The comparative morphology of cephalic structures in the superfamily Filarioidea (Nematoda)

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Abstract
Cephalic structures in 62 genera of the nematode superfamily Filarioidea are compared. Vestiges of pseudolabia do not occur in filarioids and the persistence of internal cephalic papillae in some genera indicates that this superfamily branched off early from a stem which also gave rise to the Spiruroidea. The hypothesis of Chabaud (1955) on the formation of papillae patterns in filarioids is followed generally but Pseudofilaria is regarded as the most primitive type which links the filarioids to the spiruroid family Thelaziidae. The condition of only four papillae, known conclusively only in Splendidofilaria, may have resulted from fusion of pairs of external papillae since in several genera there is a tendency towards fusion. Tetracheilonema, Litomosoides, and perhaps other genera (e.g. Icosiella, Setaria equina) apparently fall outside the main line of evolution which led to the commonest pattern of four pairs of submedian papillae (external papillae). Study of the cephalic ends of filarioids reveals characters useful in the identification of species.