entirely parasitic. Strongyline host-parasite evolution is discussed, and a revised classification provided for the suborder. A new subfam., OSLEROIDINAE (family Metastrongylidae) is proposed.